An improved system for gene replacement and xylE fusion analysis in Pseudomonas aeruginosa

Impact of alginate conditioning film on deposition kinetics of motile and nonmotile Pseudomonas aeruginosa strains

The initial deposition of bacteria in most aquatic systems is affected by the presence of a conditioning film adsorbed at the liquid-solid interface. Due to the inherent complexity of such films, their impact on bacterial deposition remains poorly defined. The aim of this study was to gain a better understanding of the effect of a conditioning film on the deposition of motile and nonmotile Pseudomonas aeruginosa cells in a radial stagnation point flow system. A well-defined alginate film was used as a model conditioning film because of its polysaccharide and polyelectrolyte nature. Deposition experiments under favorable (nonrepulsive) conditions demonstrated the importance of swimming motility for cell transport towards the substrate. The impact of the flagella of motile cells on deposition is dependent on the presence of the conditioning film. We showed that on a clean substrate surface, electrostatic repulsion governs bacterial deposition and the presence of flagella increases cell deposition. However, our results suggest that steric interactions between flagella and extended polyelectrolytes of the conditioning film hinder cell deposition. At a high ionic strength (100 mM), active swimming motility and changes in alginate film structure suppressed the steric barrier and allowed conditions favorable for deposition. We demonstrated that bacterial deposition is highly influenced by cell motility and the structure of the conditioning film, which are both dependent on ionic strength.

l-Methionine sulfoximine, but not phosphinothricin, is a substrate for an acetyltransferase (gene PA4866) from Pseudomonas aeruginosa: structural and functional studies

The gene PA4866 from Pseudomonas aeruginosa is documented in the Pseudomonas genome database as encoding a 172 amino acid hypothetical acetyltransferase. We and others have described the 3D structure of this protein (termed pita) [Davies et al. (2005) Proteins: Struct., Funct., Bioinf. 61, 677-679; Nocek et al., unpublished results], and structures have also been reported for homologues from Agrobacterium tumefaciens (Rajashankar et al., unpublished results) and Bacillus subtilis [Badger et al. (2005) Proteins: Struct., Funct., Bioinf. 60, 787-796]. Pita homologues are found in a large number of bacterial genomes, and while the majority of these have been assigned putative phosphinothricin acetyltransferase activity, their true function is unknown. In this paper we report that pita has no activity toward phosphinothricin. Instead, we demonstrate that pita acts as an acetyltransferase using the glutamate analogues l-methionine sulfoximine and l-methionine sulfone as substrates, with Km(app) values of 1.3 +/- 0.21 and 1.3 +/- 0.13 mM and kcat(app) values of 505 +/- 43 and 610 +/- 23 s-1 for l-methionine sulfoximine and l-methionine sulfone, respectively. A high-resolution (1.55 A) crystal structure of pita in complex with one of these substrates (l-methionine sulfoximine) has been solved, revealing the mode of its interaction with the enzyme. Comparison with the apoenzyme structure has also revealed how certain active site residues undergo a conformational change upon substrate binding. To investigate the role of pita in P. aeruginosa, a mutant strain, Depp4, in which pita was inactivated through an in-frame deletion, was constructed by allelic exchange. Growth of strain Depp4 in the absence of glutamine was inhibited by l-methionine sulfoximine, suggesting a role for pita in protecting glutamine synthetase from inhibition.

Autoinduction of RpoS biosynthesis in the biocontrol strain Pseudomonas sp. M18

The rpoS gene from Pseudomonas sp. M18, which encodes predicted protein (an alternative sigma factor s, sigma(S), or sigma(38)) with 99.5% sequence identity with RpoS from Pseudomonas aeruginosa PAO1, was first cloned. In order to investigate the mechanism of rpoS expression, an rpoS null mutant, named M18S, was constructed with insertion of aacC1 cassette bearing a gentamycin resistance gene. With introduction of a plasmid containing an rpoS'-'lacZ translational fusion (pMERS) to wild-type strain M18 or M18S, it was first found that beta-galactosidase activity expressed in strain M18S (pMERS) decreased to fourfold of that expressed in the strain M18 (pMERS). When strain M18S (pMERS) was introduced with another plasmid pBBS containing the wild-type rpoS gene, its beta-galactosidase expression level was enhanced and almost restored to that in strain M18 (pMERS). Similarly, expression of beta-galactosidase from a chromosomal fusion of the promoter of the wild-type rpoS gene with lacZ (rpoS-lacZ) was enhanced fivefold in the presence of a plasmid with the wild-type rpoS gene. With these findings, it is suggested that RpoS sigma factor may be involved in autoinducing its own gene expression in Pseudomonas sp. M18.

Tn5530 from Burkholderia cepacia strain 2a encodes a chloride channel protein essential for the catabolism of 2,4-dichlorophenoxyacetic acid

Chloride channel proteins (ClC) are found in living systems where they transport chloride ions across cell membranes. Recently, the structure/function of two prokaryotic ClC has been determined but little is known about the role of these proteins in the microbial metabolism of chlorinated compounds. Here we show that transposon Tn5530 from Burkholderia cepacia strain 2a encodes a ClC protein (BcClC) which is responsible for expelling Cl(-) ions generated during the catabolism of 2,4-dichlorophenoxyacetic acid (a chlorinated herbicide). We found that BcClC has the ability to transport Cl(-) ions across reconstituted proteoliposome membranes. We created two mutants in which the intrachannel glutamate residue of the protein, known to be responsible for opening and closing the channel (i.e. gating), was changed in order to create constitutively open and closed forms. We observed that cells carrying the closed-channel protein accumulated Cl(-) ions intracellularly leading to a decrease in intracellular pH, cell stasis and death. Further, we established that BcClC has the same gating mechanism as that reported for the ClC protein from Salmonella typhimurium. Our results show that the physiological role of ClC is to maintain cellular homeostasis which can be impaired by the catabolism of chlorinated compounds.

BdlA, a chemotaxis regulator essential for biofilm dispersion in Pseudomonas aeruginosa

Multiple environmental cues have been shown to trigger biofilm detachment, the transition from surface-attached, highly organized communities known as biofilms to the motile lifestyle. The goal of this study was to identify a gene product involved in sensing environmental cues that trigger biofilm dispersion in Pseudomonas aeruginosa. To do so, we focused on novel putative chemotaxis transducer proteins that could potentially be involved in environmental sensing. We identified a locus encoding such a protein that played a role in detachment, as indicated by the observation that an isogenic mutant biofilm could not disperse in response to a variety of environmental cues. The locus was termed bdlA for biofilm dispersion locus. The BdlA protein harbors an MCP (methyl-accepting chemotaxis protein) domain and two PAS (Per-Arnt-Sint) domains that have been shown to be essential for responding to environmental signals in other proteins. The dispersion-deficient phenotype of the bdlA mutant was confirmed by treatment with the biocide H(2)O(2) and by microscopic observations. The dispersion response was independent of motility. bdlA mutant biofilms were found to have increased adherent properties and increased intracellular levels of cyclic di-GMP (c-di-GMP). Our findings suggest that BdlA may be a link between sensing environmental cues, c-di-GMP levels, and detachment. Based on our findings, a possible involvement of BdlA in a signaling cascade resulting in biofilm dispersion is discussed.

Characterization of OpdH, a Pseudomonas aeruginosa porin involved in the uptake of tricarboxylates

The Pseudomonas aeruginosa outer membrane is intrinsically impermeable to many classes of antibiotics, due in part to its relative lack of general uptake pathways. Instead, this organism relies on a large number of substrate-specific uptake porins. Included in this group are the 19 members of the OprD family, which are involved in the uptake of a diverse array of metabolites. One of these porins, OpdH, has been implicated in the uptake of cis-aconitate. Here we demonstrate that this porin may also enable P. aeruginosa to take up other tricarboxylates. Isocitrate and citrate strongly and specifically induced the opdH gene via a mechanism involving derepression by the putative two-component regulatory system PA0756-PA0757. Planar bilayer analysis of purified OpdH demonstrated that it was a channel-forming protein with a large single-channel conductance (230 pS in 1 M KCl; 10-fold higher than that of OprD); however, we were unable to demonstrate the presence of a tricarboxylate binding site within the channel. Thus, these data suggest that the requirement for OpdH for efficient growth on tricarboxylates was likely due to the specific expression of this large-channel porin under particular growth conditions.

Role of Pseudomonas aeruginosa dinB-encoded DNA polymerase IV in mutagenesis

Pseudomonas aeruginosa is a human opportunistic pathogen that chronically infects the lungs of cystic fibrosis patients and is the leading cause of morbidity and mortality of people afflicted with this disease. A striking correlation between mutagenesis and the persistence of P. aeruginosa has been reported. In other well-studied organisms, error-prone replication by Y family DNA polymerases contributes significantly to mutagenesis. Based on an analysis of the PAO1 genome sequence, P. aeruginosa contains a single Y family DNA polymerase encoded by the dinB gene. As part of an effort to understand the mechanisms of mutagenesis in P. aeruginosa, we have cloned the dinB gene of P. aeruginosa and utilized a combination of genetic and biochemical approaches to characterize the activity and regulation of the P. aeruginosa DinB protein (DinB(Pa)). Our results indicate that DinB(Pa) is a distributive DNA polymerase that lacks intrinsic proofreading activity in vitro. Modest overexpression of DinB(Pa) from a plasmid conferred a mutator phenotype in both Escherichia coli and P. aeruginosa. An examination of this mutator phenotype indicated that DinB(Pa) has a propensity to promote C-->A transversions and -1 frameshift mutations within poly(dGMP) and poly(dAMP) runs. The characterization of lexA+ and DeltalexA::aacC1 P. aeruginosa strains, together with in vitro DNA binding assays utilizing cell extracts or purified P. aeruginosa LexA protein (LexA(Pa)), indicated that the transcription of the dinB gene is regulated as part of an SOS-like response. The deletion of the dinB(Pa) gene sensitized P. aeruginosa to nitrofurazone and 4-nitroquinoline-1-oxide, consistent with a role for DinB(Pa) in translesion DNA synthesis over N2-dG adducts. Finally, P. aeruginosa exhibited a UV-inducible mutator phenotype that was independent of dinB(Pa) function and instead required polA and polC, which encode DNA polymerase I and the second DNA polymerase III enzyme, respectively. Possible roles of the P. aeruginosa dinB, polA, and polC gene products in mutagenesis are discussed.

2,3-butanediol synthesis and the emergence of the Vibrio cholerae El Tor biotype

Vibrio cholerae is an aquatic bacterium that causes the severe diarrheal disease cholera. V. cholerae strains of the O1 serogroup exist as two biotypes, classical and El Tor. Toxigenic strains of the El Tor biotype emerged to cause the seventh pandemic of cholera in 1961 and subsequently displaced strains of the classical biotype both in the environment and as a cause of cholera within a decade. The factors that drove emergence of the El Tor biotype and the displacement of the classical biotype are unknown. Here, we show a unique difference in carbohydrate metabolism between these two biotypes. When grown with added carbohydrates, classical biotype strains generated a sharp decrease in medium pH, resulting in loss of viability. However, growth of El Tor biotype strain N16961 was enhanced due to its ability to produce 2,3-butanediol, a neutral fermentation end product, and suppress the accumulation of organic acids. An N16961 mutant (SSY01) defective in 2,3-butanediol synthesis showed the same defect in growth that classical biotype strains show in media rich in carbohydrates. Importantly, the SSY01 mutant was attenuated in its ability to colonize the intestines of infant mice, suggesting that host carbohydrates may be available to V. cholerae within the intestinal environment. Similarly, the SSY01 mutant failed to develop biofilms when utilizing N-acetyl-D-glucosamine as a carbon source. Because growth on N-acetyl-D-glucosamine likely reflects the ability of a strain to grow on chitin in certain aquatic environments, we conclude that the strains of classical biotype are likely defective compared to those of El Tor in growth in any environmental niche that is rich in chitin and/or other metabolizable carbohydrates. We propose that the ability to metabolize sugars without production of acid by-products might account for the improved evolutionary fitness of the V. cholerae El Tor biotype compared to that of the classical biotype both as a global cause of cholera and as an environmental organism.

The type III pseudomonal exotoxin U activates the c-Jun NH2-terminal kinase pathway and increases human epithelial interleukin-8 production

Microbial interactions with host cell signaling pathways are key determinants of the host cell response to infection. Many toxins secreted by bacterial type III secretion systems either stimulate or inhibit the host inflammatory response. We investigated the role of type III secreted toxins of the lung pathogen Pseudomonas aeruginosa in the inflammatory response of human respiratory epithelial cells to infection. Using bacteria with specific gene deletions, we found that interleukin-8 production by these cells was almost entirely dependent on bacterial type III secretion of exotoxin U (ExoU), a phospholipase, although other bacterial factors are involved. ExoU activated the c-Jun NH(2)-terminal kinase pathway, stimulating the phosphorylation and activation of mitogen-activated kinase kinase 4, c-Jun NH(2)-terminal kinase, and c-Jun. This in turn increased levels of transcriptionally competent activator protein-1. Although this pathway was dependent on the lipase activity of ExoU, it was independent of cell death. Activation of mitogen-activated kinase signaling by ExoU in this fashion is a novel mechanism by which a bacterial product can initiate a host inflammatory response, and it may result in increased epithelial permeability and bacterial spread.

Analysis of FimX, a phosphodiesterase that governs twitching motility in Pseudomonas aeruginosa

Type IV pili (Tfp) are polar surface structures of Pseudomonas aeruginosa required for twitching motility, biofilm formation and adherence. One protein required for the assembly of tfp is FimX, which possesses both GGDEF and EAL domains characteristic of diguanylate cyclases and phosphodiesterases respectively. In this work we demonstrate that FimX has phosphodiesterase activity towards bis-(3'-5')-cyclic dimeric guanosine monophosphate (c-di-GMP), but does not show diguanylate cyclase activity. Instead, the imperfect GGDEF domain of FimX likely serves to activate phosphodiesterase activity when bound to GTP, as has recently been described for the Caulobacter crescentus composite GGDEF-EAL protein, CC3396. Bacteria expressing FimX in which either the GGDEF or EAL domain is deleted or mutated have phenotypes indistinguishable from a DeltafimX strain, demonstrating the importance of both domains to function. Previous work has shown that FimX localizes to the bacterial pole. In this work we show that restriction of FimX to a single pole requires intact GGDEF and EAL domains. Deletion of the amino-terminal REC domain of FimX, which contains a putative polar localization signal, results in a protein that still supports intermediate levels of pilus assembly and function. RFP-FimXDeltaREC, unlike RFP-FimX, is no longer localized to the bacterial pole, while transmission electron microscopy shows that surface pili can originate from non-polar sites in this mutant. Although DeltafimX mutants show limited in vitro cytotoxicity, they are as virulent as the wild-type strain in a murine model of acute pneumonia.

Molecular validation of LpxC as an antibacterial drug target in Pseudomonas aeruginosa

LpxC [UDP-3-O-(R-3-hydroxymyristoyl)-GlcNAc deacetylase] is a metalloamidase that catalyzes the first committed step in the biosynthesis of the lipid A component of lipopolysaccharide. A previous study (H. R. Onishi, B. A. Pelak, L. S. Gerckens, L. L. Silver, F. M. Kahan, M. H. Chen, A. A. Patchett, S. M. Galloway, S. A. Hyland, M. S. Anderson, and C. R. H. Raetz, Science 274:980-982, 1996) identified a series of synthetic LpxC-inhibitory molecules that were bactericidal for Escherichia coli. These molecules did not inhibit the growth of Pseudomonas aeruginosa and were therefore not developed further as antibacterial drugs. The inactivity of the LpxC inhibitors for P. aeruginosa raised the possibility that LpxC activity might not be essential for all gram-negative bacteria. By placing the lpxC gene of P. aeruginosa under tight control of an arabinose-inducible promoter, we demonstrated the essentiality of LpxC activity for P. aeruginosa. It was found that compound L-161,240, the most potent inhibitor from the previous study, was active against a P. aeruginosa construct in which the endogenous lpxC gene was inactivated and in which LpxC activity was supplied by the lpxC gene from E. coli. Conversely, an E. coli construct in which growth was dependent on the P. aeruginosa lpxC gene was resistant to the compound. The differential activities of L-161,240 against the two bacterial species are thus the result primarily of greater potency toward the E. coli enzyme rather than of differences in the intrinsic resistance of the bacteria toward antibacterial compounds due to permeability or efflux. These data validate P. aeruginosa LpxC as a target for novel antibiotic drugs and should help direct the design of inhibitors against clinically important gram-negative bacteria.

Pseudomonas aeruginosa 1244 pilin glycosylation: glycan substrate recognition

The pilin of Pseudomonas aeruginosa 1244 is glycosylated with an oligosaccharide that is structurally identical to the O-antigen repeating unit of this organism. Concordantly, the metabolic source of the pilin glycan is the O-antigen biosynthetic pathway. The present study was conducted to investigate glycan substrate recognition in the 1244 pilin glycosylation reaction. Comparative structural analysis of O subunits that had been previously shown to be compatible with the 1244 glycosylation machinery revealed similarities among sugars at the presumed reducing termini of these oligosaccharides. We therefore hypothesized that the glycosylation substrate was within the sugar at the reducing end of the glycan precursor. Since much is known of PA103 O-antigen genetics and because the sugars at the reducing termini of the O7 (strain 1244) and O11 (strain PA103) are identical (beta-N-acetyl fucosamine), we utilized PA103 and strains that express lipopolysaccharide (LPS) with a truncated O-antigen subunit to test our hypothesis. LPS from a strain mutated in the wbjE gene produced an incomplete O subunit, consisting only of the monosaccharide at the reducing end (beta-d-N-acetyl fucosamine), indicating that this moiety contained substrate recognition elements for WaaL. Expression of pilAO(1244) in PA103 wbjE::aacC1, followed by Western blotting of extracts of these cells, indicated that pilin produced has been modified by the addition of material consistent with a single N-acetyl fucosamine. This was confirmed by analyzing endopeptidase-treated pilin by mass spectrometry. These data suggest that the pilin glycosylation substrate recognition features lie within the reducing-end moiety of the O repeat and that structures of the remaining sugars are irrelevant.

Metabolic engineering of strains of Ralstonia eutropha and Pseudomonas putida for biotechnological production of 2-methylcitric acid

In this study strains of Ralstonia eutropha H16 and Pseudomonas putida KT2440 were engineered which are suitable for biotechnological production of 2-methylcitric acid (2MC). Analysis of a previous mutant of R. eutropha able to accumulate 2MC recommended this strain as a candidate for fermentative production of 2MC. This knowledge was used for construction of strains of R. eutropha H16 and P. putida KT2440 capable of enhanced production of 2MC. In both bacteria the chromosomal genes encoding the 2-methyl-cis-aconitate hydratase (acnM) were disrupted by directed insertion of a copy of an additional 2-methylcitrate synthase gene (prpC) yielding strains R. eutropha DeltaacnM(Re)OmegaKmprpC(Pp) and P. putida DeltaacnM(Pp)OmegaKmprpC(Re). In both strains 2-methylcitrate synthase was expressed under control of the constitutive kanamycin-resistance gene (OmegaKm) resulting in up to 20-fold higher specific 2-methylcitrate synthase activities in comparison to the wild type. The disruption of the acnM gene by insertion of prpC led to a propionate- and levulinate-negative phenotype of the engineered strains, and analysis of supernatant of these strains revealed overproduction and accumulation of 2MC in the medium. A two stage cultivation regime comprising an exponential growth phase and a 2MC production phase was developed and applied to both engineered strains for optimum production of 2MC. Whereas gluconate, fructose or succinate were provided as carbon source for the exponential growth phase, a combination of propionate or levulinate as precursor substrate for provision of propionyl-CoA and succinate or fumarate as precursor substrate for provision of oxaloacetate were used in the production phase to make sure that the 2-methylcitrate synthase was provided with their substrates. Employing the optimised feeding regime P. putida DeltaacnM(Pp)OmegaKmprpC(Re) and R. eutropha DeltaacnM(Re)OmegaKmprpC(Pp) produced 2MC up to maximal concentrations of 7.2 g/L or 26.5 mM and 19.2 g/L or 70.5 mM, respectively, during 144 h of cultivation.

Role of the Pseudomonas aeruginosa PlcH Tat signal peptide in protein secretion, transcription, and cross-species Tat secretion system compatibility

The secretion of PlcH and its homolog PlcN of Pseudomonas aeruginosa through the inner membrane depends upon a functional twin arginine translocase (Tat) system and a Tat signal sequence. Conserved twin arginine (Arg) residues within the Tat signal sequence consensus motif (S/TRRxFLK) are considered essential for the secretion of Tat substrates, but some exceptions (e.g., Lys and Arg) to the twin Arg residues in this motif have been noted. The roles of all three Arg residues within the PlcH RRRTFLK consensus motif were examined. Data are presented which indicate that Arg-9 and Arg-10 are essential for PlcH secretion across the inner membrane, but the mutation of Arg-8 (e.g., to Ala or Ser) had no observable effect on the localization of PlcH. In the signal sequence of PlcH and in all of its homologs in other bacteria, there are basic amino acid residues (Arg, Lys, and Gln) immediately adjacent to the signal peptidase cleavage site (Ala-X-Ala) that are not seen in Sec-dependent signal sequences. The mutation of these basic residues to Ala caused slightly decreased levels of extracellular PlcH, but normal localization was still observed. Deletion of the entire Tat signal sequence of PlcH not only resulted in the absence of detectable extracellular PlcH activity and protein but also caused a substantial decrease in the detectable level of plcH mRNA. Finally, data are presented which indicate that P. aeruginosa PlcH exhibits cross-species compatibility with the Escherichia coli Tat secretion machinery, but only when the E. coli Tat machinery is expressed in a P. aeruginosa host.

lbtA and lbtB are required for production of the Legionella pneumophila siderophore legiobactin

Under iron stress, Legionella pneumophila secretes legiobactin, a nonclassical siderophore that is reactive in the chrome azurol S (CAS) assay. Here, we have optimized conditions for legiobactin expression, shown its biological activity, and identified two genes, lbtA and lbtB, which are involved in legiobactin production. lbtA appears to be iron repressed and encodes a protein that has significant homology with siderophore synthetases, and FrgA, a previously described iron-regulated protein of L. pneumophila. lbtB encodes a protein homologous with members of the major facilitator superfamily of multidrug efflux pumps. Mutants lacking lbtA or lbtB were defective for legiobactin, producing 40 to 70% less CAS reactivity in deferrated chemically defined medium (CDM). In bioassays, mutant CDM culture supernatants, unlike those of the wild type, did not support growth of iron-limited wild-type bacteria in 2',2'-dipyridyl-containing buffered charcoal yeast extract (BCYE) agar and a ferrous iron transport mutant on BCYE agar without added iron. The lbtA mutant was modestly defective for growth in deferrated CDM containing the iron chelator citrate, indicating that legiobactin is required in conditions of severe iron limitation. Complementation of the lbt mutants restored both siderophore expression, as measured by the CAS assay and bioassays, and bacterial growth in deferrated, citrate-containing media. The lbtA mutant replicated as the wild type did in macrophages, amoebae, and the lungs of mice. However, L. pneumophila expresses lbtA in the macrophage, suggesting that legiobactin, though not required, may play a dispensable role in intracellular growth. The discovery of lbtAB represents the first identification of genes required for L. pneumophila siderophore expression.

Role of the novel OprD family of porins in nutrient uptake in Pseudomonas aeruginosa

To circumvent the permeability barrier of its outer membrane, Pseudomonas aeruginosa has evolved a series of specific porins. These channels have binding sites for related classes of molecules that facilitate uptake under nutrient-limited conditions. Here, we report on the identification of a 19-member family of porins similar to the basic-amino-acid-specific porin OprD. The members of this family fell into one of two phylogenetically distinct clusters, one bearing high similarity to OprD and the other bearing most similarity to the putative phenylacetic acid uptake porin PhaK of Pseudomonas putida. Analysis of the genome context, operon arrangement, and regulation of the PhaK-like porin OpdK indicated that it might be involved in vanillate uptake. This result was confirmed by demonstrating that an opdK mutant had a deficiency in the ability to grow on vanillate as a carbon source. To extrapolate these data to other paralogues within this family, the substrate specificities of 6 of the 17 remaining OprD homologues were inferred using an approach similar to that used with opdK. The specificities determined were as follows: OpdP, glycine-glutamate; OpdC, histidine; OpdB, proline; OpdT, tyrosine; OpdH, cis-aconitate; and OpdO, pyroglutamate. Thus, members of the OprD subfamily took up amino acids and related molecules, and those characterized members most similar to PhaK were responsible for the uptake of a diverse array of organic acids. These results imply that there is a functional basis for the phylogenetic clustering of these proteins and provide a framework for studying OprD homologues in other organisms.

PsrA is a positive transcriptional regulator of the type III secretion system in Pseudomonas aeruginosa

The type III secretion system (TTSS) of Pseudomonas aeruginosa is induced in vivo upon contact with eukaryotic cells and in vitro by calcium depletion in culture medium. We have observed a previously identified protein, PsrA, necessary for full activation of TTSS gene expression in P. aeruginosa. Electrophoretic mobility shift assays showed that recombinant PsrA could bind to the exsCEBA promoter region. A mutant with a deletion in the psrA gene was constructed. Using transcriptional fusions, we demonstrated that PsrA is required for the full activation of transcription of the TTSS regulatory operon exsCEBA and effector exoS, although the deletion mutant still responded to calcium depletion, to serum, and to host cell contact. The psrA mutant showed a marked decrease in the secretion of the type III effectors and weak resistance to phagocyte-like PLB-985 cells. The defect in TTSS transcription and secretion in the psrA mutant could be complemented by expression in trans of psrA. PsrA was previously identified as a transcriptional activator of RpoS, a central regulator during stationary phase. We confirmed with our strain that RpoS has a negative effect on TTSS gene expression. Taken altogether, these results suggest that PsrA is a newly identified activator that is involved in the expression of the TTSS by enhancing the exsCEBA transcriptional level.

Pseudomonas aeruginosa AmpR is a global transcriptional factor that regulates expression of AmpC and PoxB beta-lactamases, proteases, quorum sensing, and other virulence factors

In members of the family Enterobacteriaceae, ampC, which encodes a beta-lactamase, is regulated by an upstream, divergently transcribed gene, ampR. However, in Pseudomonas aeruginosa, the regulation of ampC is not understood. In this study, we compared the characteristics of a P. aeruginosa ampR mutant, PAOampR, with that of an isogenic ampR+ parent. The ampR mutation greatly altered AmpC production. In the absence of antibiotic, PAOampR expressed increased basal beta-lactamase levels. However, this increase was not followed by a concomitant increase in the P(ampC) promoter activity. The discrepancy in protein and transcription analyses led us to discover the presence of another chromosomal AmpR-regulated beta-lactamase, PoxB. We found that the expression of P. aeruginosa ampR greatly altered the beta-lactamase production from ampC and poxB in Escherichia coli: it up-regulated AmpC but down-regulated PoxB activities. In addition, the constitutive P(ampR) promoter activity in PAOampR indicated that AmpR did not autoregulate in the absence or presence of inducers. We further demonstrated that AmpR is a global regulator because the strain carrying the ampR mutation produced higher levels of pyocyanin and LasA protease and lower levels of LasB elastase than the wild-type strain. The increase in LasA levels was positively correlated with the P(lasA), P(lasI), and P(lasR) expression. The reduction in the LasB activity was positively correlated with the P(rhlR) expression. Thus, AmpR plays a dual role, positively regulating the ampC, lasB, and rhlR expression levels and negatively regulating the poxB, lasA, lasI, and lasR expression levels.

Influence of pilin glycosylation on Pseudomonas aeruginosa 1244 pilus function

The opportunistic pathogen Pseudomonas aeruginosa is a leading cause of nosocomial pneumonia. Among its virulence factors, the type IV pili of P. aeruginosa strain 1244 contain a covalently linked, three-sugar glycan of previously unknown significance. The work described in this paper was carried out to determine the influence of the P. aeruginosa 1244 pilin glycan on pilus function, as well as a possible role in pathogenesis. To accomplish this, a deletion was introduced into the pilO gene of this organism. The isogenic knockout strain produced, 1244G7, was unable to glycosylate pilin but could produce pili normal in appearance and quantity. In addition, this strain had somewhat reduced twitching motility, was sensitive to pilus-specific bacteriophages, and could form a normal biofilm. Analysis of whole cells and isolated pili from wild-type P. aeruginosa strain 1244 by transmission electron microscopy with a glycan-specific immunogold label showed that this saccharide was distributed evenly over the fiber surface. The presence of the pilin glycan reduced the hydrophobicity of purified pili as well as whole cells. With regard to pathogenicity, P. aeruginosa strains producing glycosylated pili were commonly found among clinical isolates and particularly among those strains isolated from sputum. Competition index analysis using a mouse respiratory model comparing strains 1244 and 1244G7 indicated that the presence of the pilin glycan allowed for significantly greater survival in the lung environment. These results collectively suggest that the pilin glycan is a significant virulence factor and may aid in the establishment of infection.

Application of a KDPG-aldolase gene-dependent addiction system for enhanced production of cyanophycin in Ralstonia eutropha strain H16

Two different recombinant plasmids both containing the cyanophycin synthetase gene (cphA) of Synechocystis sp. strain PCC6308 but differing concerning the resistance marker gene were tested for their suitability to produce high amounts of cyanophycin in recombinant strains of Ralstonia eutropha. Various cultivation experiments at the 30-L scale revealed very low cyanophycin contents of the cells ranging from 4.6% to 6.2% (w/w) of cellular dry weight (CDW) only, most probably because most cells had lost the corresponding plasmid during cultivation. To establish a cost effective and high efficient system for production of cyanophycin at larger scales using recombinant strains of R. eutropha, we applied two strategies: First, we integrated cphA into the dispensable chromosomal l-lactate dehydrogenase gene (ldh) of R. eutropha. Depending on the cultivation conditions used, relatively low cyanophycin contents between 2.2% and 7.7% (w/w) of CDW were reproducibly detected, which might be due to weak expression or low gene dosage in the single cphA copy strain of R. eutropha. In a second strategy we constructed a KDPG-aldolase gene (eda)-dependent addiction system, which combined features of a multi-copy plasmid with stabilized expression of cphA. Flasks experiments revealed that the cells accumulated extraordinarily high amounts of cyanophycin between 26.9% and 40.0% (w/w) of CDW even under cultivation conditions lacking cyanophycin precursor substrates or plasmid stabilizing antibiotics. Cyanophycin contents of up to 40.0% (w/w) of CDW were also obtained at a 30-L scale or a 500-L pilot-plant scale under such non-selective conditions. This demonstrates impressively that the stabilizing effect of the constructed eda-dependent addiction system can be used for production of enhanced amounts of cyanophycin at a larger scale in recombinant strains of R. eutropha.

Evidence that WbpD is an N-acetyltransferase belonging to the hexapeptide acyltransferase superfamily and an important protein for O-antigen biosynthesis in Pseudomonas aeruginosa PAO1

Di-N-acetylated uronic acid residues are unique sugar moieties observed in the lipopolysaccharides (LPS) of respiratory pathogens including several serotypes of Pseudomonas aeruginosa and several species of Bordetella. WbpD of P. aeruginosa PAO1 (serotype O5) is a putative 3-N-acetyltransferase that has been implicated in the biosynthesis of UDP-2,3-diacetamido-2,3-dideoxy-d-mannuronic acid [UDP-d-Man(2NAc3NAc)A], a precursor for the d-Man(2NAc3NAc)A residues in the B-band O antigen of this bacterium. A chromosomal knockout mutant of wbpD is incapable of producing either long-chain B-band O antigen (> or = 2 repeating units) or semi-rough LPS (lipid A-core + one repeat). Adding wbpD in trans restored LPS production to the wild-type level; this indicates that wbpD is important for biosynthesis of individual B-band O-antigen repeating units. WbpD contains left-handed beta-helical (LbetaH) structure as observed by Conserved Domain analysis and in silico secondary and tertiary structure predictions. This feature suggested that WbpD belongs to the hexapeptide acyltransferase (HexAT) superfamily of enzymes. WbpD was overexpressed as an N-terminally histidine-tagged fusion protein (His6-WbpD) and purified to > 95% purity. The protein was subjected to Far-UV circular dichroism spectroscopy, and the data revealed that WbpD contains left-handed helical structure, which substantiated in silico predictions made earlier. Results from SDS-PAGE, matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF) mass spectrometry (MS), and gel filtration analyses indicated that His6-WbpD has trimeric organization, consistent with the quaternary structure of HexATs. The binding of acetyl-CoA by WbpD was demonstrated by MALDI-TOF MS, suggesting that WbpD is an acetyltransferase that utilizes a direct-transfer reaction mechanism. Incubation of WbpD with acetyl-CoA significantly enhanced the stability of the protein and prevented precipitation over a course of 14 days. As a substrate for studying the enzymatic activity of WbpD is unavailable at present, a structure-based model for the LbetaH domain of WbpD was generated. Comparisons between this model and the LbetaH domains of known HexATs suggested that Lys136 plays a role in acetyl-CoA binding. A K136A site-directed mutant construct could only partially complement the wbpD knockout, and this mutation also reduced the stabilizing effects of acetyl-CoA, while a K136R mutation showed no discernible effect on complementation of the wbpD mutant or the stabilizing effects of acetyl-CoA on the purified mutant protein. A modified pathway was proposed for the biosynthesis of UDP-d-Man(2NAc3NAc)A, in which WbpD is involved in the catalysis of the fourth step by acting as a UDP-2-acetamido-3-amino-2,3-dideoxy-d-glucuronic acid 3-N-acetyltransferase.

Physiological roles of three Na+/H+ antiporters in the halophilic bacterium Vibrio parahaemolyticus

Vibrio parahaemolyticus mutants lacking three Na+/H+ antiporters (NhaA, NhaB, NhaD) were constructed. The DeltanhaA strains showed significantly higher sensitivity to LiCl regarding their growth compared to the parental strain. The DeltanhaA and DeltanhaB strains exhibited higher sensitivities to LiCl. The mutant XACabd lacking all of the three antiporters could not grow in the presence of 500 mM LiCl at pH 7.0, or 50 mM at pH 8.5. The XACabd mutant was also sensitive to 1.0 M NaCl at pH 8.5. These results suggest that Na+/H+ antiporters, especially NhaA, are responsible for resistance to LiCl and to high concentrations of NaCl. Reduced Na+/H+ and Li+/H+ antiport activities were observed with everted membrane vesicles of DeltanhaB strains. However, Li+/H+ antiport activities of DeltanhaB strains were two times higher than those of DeltanhaA strains when cells were cultured at pH 8.5. It seems that expression of nhaA and nhaB is dependent on medium pH to some extent. In addition, HQNO (2-heptyl-4-hydroxyquinoline N-oxide), which is a potent inhibitor of the respiratory Na+ pump, inhibited growth of XACabd, but not of the wild type strain. Moreover, survival rate of XACabd under hypoosmotic stress was lower than that of wild type strain. It is likely that the Na+/H+ antiporters are involved in osmoregulation under hypoosmotic stress. Based on these findings, we propose that the Na+/H+ antiporters cooperate with the respiratory Na+ pump in ionic homeostasis in V. parahaemolyticus.

Adaptations of Pseudomonas aeruginosa to the cystic fibrosis lung environment can include deregulation of zwf, encoding glucose-6-phosphate dehydrogenase

Cystic fibrosis (CF) patients are highly susceptible to chronic pulmonary disease caused by mucoid Pseudomonas aeruginosa strains that overproduce the exopolysaccharide alginate. We showed here that a mutation in zwf, encoding glucose-6-phosphate dehydrogenase (G6PDH), leads to a approximately 90% reduction in alginate production in the mucoid, CF isolate, P. aeruginosa FRD1. The main regulator of alginate, sigma-22 encoded by algT (algU), plays a small but demonstrable role in the induction of zwf expression in P. aeruginosa. However, G6PDH activity and zwf expression were higher in FRD1 strains than in PAO1 strains. In PAO1, zwf expression and G6PDH activity are known to be subject to catabolite repression by succinate. In contrast, FRD1 zwf expression and G6PDH activity were shown to be refractory to such catabolite repression. This was apparently not due to a defect in the catabolite repression control (Crc) protein. Such relaxed control of zwf was found to be common among several examined CF isolates but was not seen in other strains of clinical and environmental origin. Two sets of clonal isolates from individual CF patient indicated that the resident P. aeruginosa strain underwent an adaptive change that deregulated zwf expression. We hypothesized that high-level, unregulated G6PDH activity provided a survival advantage to P. aeruginosa within the lung environment. Interestingly, zwf expression in P. aeruginosa was shown to be required for its resistance to human sputum. This study illustrates that adaptation to the CF pulmonary environment by P. aeruginosa can include altered regulation of basic metabolic activities, including carbon catabolism.

Identification of Erwinia amylovora genes induced during infection of immature pear tissue

The enterobacterium Erwinia amylovora is a devastating plant pathogen causing necrotrophic fire blight disease of apple, pear, and other rosaceous plants. In this study, we used a modified in vivo expression technology system to identify E. amylovora genes that are activated during infection of immature pear tissue, a process that requires the major pathogenicity factors of this organism. We identified 394 unique pear fruit-induced (pfi) genes on the basis of sequence similarity to known genes and separated them into nine putative function groups including host-microbe interactions (3.8%), stress response (5.3%), regulation (11.9%), cell surface (8.9%), transport (13.5%), mobile elements (1.0%), metabolism (20.3%), nutrient acquisition and synthesis (15.5%), and unknown or hypothetical proteins (19.8%). Known virulence genes, including hrp/hrc components of the type III secretion system, the major effector gene dspE, type II secretion, levansucrase (lsc), and regulators of levansucrase and amylovoran biosynthesis, were upregulated during pear tissue infection. Known virulence factors previously identified in E. (Pectobacterium) carotovora and Pseudomonas syringae were identified for the first time in E. amylovora and included HecA hemagglutinin family adhesion, Peh polygalacturonase, new effector HopPtoC(EA), and membrane-bound lytic murein transglycosylase MltE(EA). An insertional mutation within hopPtoC(EA) did not result in reduced virulence; however, an mltE(EA) knockout mutant was reduced in virulence and growth in immature pears. This study suggests that E. amylovora utilizes a variety of strategies during plant infection and to overcome the stressful and poor nutritional environment of its plant hosts.

Expression of the psl operon in Pseudomonas aeruginosa PAO1 biofilms: PslA performs an essential function in biofilm formation

The psl gene cluster, comprising 15 cotranscribed genes from Pseudomonas aeruginosa, was recently identified as being involved in exopolysaccharide biosynthesis and biofilm formation. In this study, we investigated the regulation of the psl gene cluster and the function of the first gene in this cluster, the pslA gene. PslA shows strong similarities to UDP-glucose lipid carriers. An isogenic marker-free pslA deletion mutant of P. aeruginosa PAO1 deficient in attachment and biofilm formation was used for complementation studies. The expression of only the pslA gene, comprising a coding region of 1,437 bp, restored the biofilm-forming phenotype of the wild type, indicating that PslA is required for biofilm formation by nonmucoid P. aeruginosa. The promoter region of the psl gene cluster, which encodes PslA-PslO, was identified by rapid amplification of cDNA 5' ends. Promoter assays using transcriptional fusions to lacZ and gfp indicated a constitutive expression of the psl cluster in planktonic cells and a highly regulated and localized expression in biofilms, respectively. Expression of the psl cluster in biofilms was almost exclusively found in the centers of microcolonies, as revealed by confocal laser scanning microscopy. These data suggest that constitutive expression of the psl operon enables efficient attachment to surfaces and that regulated localized psl operon expression is required for biofilm differentiation.

Global analysis of cellular factors and responses involved in Pseudomonas aeruginosa resistance to arsenite

The impact of arsenite [As(III)] on several levels of cellular metabolism and gene regulation was examined in Pseudomonas aeruginosa. P. aeruginosa isogenic mutants devoid of antioxidant enzymes or defective in various metabolic pathways, DNA repair systems, metal storage proteins, global regulators, or quorum sensing circuitry were examined for their sensitivity to As(III). Mutants lacking the As(III) translocator (ArsB), superoxide dismutase (SOD), catabolite repression control protein (Crc), or glutathione reductase (Gor) were more sensitive to As(III) than wild-type bacteria. The MICs of As(III) under aerobic conditions were 0.2, 0.3, 0.8, and 1.9 mM for arsB, sodA sodB, crc, and gor mutants, respectively, and were 1.5- to 13-fold less than the MIC for the wild-type strain. A two-dimensional gel/matrix-assisted laser desorption ionization-time of flight analysis of As(III)-treated wild-type bacteria showed significantly (>40-fold) increased levels of a heat shock protein (IbpA) and a putative allo-threonine aldolase (GlyI). Smaller increases (up to 3.1-fold) in expression were observed for acetyl-coenzyme A acetyltransferase (AtoB), a probable aldehyde dehydrogenase (KauB), ribosomal protein L25 (RplY), and the probable DNA-binding stress protein (PA0962). In contrast, decreased levels of a heme oxygenase (HemO/PigA) were found upon As(III) treatment. Isogenic mutants were successfully constructed for six of the eight genes encoding the aforementioned proteins. When treated with sublethal concentrations of As(III), each mutant revealed a marginal to significant lag period prior to resumption of apparent normal growth compared to that observed in the wild-type strain. Our results suggest that As(III) exposure results in an oxidative stress-like response in P. aeruginosa, although activities of classic oxidative stress enzymes are not increased. Instead, relief from As(III)-based oxidative stress is accomplished from the collective activities of ArsB, glutathione reductase, and the global regulator Crc. SOD appears to be involved, but its function may be in the protection of superoxide-sensitive sulfhydryl groups.

Two-component signal transduction pathways regulating growth and cell cycle progression in a bacterium: a system-level analysis

Two-component signal transduction systems, comprised of histidine kinases and their response regulator substrates, are the predominant means by which bacteria sense and respond to extracellular signals. These systems allow cells to adapt to prevailing conditions by modifying cellular physiology, including initiating programs of gene expression, catalyzing reactions, or modifying protein–protein interactions. These signaling pathways have also been demonstrated to play a role in coordinating bacterial cell cycle progression and development. Here we report a system-level investigation of two-component pathways in the model organism Caulobacter crescentus. First, by a comprehensive deletion analysis we show that at least 39 of the 106 two-component genes are required for cell cycle progression, growth, or morphogenesis. These include nine genes essential for growth or viability of the organism. We then use a systematic biochemical approach, called phosphotransfer profiling, to map the connectivity of histidine kinases and response regulators. Combining these genetic and biochemical approaches, we identify a new, highly conserved essential signaling pathway from the histidine kinase CenK to the response regulator CenR, which plays a critical role in controlling cell envelope biogenesis and structure. Depletion of either cenK or cenR leads to an unusual, severe blebbing of cell envelope material, whereas constitutive activation of the pathway compromises cell envelope integrity, resulting in cell lysis and death. We propose that the CenK–CenR pathway may be a suitable target for new antibiotic development, given previous successes in targeting the bacterial cell wall. Finally, the ability of our in vitro phosphotransfer profiling method to identify signaling pathways that operate in vivo takes advantage of an observation that histidine kinases are endowed with a global kinetic preference for their cognate response regulators. We propose that this system-wide selectivity insulates two-component pathways from one another, preventing unwanted cross-talk.

Histidine kinases and their (sensory) response regulators are screened for in C. crescentus. Follow-up experiments determine several essential components, including one pair critical for cell envelope biogenesis and structure.

Occurrence and expression of tricarboxylate synthases in Ralstonia eutropha

2-Methylcitrate synthase (2-MCS1) and citrate synthase (CS) of Ralstonia eutropha strain H16 were separated by affinity chromatography and analyzed for their substrate specificities. 2-MCS1 used not only the primary substrate propionyl-CoA but also acetyl-CoA and, at a low rate, even butyryl-CoA and valeryl-CoA for condensation with oxaloacetate. The KM values for propionyl-CoA and acetyl-CoA were 0.061 or 0.35 mM, respectively. This enzyme is therefore a competitor for acetyl-CoA during biosynthesis of poly(3-hydroxybutyrate) (PHB) and has to be taken into account if metabolic fluxes are calculated for PHB biosynthesis. In contrast, CS could not use propionyl-CoA as a substrate. The gene-encoding CS (cisY) of R. eutropha was cloned and encodes for a protein consisting of 433 amino acids with a calculated molecular weight of 48,600 Da; it is not truncated in the N-terminal region. Furthermore, a gene encoding a second functionally active 2-methylcitrate synthase (2-MCS2, prpC2) was identified in the genome of R. eutropha. The latter was localized in a gene cluster with genes for an NAD(H)-dependent malate dehydrogenase and a putative citrate lyase. RT-PCR analysis of R. eutropha growing on different carbon sources revealed the transcription of prpC2. In addition, cells of recombinant Escherichia coli strains harboring prpC2 of R. eutropha exhibited high 2-MCS activity of 0.544 U mg-1. A prpC2 knockout mutant of R. eutropha exhibited an identical phenotype as the wild type if grown on different media. 2-MCS2 seems to be dispensable, and a function could not be revealed for this enzyme.

A novel sensor kinase-response regulator hybrid regulates type III secretion and is required for virulence in Pseudomonas aeruginosa

The type III secretion system (TTSS) of Pseudomonas aeruginosa is induced by contact with eukaryotic cells and by growth in low-calcium media. We have identified a protein, RtsM, that is necessary for expression of the TTSS genes in P. aeruginosa. RtsM possesses both histidine kinase and response regulator domains common to two-component signalling proteins, as well as a large predicted periplasmic domain and seven transmembrane domains. Deletion of rtsM resulted in a defect in production and secretion of the type III effectors. Northern blot analysis revealed that mRNAs encoding the effectors ExoT and ExoU are absent in the DeltartsM strain under TTSS-inducing conditions. Using transcriptional fusions, we demonstrated that RtsM is required for transcription of the operons encoding the TTSS effectors and apparatus in response to calcium limitation or to host cell contact. The operon encoding the TTSS regulator ExsA does not respond to calcium limitation, but the basal transcription rate of this operon was lower in deltartsM than in the wild-type parent, PA103. The defect in TTSS effector production and secretion of deltartsM could be complemented by overexpressing ExsA or Vfr, two transcriptional activators involved in TTSS regulation. DeltartsM was markedly less virulent than PA103 in a murine model of acute pneumonia, demonstrating that RtsM is required in vivo. We propose that RtsM is a sensor protein at the start of a signalling cascade that induces expression of the TTSS in response to environmental signals.

Characterization of starvation-induced dispersion in Pseudomonas putida biofilms

The biofilm lifestyle, where microbial cells are aggregated because of expression of cell-to-cell interconnecting compounds, is believed to be of paramount importance to microbes in the environment. Because microbes must be able to alternate between sessile and planktonic states, it is anticipated that they must be able to regulate their ability to form biofilm and to dissolve biofilm. We present an investigation of a biofilm dissolution process occurring in flow-chamber-grown Pseudomonas putida biofilms. Local starvation-induced biofilm dissolution appears to be an integrated part of P. putida biofilm development that causes characteristic structural rearrangements. Rapid global dissolution of entire P. putida biofilms was shown to occur in response to carbon starvation. Genetic analysis suggested that the adjacent P. putida genes PP0164 and PP0165 play a role in P. putida biofilm formation and dissolution. PP0164 encodes a putative periplasmic protein of previously unknown function, and PP0164 mutant bacteria are sticky, and unable to reduce their adhesiveness and dissolve their biofilm in response to carbon starvation. PP0165 encodes a putative transmembrane protein containing GGDEF and EAL domains, and PP0165 mutant bacteria are unable to increase their adhesiveness and form biofilm. We suggest that the PP0164 and PP0165 proteins are involved in the regulation of the adhesiveness of the bacteria; the PP0165 protein through c-di-GMP signalling, and the PP0164 protein as a transducer of the signal.

Calcium-induced virulence factors associated with the extracellular matrix of mucoid Pseudomonas aeruginosa biofilms

Pseudomonas aeruginosa colonizes the pulmonary tissue of patients with cystic fibrosis (CF), leading to biofilm-associated infections. The pulmonary fluid of CF patients usually contains elevated concentrations of cations and may contain the P. aeruginosa redox-active pigment pyocyanin, which is known to disrupt calcium homeostasis of host cells. Since divalent cations are important bridging ions for bacterial polysaccharides and since they may play regulatory roles in bacterial gene expression, we investigated the effect of calcium ions on the extracellular matrix constituents of P. aeruginosa biofilms. For mucoid strain P. aeruginosa FRD1, calcium addition (1.0 and 10 mM as CaCl(2)) resulted in biofilms that were at least 10-fold thicker than biofilms without added calcium. Scanning confocal laser microscopy showed increased spacing between cells for the thick biofilms, and Fourier transform infrared spectroscopy revealed that the material between cells is primarily alginate. An algD transcriptional reporter demonstrated that calcium addition caused an eightfold increase in alg gene expression in FRD1 biofilms. Calcium addition also resulted in increased amounts of three extracellular proteases (AprA, LasB, and PrpL). Immunoblots of the biofilm extracellular material established that AprA was harbored within the biofilm extracellular matrix. An aprA deletion mutation and a mutation in gene for a putative P. aeruginosa calmodulin-like protein did not significantly affect calcium-induced biofilm structure. Two-dimensional gel electrophoresis showed increased amounts of phenazine biosynthetic proteins in FRD1 biofilms and in calcium-amended planktonic cultures. Spectrochemical analyses showed that the calcium addition causes a three- to fivefold increase in pyocyanin production. These results demonstrate that calcium addition affects the structure and extracellular matrix composition of mucoid P. aeruginosa biofilms, through increased expression and stability of bacterial extracellular products. The calcium-induced extracellular matrix of mucoid P. aeruginosa consists primarily of the virulence factor alginate and also harbors extracellular proteases and perhaps pyocyanin, a biomolecule that may further disrupt cellular calcium levels.

Mutations in the cueA gene encoding a copper homeostasis P-type ATPase reduce the pathogenicity of Pseudomonas aeruginosa in mice

A sequencing project identified a putative copper homeostasis gene, cueA, in Pseudomonas aeruginosa strain PAO1. Strains with mutations of the cueA gene, encoding a P-type ATPase linked to copper homeostasis in P. putida, displayed greater sensitivity to copper compared to wild-type bacteria using MIC determinations and in vitro passage in growth media with different concentrations of copper added. An LD50 assay showed a cueA deletion mutant was 50-fold more attenuated than wild-type strain PAO1 bacteria. Complementation of the cueA mutation restored in vitro tolerance to copper and virulence in a systemic model of infection to near wild-type levels. Competition assays between cueA mutants and wild-type P. aeruginosa strains demonstrated 20-fold attenuation by the cueA mutants within spleens of mice. This data suggests the P. aeruginosa CueA protein may be important in maintaining copper homeostasis both in vitro and in vivo.

Alginate lyase (AlgL) activity is required for alginate biosynthesis in Pseudomonas aeruginosa

To determine whether AlgL's lyase activity is required for alginate production in Pseudomonas aeruginosa, an algLdelta::Gm(r) mutant (FRD-MA7) was created. algL complementation of FRD-MA7 restored alginate production, but algL constructs containing mutations inactivating lyase activity did not, demonstrating that the enzymatic activity of AlgL is required for alginate production.

Functional characterization of WaaL, a ligase associated with linking O-antigen polysaccharide to the core of Pseudomonas aeruginosa lipopolysaccharide

The O antigen of Pseudomonas aeruginosa B-band lipopolysaccharide is synthesized by assembling O-antigen-repeat units at the cytoplasmic face of the inner membrane by nonprocessive glycosyltransferases, followed by polymerization on the periplasmic face. The completed chains are covalently attached to lipid A core by the O-antigen ligase, WaaL. In P. aeruginosa the process of ligating these O-antigen molecules to lipid A core is not clearly defined, and an O-antigen ligase has not been identified until this study. Using the sequence of waaL from Salmonella enterica as a template in a BLAST search, a putative waaL gene was identified in the P. aeruginosa genome. The candidate gene was amplified and cloned, and a chromosomal knockout of PAO1 waaL was generated. Lipopolysaccharide (LPS) from this mutant is devoid of B-band O-polysaccharides and semirough (SR-LPS, or core-plus-one O-antigen). The mutant PAO1waaL is also deficient in the production of A-band polysaccharide, a homopolymer of D-rhamnose. Complementation of the mutant with pPAJL4 containing waaL restored the production of both A-band and B-band O antigens as well as SR-LPS, indicating that the knockout was nonpolar and waaL is required for the attachment of O-antigen repeat units to the core. Mutation of waaL in PAO1 and PA14, respectively, could be complemented with waaL from either strain to restore wild-type LPS production. The waaL mutation also drastically affected the swimming and twitching motilities of the bacteria. These results demonstrate that waaL in P. aeruginosa encodes a functional O-antigen ligase that is important for cell wall integrity and motility of the bacteria.

Pseudomonas aeruginosa fimL regulates multiple virulence functions by intersecting with Vfr-modulated pathways

Virulence of Pseudomonas aeruginosa involves the co-ordinate expression of a range of factors including type IV pili (tfp), the type III secretion system (TTSS) and quorum sensing. Tfp are required for twitching motility, efficient biofilm formation, and for adhesion and type III secretion (TTS)-mediated damage to mammalian cells. We describe a novel gene (fimL) that is required for tfp biogenesis and function, for TTS and for normal biofilm development in P. aeruginosa. The predicted product of fimL is homologous to the N-terminal domain of ChpA, except that its putative histidine and threonine phosphotransfer sites have been replaced with glutamine. fimL mutants resemble vfr mutants in many aspects including increased autolysis, reduced levels of surface-assembled tfp and diminished production of type III secreted effectors. Expression of vfr in trans can complement fimL mutants. vfr transcription and production is reduced in fimL mutants whereas cAMP levels are unaffected. Deletion and insertion mutants of fimL frequently revert to wild-type phenotypes suggesting that an extragenic suppressor mutation is able to overcome the loss of fimL. vfr transcription and production, as well as cAMP levels, are elevated in these revertants, while Pseudomonas quinolone signal (PQS) production is reduced. These results suggest that the site(s) of spontaneous mutation is in a gene(s) which lies upstream of vfr transcription, cAMP, production, and PQS synthesis. Our studies indicate that Vfr and FimL are components of intersecting pathways that control twitching motility, TTSS and autolysis in P. aeruginosa.

Combined sacB-based negative selection and cre-lox antibiotic marker recycling for efficient gene deletion in pseudomonas aeruginosa

The complete genome of the bacterial pathogen Pseudomonas aeruginosa has now been sequenced, allowing gene deletion, one of the most frequently used methods in gene function study, to be fully exploited. In this study, we combine the sacB-based negative selection system with a cre-lox antibiotic marker recycling method. This methodology allows allelic exchange between a target gene and a gentamicin cassette flanked by the two lox sequences. A tetracycline plasmid expressing the cre recombinase is then introduced in the mutant strain to catalyze the excision of the lox-flanked resistance marker. We demonstrate here the efficiency of the combination of these two methods in P. aeruginosa by successively deleting ExoS and ExoT, which are two genetically independent toxins of the type-three secretion system (TTSS). This functional cre-lox recycling antibiotic marker system can create P. aeruginosa strains with multiple mutations without modifying the antibiotic resistance profile when compared to the parental strain.

Aerogenic vaccination with a Burkholderia mallei auxotroph protects against aerosol-initiated glanders in mice

Burkholderia mallei is an obligate mammalian pathogen that causes the zoonotic disease glanders. Two live attenuated B. mallei strains, a capsule mutant and a branched-chain amino acid auxotroph, were evaluated for use as vaccines against aerosol-initiated glanders in mice. Animals were aerogenically vaccinated and serum samples were obtained before aerosol challenge with a high-dose (>300 times the LD50) of B. mallei ATCC 23344. Mice vaccinated with the capsule mutant developed a Th2-like Ig subclass antibody response and none survived beyond 5 days. In comparison, the auxotrophic mutant elicited a Th1-like Ig subclass antibody response and 25% of the animals survived for 1 month postchallenge. After a low-dose (5 times the LD50) aerosol challenge, the survival rates of auxotroph-vaccinated and unvaccinated animals were 50 and 0%, respectively. Thus, live attenuated strains that promote a Th1-like Ig response may serve as promising vaccine candidates against aerosol infection with B. mallei.

The Pseudomonas aeruginosa PA14 type III secretion system is expressed but not essential to virulence in the Caenorhabditis elegans-P. aeruginosa pathogenicity model

The Pseudomonas aeruginosa type III secretion system (TTSS), enabling direct injection of toxins into host cells, has been shown to be crucial to virulence in several models of P. aeruginosa pathogenesis. Using the strain PA14 and its isogenic mutant, PA14exsA, we investigated the role of the TTSS during infection of the nematode Caenorhabditis elegans. Although C. elegans N2 was killed by PA14 in an infection like process over 48 to 72 h the same effect was observed following infection with PA14exsA, implying that a functional TTSS was not essential for virulence. This was despite the TTSS being actively expressed during C. elegans infection as demonstrated by the use of green fluorescent reporter constructs and RT-PCR. However, compared to the wild type PA14, PA14exsA did display a reduced rate of killing of C. elegans strain AU1 which harbours a mutation in the sek-1 gene encoding a MAP kinase involved in nematode innate immunity. A fuller understanding of the mechanism of resistance to type III attack in C. elegans may lead to the identification and development of novel therapeutic targets affording protection to TTSS products in man.

The Legionella pneumophila tatB gene facilitates secretion of phospholipase C, growth under iron-limiting conditions, and intracellular infection

Our previous mutational analysis of Legionella pneumophila demonstrated a role for type II protein (Lsp) secretion and iron acquisition in intracellular infection and virulence. In gram-negative bacteria, the twin-arginine translocation (Tat) pathway is involved in secretion of proteins, including components of respiratory complexes, across the inner membrane to the periplasm. To assess the significance of Tat for L. pneumophila, tatB mutants were characterized. The mutants exhibited normal growth in standard media but grew slowly under low-iron conditions. They were also impaired in the Nadi assay, indicating that the function of cytochrome c oxidase is influenced by tatB. Consistent with this observation, a subunit of the cytochrome c reductase was shown to be a Tat substrate. Supernatants of the tatB mutants showed a 30% reduction in phospholipase C activity while maintaining normal levels of other Lsp secreted activities. When tested for infection of U937 macrophages, the tatB mutants showed a 10-fold reduction in growth. Double mutants lacking tatB and Lsp secretion were even more defective, suggesting tatB has an intracellular role that is independent of Lsp. tatB mutants were also impaired 20-fold in Hartmannella vermiformis amoebae cultured in the presence of an iron chelator. All mutant phenotypes were complemented by reintroduction of an intact tatB. Thus, L. pneumophila tatB plays a role in the formation of a respiratory complex, growth under low-iron conditions, the secretion of a phospholipase C activity, and intracellular infection.

Screening for quorum-sensing inhibitors (QSI) by use of a novel genetic system, the QSI selector

With the widespread appearance of antibiotic-resistant bacteria, there is an increasing demand for novel strategies to control infectious diseases. Furthermore, it has become apparent that the bacterial life style also contributes significantly to this problem. Bacteria living in the biofilm mode of growth tolerate conventional antimicrobial treatments. The discovery that many bacteria use quorum-sensing (QS) systems to coordinate virulence and biofilm development has pointed out a new, promising target for antimicrobial drugs. We constructed a collection of screening systems, QS inhibitor (QSI) selectors, which enabled us to identify a number of novel QSIs among natural and synthetic compound libraries. The two most active were garlic extract and 4-nitro-pyridine-N-oxide (4-NPO). GeneChip-based transcriptome analysis revealed that garlic extract and 4-NPO had specificity for QS-controlled virulence genes in Pseudomonas aeruginosa. These two QSIs also significantly reduced P. aeruginosa biofilm tolerance to tobramycin treatment as well as virulence in a Caenorhabditis elegans pathogenesis model.

Uranyl precipitation by Pseudomonas aeruginosa via controlled polyphosphate metabolism

The polyphosphate kinase gene from Pseudomonas aeruginosa was overexpressed in its native host, resulting in the accumulation of 100 times the polyphosphate seen with control strains. Degradation of this polyphosphate was induced by carbon starvation conditions, resulting in phosphate release into the medium. The mechanism of polyphosphate degradation is not clearly understood, but it appears to be associated with glycogen degradation. Upon suspension of the cells in 1 mM uranyl nitrate, nearly all polyphosphate that had accumulated was degraded within 48 h, resulting in the removal of nearly 80% of the uranyl ion and >95% of lesser-concentrated solutions. Electron microscopy, energy-dispersive X-ray spectroscopy, and time-resolved laser-induced fluorescence spectroscopy (TRLFS) suggest that this removal was due to the precipitation of uranyl phosphate at the cell membrane. TRLFS also indicated that uranyl was initially sorbed to the cell as uranyl hydroxide and was then precipitated as uranyl phosphate as phosphate was released from the cell. Lethal doses of radiation did not halt phosphate secretion from polyphosphate-filled cells under carbon starvation conditions.

Functional regions of the Pseudomonas aeruginosa cytotoxin ExoU

ExoU, a potent patatin-like phospholipase, causes rapid cell death following its injection into host cells by the Pseudomonas aeruginosa type III secretion system. To better define regions of ExoU required for cytotoxicity, transposon-based linker insertion mutagenesis followed by site-directed mutagenesis of individual residues was employed by using a Saccharomyces cerevisiae model system. Random insertion of five amino acids identified multiple regions within ExoU that are required for cell killing. Five regions were chosen for further characterization: three corresponded to the oxyanion hole, hydrolase motif, and catalytic aspartate motif of the patatin-like domain within the N-terminal half of ExoU; one corresponded to an uncharacterized part of the patatin-like domain; and one corresponded to a region near the C terminus. Specific individual amino acid substitutions in each of the four N-terminal regions prevented killing of yeast and significantly reduced phospholipase activity. Whereas five amino acid insertions in the fifth region near the C terminus markedly reduced cytotoxicity and phospholipase activity, substitution of individual amino acids did not abolish either activity. To determine whether each of the five identified regions of ExoU was also essential for cytotoxicity in human cells, representative mutant forms of ExoU fused to green fluorescent protein were expressed in HeLa cells. These variants of ExoU were readily visualized and caused minimal cytotoxicity to HeLa cells, while wild-type ExoU fused to green fluorescent protein induced significant cell lysis and no detectable fluorescence. Thus, a minimum of five regions, including one which is well removed from the patatin-like domain, are required for the cytotoxicity and phospholipase activity of ExoU.

Relative contributions of Pseudomonas aeruginosa ExoU, ExoS, and ExoT to virulence in the lung

Pseudomonas aeruginosa uses a type III secretion system to promote development of severe disease, particularly in patients with impaired immune defenses. While the biochemical and enzymatic functions of ExoU, ExoS, and ExoT, three effector proteins secreted by this system, are well defined, the relative roles of each protein in the pathogenesis of acute infections is not clearly understood. Since ExoU and ExoS are usually not secreted by the same strain, it has been difficult to directly compare the effects of these proteins during infection. In the work described here, several isogenic mutants of a bacterial strain that naturally secretes ExoU, ExoS, and ExoT were generated to carefully evaluate the relative contribution of each effector protein to pathogenesis in a mouse model of acute pneumonia. Measurements of mortality, bacterial persistence in the lung, and dissemination indicated that secretion of ExoU had the greatest impact on virulence while secretion of ExoS had an intermediate effect and ExoT had a minor effect. It is of note that these results conclusively show for the first time that ExoS is a virulence factor. Infection with isogenic mutants secreting wild-type ExoS, ExoS defective in GTPase-activating protein (GAP) activity, or ExoS defective in ADP-ribosyltransferase activity demonstrated that the virulence of ExoS was largely dependent on its ADP-ribosyltransferase activity. The GAP activity of this protein had only a minor effect in vivo. The relative virulence associated with each of these type III effector proteins may have important prognostic implications for patients infected with P. aeruginosa.

Polarisation of type III translocation by Pseudomonas aeruginosa requires PcrG, PcrV and PopN

Type III secretion (TTS) mediated translocation of exoenzymes is a key virulence strategy utilised by the opportunistic pathogen Pseudomonas aeruginosa to deliver exoenzyme effectors into the eukaryotic cell. We have previously shown that type III mediated translocation is a contact dependent process, which requires the secreted translocator proteins PcrV, PopB and PopD. To further analyse this mechanism, HeLa cells were infected with the wild-type strain PAK as well as isogenic pcrV, popB, popD, pcrG and popN mutants. In the presence of eukaryotic cells, expression of exoenzyme S (ExoS) increased. When cells were infected with the wild-type strain PAK no ExoS was detected in the tissue culture medium. This confirms that ExoS translocation by P. aeruginosa occurs by a polarised mechanism. In contrast, high levels of ExoS were recovered in the tissue culture medium when cells were infected with pcrG, pcrV and popN mutants. Additionally, ExoS expression levels were higher for these mutants regardless of inducing conditions. This suggests that PcrG, PcrV and PopN are involved in negative regulation of ExoS expression and secretion, and are required to ensure polarised delivery of effectors into target cells.

AlgX is a periplasmic protein required for alginate biosynthesis in Pseudomonas aeruginosa

Alginate, an exopolysaccharide produced by Pseudomonas aeruginosa, provides the bacterium with a selective advantage that makes it difficult to eradicate from the lungs of cystic fibrosis (CF) patients. Previous studies identified a gene, algX, within the alginate biosynthetic gene cluster on the P. aeruginosa chromosome. By probing cell fractions with anti-AlgX antibodies in a Western blot, AlgX was localized within the periplasm. Consistent with these results is the presence of a 26-amino-acid signal sequence. To examine the requirement for AlgX in alginate biosynthesis, part of algX in P. aeruginosa strain FRD1::pJLS3 was replaced with a nonpolar gentamicin resistance cassette. The resulting algXDelta::Gm mutant was verified by PCR and Western blot analysis and was phenotypically nonmucoid (non-alginate producing). The algXDelta::Gm mutant was restored to the mucoid phenotype with wild-type P. aeruginosa algX provided on a plasmid. The algXDelta::Gm mutant was found to secrete dialyzable oligouronic acids of various lengths. Mass spectroscopy and Dionex chromatography indicated that the dialyzable uronic acids are mainly mannuronic acid dimers resulting from alginate lyase (AlgL) degradation of polymannuronic acid. These studies suggest that AlgX is part of a protein scaffold that surrounds and protects newly formed polymers from AlgL degradation as they are transported within the periplasm for further modification and eventual transport out of the cell.

The siderophore-mediated iron acquisition systems of Acinetobacter baumannii ATCC 19606 and Vibrio anguillarum 775 are structurally and functionally related

TheAcinetobacter baumanniitype strain, ATCC 19606, secretes acinetobactin, a catechol siderophore highly related to the iron chelator anguibactin produced by the fish pathogenVibrio anguillarum(Listonella anguillarum). This paper reports the initial characterization of the genes and gene products involved in the acinetobactin-mediated iron-acquisition process. Insertional mutagenesis resulted in the isolation of several derivatives whose ability to grow in medium containing the iron chelator 2,2′-dipyridyl was affected. One of the insertions disrupted a gene encoding a predicted outer-membrane protein, named BauA, highly similar to FatA, the receptor for ferric anguibactin. Immunological relatedness of BauA with FatA was confirmed by Western blot analysis. Another transposon insertion was mapped to a gene encoding a protein highly similar to FatD, the permease component of the anguibactin transport system. Further DNA sequencing and nucleotide sequence analysis revealed that theseA. baumannii19606 genes are part of a polycistronic locus that contains thebauDCEBAORFs. While the translation products ofbauD, -C, -Band -Aare highly related to theV. anguillarumFatDCBA iron-transport proteins, the product ofbauEis related to the ATPase component of Gram-positive ATP-binding cassette (ABC) transport systems. This entire locus is flanked by genes encoding predicted proteins related to AngU and AngN,V. anguillarumproteins required for the biosynthesis of anguibactin. These protein similarities, as well as the structural similarity of anguibactin and acinetobactin, suggested that these two siderophores could be utilized by both bacterial strains, a possibility that was confirmed by siderophore utilization bioassays. Taken together, these results demonstrate that these pathogens, which cause serious infections in unrelated hosts, express very similar siderophore-mediated iron-acquisition systems.

Functional characterization of an aminotransferase required for pyoverdine siderophore biosynthesis in Pseudomonas aeruginosa PAO1

The fluorescent dihydroxyquinoline chromophore of the pyoverdine siderophore in Pseudomonas is a condensation product of D-tyrosine and l-2,4-diaminobutyrate. Both pvdH and asd (encoding aspartate beta-semialdehyde dehydrogenase) knockout mutants of Pseudomonas aeruginosa PAO1 were unable to synthesize pyoverdine under iron-limiting conditions in the absence of l-2,4-diaminobutyrate in the culture media. The pvdH gene was subcloned, and the gene product was hyperexpressed and purified from P. aeruginosa PAO1. PvdH was found to catalyze an aminotransferase reaction, interconverting aspartate beta-semialdehyde and l-2,4-diaminobutyrate. Steady-state kinetic analysis with a novel coupled assay established that the enzyme adopts a ping-pong kinetic mechanism and has the highest specificity for alpha-ketoglutarate. The specificity of the enzyme toward the smaller keto acid pyruvate is 41-fold lower. The enzyme has negligible activity toward other keto acids tested. Homologues of PvdH were present in the genomes of other Pseudomonas spp. These homologues were found in the DNA loci of the corresponding genomes that contain other pyoverdine synthesis genes. This suggests that there is a general mechanism of l-2,4-diaminobutyrate synthesis in Pseudomonas strains that produce the pyoverdine siderophore.

The global regulator genes from biocontrol strain Serratia plymuthica IC1270: cloning, sequencing, and functional studies

The biocontrol activity of various fluorescent pseudomonads towards plant-pathogenic fungi is dependent upon the GacA/GacS-type two-component system of global regulators and the RpoS transcription sigma factor. In particular, these components are required for the production of antifungal antibiotics and exoenzymes. To investigate the effects of these global regulators on the expression of biocontrol factors by plant-associated bacteria other than Pseudomonas spp., gacA/gacS and rpoS homologues were cloned from biocontrol strain IC1270 of Serratia plymuthica, which produces a set of antifungal compounds, including chitinolytic enzymes and the antibiotic pyrrolnitrin. The nucleotide and deduced protein sequence alignments of the cloned gacA/gacS-like genes-tentatively designated grrA (global response regulation activator) and grrS (global response regulation sensor) and of the cloned rpoS gene revealed 64 to 93% identity with matching genes and proteins of the enteric bacteria Escherichia coli, Pectobacterium carotovora subsp. carotovora, and Serratia marcescens. grrA, grrS, and rpoS gene replacement mutants of strain IC1270 were deficient in the production of pyrrolnitrin, an exoprotease, and N-acylhomoserine lactone quorum-sensing signal molecules. However, neither mutant appeared to differ from the parental strain in the production of siderophores, and only grrA and grrS mutants were deficient in the production of a 58-kDa endochitinase, representing the involvement of other sigma factors in the regulation of strain IC1270's chitinolytic activity. Compared to the parental strain, the grrA, grrS, and rpoS mutants were markedly less capable of suppressing Rhizoctonia solani and Pythium aphanidermatum under greenhouse conditions, indicating the dependence of strain IC1270's biocontrol property on the GrrA/GrrS and RpoS global regulators.

Pseudomonas aeruginosa dihydroorotases: a tale of three pyrCs

Pseudomonas aeruginosa PAO1 was shown to contain three pyrC sequences. Two of these genes, designated pyrC (PA3527) and pyrC2 (PA5541), encode polypeptides with dihydroorotase (DHOase) activity, while the third, pyrC' (PA0401), encodes a DHOase-like polypeptide that lacks DHOase activity, but is necessary for the structure and function of ATCase. Both pyrC and pyrC2 were cloned and complemented an Escherichia coli pyrC mutant. In addition, pyrC and pyrC2 were individually inactivated in P. aeruginosa by homologous exchange with a mutated allele of each. The resulting mutant strains were prototrophic. A pyrC, pyrC2 double mutant was also constructed, and this strain had an absolute requirement for pyrimidines. The transcriptional activity of pyrC and pyrC2 was measured using lacZ promoter fusions. While pyrC was found to be constitutively expressed, pyrC2 was expressed only in the pyrC mutant background. An in vitro transcriptional/translational system was used to estimate the size of the pyrC2 gene product. The expressed polypeptide was approximately 47 kDa, which is in keeping with the theoretical molecular mass of 48 kDa, making it the largest prokaryotic DHOase polypeptide identified to date. To our knowledge, this is the first report of a true DHOase mutant in P. aeruginosa and also the first confirmation that pyrC2 encodes a polypeptide with DHOase activity.

Identification of psl, a locus encoding a potential exopolysaccharide that is essential for Pseudomonas aeruginosa PAO1 biofilm formation

Bacteria inhabiting biofilms usually produce one or more polysaccharides that provide a hydrated scaffolding to stabilize and reinforce the structure of the biofilm, mediate cell-cell and cell-surface interactions, and provide protection from biocides and antimicrobial agents. Historically, alginate has been considered the major exopolysaccharide of the Pseudomonas aeruginosa biofilm matrix, with minimal regard to the different functions polysaccharides execute. Recent chemical and genetic studies have demonstrated that alginate is not involved in the initiation of biofilm formation in P. aeruginosa strains PAO1 and PA14. We hypothesized that there is at least one other polysaccharide gene cluster involved in biofilm development. Two separate clusters of genes with homology to exopolysaccharide biosynthetic functions were identified from the annotated PAO1 genome. Reverse genetics was employed to generate mutations in genes from these clusters. We discovered that one group of genes, designated psl, are important for biofilm initiation. A PAO1 strain with a disruption of the first two genes of the psl cluster (PA2231 and PA2232) was severely compromised in biofilm initiation, as confirmed by static microtiter and continuous culture flow cell and tubing biofilm assays. This impaired biofilm phenotype could be complemented with the wild-type psl sequences and was not due to defects in motility or lipopolysaccharide biosynthesis. These results implicate an as yet unknown exopolysaccharide as being required for the formation of the biofilm matrix. Understanding psl-encoded exopolysaccharide expression and protection in biofilms will provide insight into the pathogenesis of P. aeruginosa in cystic fibrosis and other infections involving biofilms.