RpoS and RpoN are involved in the growth-dependent regulation of rfaH transcription and O antigen expression in Salmonella enterica serovar Typhi

RpoN is required for the motility and contributes to the killing ability of Plesiomonas shigelloides

BACKGROUND

RpoN, also known as σ⁵⁴, first reported in Escherichia coli, is a subunit of RNA polymerase that strictly controls the expression of different genes by identifying specific promoter elements. RpoN has an important regulatory function in carbon and nitrogen metabolism and participates in the regulation of flagellar synthesis, bacterial motility and virulence. However, little is known about the effect of RpoN in Plesiomonas shigelloides.

RESULTS

To identify pathways controlled by RpoN, RNA sequencing (RNA-Seq) of the WT and the rpoN deletion strain was carried out for comparison. The RNA-seq results showed that RpoN regulates ~ 13.2% of the P. shigelloides transcriptome, involves amino acid transport and metabolism, glycerophospholipid metabolism, pantothenate and CoA biosynthesis, ribosome biosynthesis, flagellar assembly and bacterial secretion system. Furthermore, we verified the results of RNA-seq using quantitative real-time reverse transcription PCR, which indicated that the absence of rpoN caused downregulation of more than half of the polar and lateral flagella genes in P. shigelloides, and the ΔrpoN mutant was also non-motile and lacked flagella. In the present study, the ability of the ΔrpoN mutant to kill E. coli MG1655 was reduced by 54.6% compared with that of the WT, which was consistent with results in RNA-seq, which showed that the type II secretion system (T2SS-2) genes and the type VI secretion system (T6SS) genes were repressed. By contrast, the expression of type III secretion system genes was largely unchanged in the ΔrpoN mutant transcriptome and the ability of the ΔrpoN mutant to infect Caco-2 cells was also not significantly different compared with the WT.

CONCLUSIONS

We showed that RpoN is required for the motility and contributes to the killing ability of P. shigelloides and positively regulates the T6SS and T2SS-2 genes.

The Regulatory Functions of σ54 Factor in Phytopathogenic Bacteria

σ⁵⁴ factor (RpoN), a type of transcriptional regulatory factor, is widely found in pathogenic bacteria. It binds to core RNA polymerase (RNAP) and regulates the transcription of many functional genes in an enhancer-binding protein (EBP)-dependent manner. σ⁵⁴ has two conserved functional domains: the activator-interacting domain located at the N-terminal and the DNA-binding domain located at the C-terminal. RpoN directly binds to the highly conserved sequence, GGN₁₀GC, at the −24/−12 position relative to the transcription start site of target genes. In general, bacteria contain one or two RpoNs but multiple EBPs. A single RpoN can bind to different EBPs in order to regulate various biological functions. Thus, the overlapping and unique regulatory pathways of two RpoNs and multiple EBP-dependent regulatory pathways form a complex regulatory network in bacteria. However, the regulatory role of RpoN and EBPs is still poorly understood in phytopathogenic bacteria, which cause economically important crop diseases and pose a serious threat to world food security. In this review, we summarize the current knowledge on the regulatory function of RpoN, including swimming motility, flagella synthesis, bacterial growth, type IV pilus (T4Ps), twitching motility, type III secretion system (T3SS), and virulence-associated phenotypes in phytopathogenic bacteria. These findings and knowledge prove the key regulatory role of RpoN in bacterial growth and pathogenesis, as well as lay the groundwork for further elucidation of the complex regulatory network of RpoN in bacteria.

The low level of O antigen in Salmonella enterica Paratyphi A is due to inefficiency of the glycosyltransferase WbaV

The group A O antigen is the major surface polysaccharide of Salmonella enterica serovar Paratyphi A (SPA), and the focal point for most current vaccine development efforts. The SPA O-antigen repeat (O unit) is structurally similar to the group D1 O unit of S. enterica serovar Typhi, differing only in the presence of a terminal side-branch paratose (Par) in place of tyvelose (Tyv), both of which are attached by the glycosyltransferase WbaV. The two O-antigen gene clusters are also highly similar, but with a loss-of-function mutation in the group A tyv gene and the tandem amplification of wbaV in most SPA strains. In this study, we show that SPA strains consistently produce less O antigen than their group D1 counterparts and use an artificial group A strain (D1 Δtyv) to show this is due to inefficient Par attachment by WbaV. We also demonstrate that group A O-antigen production can be increased by overexpression of the wbaV gene in both the D1 Δtyv strain and two multi-wbaV SPA strains. These findings should be broadly applicable in ongoing vaccine development pipelines, where efficient isolation and purification of large quantities of O antigen is of critical importance.

The CpxAR Two-Component System Contributes to Growth, Stress Resistance, and Virulence of Actinobacillus pleuropneumoniae by Upregulating wecA Transcription

Actinobacillus pleuropneumoniae is the pathogen of porcine contagious pleuropneumonia. In A. pleuropneumoniae, the CpxAR two-component system is essential for fitness and growth. The O-antigen protrudes from the outer membrane to the exterior of the cell, and the outer membrane serves as a barrier that helps the bacteria to survive in harsh environments. WecA, a undecaprenyl phosphate GlcNAc-1-phosphate transferase, is involved in O-antigen repeating unit biosynthesis. In this study, we investigated the role of CpxAR in the expression of wecA in A. pleuropneumoniae. Our results revealed that CpxR positively regulates wecA expression by directly binding to the putative promoter region of wecA. Wild-type, ΔcpxAR, ΔwecA, and complemented strains were investigated under serum, oxidative, and osmotic stresses. The ΔcpxAR and ΔwecA strains were more susceptible to these stresses than the wild-type, but the complemented strains showed phenotypes similar to those of the wild-type. Mice infected with the ΔcpxAR and ΔwecA strains exhibited lower mortality and bacterial loads in the lung than those infected with the wild-type or complemented strains. This study reveals that the CpxAR two-component system contributes to A. pleuropneumoniae growth, stress resistance, and virulence, by upregulating expression of wecA. Our findings provide new insight into the pathogenesis of A. pleuropneumoniae.

Lipopolysaccharide O-antigens-bacterial glycans made to measure

Lipopolysaccharides are critical components of bacterial outer membranes. The more conserved lipid A part of the lipopolysaccharide molecule is a major element in the permeability barrier imposed by the outer membrane and offers a pathogen-associated molecular pattern recognized by innate immune systems. In contrast, the long-chain O-antigen polysaccharide (O-PS) shows remarkable structural diversity and fulfills a range of functions, depending on bacterial lifestyles. O-PS production is vital for the success of clinically important Gram-negative pathogens. The biological properties and functions of O-PSs are mostly independent of specific structures, but the size distribution of O-PS chains is particularly important in many contexts. Despite the vast O-PS chemical diversity, most are produced in bacterial cells by two assembly strategies, and the different mechanisms employed in these pathways to regulate chain-length distribution are emerging. Here, we review our current understanding of the mechanisms involved in regulating O-PS chain-length distribution and discuss their impact on microbial cell biology.

Cross-resistance is modular in bacteria-phage interactions

Phages shape the structure of natural bacterial communities and can be effective therapeutic agents. Bacterial resistance to phage infection, however, limits the usefulness of phage therapies and could destabilise community structures, especially if individual resistance mutations provide cross-resistance against multiple phages. We currently understand very little about the evolution of cross-resistance in bacteria–phage interactions. Here we show that the network structure of cross-resistance among spontaneous resistance mutants of Pseudomonas aeruginosa evolved against each of 27 phages is highly modular. The cross-resistance network contained both symmetric (reciprocal) and asymmetric (nonreciprocal) cross-resistance, forming two cross-resistance modules defined by high within- but low between-module cross-resistance. Mutations conferring cross-resistance within modules targeted either lipopolysaccharide or type IV pilus biosynthesis, suggesting that the modularity of cross-resistance was structured by distinct phage receptors. In contrast, between-module cross-resistance was provided by mutations affecting the alternative sigma factor, RpoN, which controls many lifestyle-associated functions, including motility, biofilm formation, and quorum sensing. Broader cross-resistance range was not associated with higher fitness costs or weaker resistance against the focal phage used to select resistance. However, mutations in rpoN, providing between-module cross-resistance, were associated with higher fitness costs than mutations associated with within-module cross-resistance, i.e., in genes encoding either lipopolysaccharide or type IV pilus biosynthesis. The observed structure of cross-resistance predicted both the frequency of resistance mutations and the ability of phage combinations to suppress bacterial growth. These findings suggest that the evolution of cross-resistance is common, is likely to play an important role in the dynamic structure of bacteria–phage communities, and could inform the design principles for phage therapy treatments.

Phage therapy is a promising alternative to antibiotics for treating bacterial infections. Yet as with antibiotics, bacteria readily evolve resistance to phage attack, including cross-resistance that protects against multiple phages at once and so limits the usefulness of phage cocktails. Here we show, using laboratory experimental evolution of resistance against 27 phages in P. aeruginosa, that cross-resistance is common and determines the ability of phage combinations to suppress bacterial growth. Using whole-genome sequencing, we show that cross-resistance is most common against multiple phages that use the same receptor but that global regulator mutations provide generalist resistance, probably by simultaneously affecting the expression of multiple different phage receptors. Future trials should test if these features of cross-resistance evolution translate to more complex in vivo environments and can therefore be exploited to design more effective phage therapies for the clinic.

Regulation of Escherichia coli Pathogenesis by Alternative Sigma Factor N

σ^N (also σ⁵⁴) is an alternative sigma factor subunit of the RNA polymerase complex that regulates the expression of genes from many different ontological groups. It is broadly conserved in the Eubacteria with major roles in nitrogen metabolism, membrane biogenesis, and motility. σ^N is encoded as the first gene of a five-gene operon including rpoN (σ^N), ptsN, hpf, rapZ, and npr that has been genetically retained among species of Escherichia, Shigella, and Salmonella. In an increasing number of bacteria, σ^N has been implicated in the control of genes essential to pathogenic behavior, including those involved in adherence, secretion, immune subversion, biofilm formation, toxin production, and resistance to both antimicrobials and biological stressors. For most pathogens how this is achieved is unknown. In enterohemorrhagic Escherichia coli (EHEC) O157, Salmonella enterica, and Borrelia burgdorferi, regulation of virulence by σ^N requires another alternative sigma factor, σ^S, yet the model by which σ^N-σ^S virulence regulation is predicted to occur is varied in each of these pathogens. In this review, the importance of σ^N to bacterial pathogenesis is introduced, and common features of σ^N-dependent virulence regulation discussed. Emphasis is placed on the molecular mechanisms underlying σ^N virulence regulation in E. coli O157. This includes a review of the structure and function of regulatory pathways connecting σ^N to virulence expression, predicted input signals for pathway stimulation, and the role for cognate σ^N activators in initiation of gene systems determining pathogenic behavior.

O-antigen chain-length distribution in Salmonella enterica serovar Enteritidis is regulated by oxygen availability

Lipopolysaccharide (LPS) consists of three covalently linked domains: the lipid A, the core region and the O antigen (OAg), consisting of repeats of an oligosaccharide. Salmonella enterica serovar Enteritidis (S. Enteritidis) produces a LPS with two OAg preferred chain lengths: a long (L)-OAg controlled by WzzSE and a very long (VL)-OAg controlled by WzzfepE. In this work, we show that OAg produced by S. Enteritidis grown in E minimal medium also presented two preferred chain-lengths. However, a simultaneous and opposing change in the production of L-OAg and VL-OAg was observed in response to oxygen availability. Biochemical and genetics analyses indicate that this process is regulated by transcriptional factors Fnr and ArcA by means of controlling the transcription of genes encoding WzzSE and WzzfepE in response to oxygen availability. Thus, our results revealed a sophisticated regulatory mechanism involved in the adaptation of S. Enteritidis to one of the main environmental cues faced by this pathogen during infection.

Transcriptional and Post-Transcriptional Modulation of SPI1 and SPI2 Expression by ppGpp, RpoS and DksA in Salmonella enterica sv Typhimurium

The expression of genes within Salmonella Pathogenicity Islands 1 and 2 (SPI1, SPI2) is required to facilitate invasion and intracellular replication respectively of S. Typhimurium in host cell lines. Control of their expression is complex and occurs via a variety of factors operating at transcriptional and post-transcriptional levels in response to the environmental stimuli found within the host. Several of the factors that modulate SPI1 and SPI2 expression are involved in the redistribution or modification of RNA polymerase (RNAP) specificity. These factors include the bacterial alarmone, ppGpp, the alternative sigma factor, RpoS, and the RNAP accessory protein, DksA. In this report we show not only how these three factors modulate SPI1 and SPI2 expression but also how they contribute to the 'phased' expression of SPI1 and SPI2 during progress through late-log and stationary phase in aerobic rich broth culture conditions. In addition, we demonstrate that the expression of at least one SPI1-encoded protein, SipC is subject to DksA-dependent post-transcriptional control.

Sigma factor N, liaison to an ntrC and rpoS dependent regulatory pathway controlling acid resistance and the LEE in enterohemorrhagic Escherichia coli

Enterohemorrhagic Escherichia coli (EHEC) is dependent on acid resistance for gastric passage and low oral infectious dose, and the locus of enterocyte effacement (LEE) for intestinal colonization. Mutation of rpoN, encoding sigma factor N (σ(N)), dramatically alters the growth-phase dependent regulation of both acid resistance and the LEE. This study reports on the determinants of σ(N)-directed acid resistance and LEE expression, and the underlying mechanism attributable to this phenotype. Glutamate-dependent acid resistance (GDAR) in TW14359ΔrpoN correlated with increased expression of the gadX-gadW regulatory circuit during exponential growth, whereas upregulation of arginine-dependent acid resistance (ADAR) genes adiA and adiC in TW14359ΔrpoN did not confer acid resistance by the ADAR mechanism. LEE regulatory (ler), structural (espA and cesT) and effector (tir) genes were downregulated in TW14359ΔrpoN, and mutation of rpoS encoding sigma factor 38 (σ(S)) in TW14359ΔrpoN restored acid resistance and LEE genes to WT levels. Stability, but not the absolute level, of σ(S) was increased in TW14359ΔrpoN; however, increased stability was not solely attributable to the GDAR and LEE expression phenotype. Complementation of TW14359ΔrpoN with a σ(N) allele that binds RNA polymerase (RNAP) but not DNA, did not restore WT levels of σ(S) stability, gadE, ler or GDAR, indicating a dependence on transcription from a σ(N) promoter(s) and not RNAP competition for the phenotype. Among a library of σ(N) enhancer binding protein mutants, only TW14359ΔntrC, inactivated for nitrogen regulatory protein NtrC, phenocopied TW14359ΔrpoN for σ(S) stability, GDAR and ler expression. The results of this study suggest that during exponential growth, NtrC-σ(N) regulate GDAR and LEE expression through downregulation of σ(S) at the post-translational level; likely by altering σ(S) stability or activity. The regulatory interplay between NtrC, other EBPs, and σ(N)-σ(S), represents a mechanism by which EHEC can coordinate GDAR, LEE expression and other cellular functions, with nitrogen availability and physiologic stimuli.

Susceptibility of a Vibrio alginolyticus rpoS mutant to environmental stresses and its expression of OMPs

Vibrio alginolyticus, one of the most important opportunistic pathogens, can be detected in human being and marine animals. Like other bacteria, V. alginolyticus is able to adapt to a variety of stressful environmental changes. The alternate sigma factor RpoS, which is a regulator during stationary phase, plays an important role in surviving under these stressful situations in many bacteria. Sequence analysis reveals a 990 bp open reading frame which is predicted to encode a 330-amino-acid protein with 68% to 96% overall identity to other reported sequences. To study the function of rpoS, the rpoS gene of V. alginolyticus VIB283 was cloned and an rpoS mutant was constructed by homologous recombination. Comparison of the study result of the wild type and the mutant showed that the mutant was more sensitive to stress conditions such as high osmolarity, oxidative stress, heat shock, and long-term starvation and that the LD(50) of the mutant strain to the zebra fish was about 2.8 times as that of the control strain. In addition, the SDS-PAGE analysis indicated that the outer membrane proteins (OMPs) existed great differences.

The PmrA/PmrB regulatory system controls the expression of the wzzfepE gene involved in the O-antigen synthesis of Salmonella enterica serovar Typhimurium

The degree of polymerization of O-antigen from Salmonella enterica serovar Typhimurium is controlled by the products of the wzz(s)(t) and wzz(fepE) genes. In the present study we investigated the role of the PmrA/PmrB regulatory system in wzz(fepE) transcription. We report that the direct binding of the PmrA regulator to a specific promoter site induces the expression of the wzz(fepE) gene. This effect increases the amount of very long (VL) O-antigen, which is required for the resistance of Salmonella to serum human complement and polymyxin B, and for the replication of the bacteria within macrophages. The results obtained here highlight functional differences between Wzz(fepE) and Wzz(st), although the genes for both proteins are regulated in a PmrA-dependent way.

Inactivation of alternative sigma factor 54 (RpoN) leads to increased acid resistance, and alters locus of enterocyte effacement (LEE) expression in Escherichia coli O157 : H7

Alternative sigma factor 54 (RpoN) is an important regulator of stress resistance and virulence genes in many bacterial species. In this study, we report on the gene expression alterations that follow rpoN inactivation in Escherichia coli O157 : H7 strain Sakai (Sakai rpoN : : kan), and the influence of RpoN on the acid resistance phenotype. Microarray gene expression profiling revealed the differential expression of 103 genes in SakairpoN : : kan relative to Sakai. This included the growth-phase-dependent upregulation of genes required for glutamate-dependent acid resistance (GDAR) ( gadA, gadB, gadC and gadE), and the downregulation of locus of enterocyte effacement (LEE) genes, which encode a type III secretion system. Upregulation of gad genes in SakairpoN : : kan during exponential growth correlated with increased GDAR and survival in a model stomach system. Complementation of SakairpoN : : kan with a cloned version of rpoN restored acid susceptibility. Genes involved in GDAR regulation, including rpoS (sigma factor 38) and gadE (acid-responsive regulator), were shown to be required for the survival of SakairpoN : : kan by the GDAR mechanism. This study describes the contribution of rpoN to acid resistance and GDAR gene regulation, and reveals RpoN to be an important regulator of stress resistance and virulence genes in E. coli O157 : H7.

Downregulation of RpoN-controlled genes protects Salmonella cells from killing by the cationic antimicrobial peptide polymyxin B

Salmonella enterica polymyxin B (PM) resistance is modulated mainly by substitutions of the acyl chains and the phosphate groups on the lipid A moiety of lipopolysaccharide. These modifications are mediated by genes under the control of the PmrA/PmrB and PhoP/PhoQ two-component regulatory systems. In this study, a deletion in the gene encoding the alternative sigma(54) factor, rpoN, was shown to increase PM resistance without affecting protamine sensitivity. The results presented here showed that the increased polymyxin resistance observed in the DeltarpoN mutant occurs through a PmrA/PhoP-independent pathway. Downregulation of one or more genes belonging to the RpoN regulon may provide an additional mechanism of defence against membrane-permeabilizing antimicrobial peptides that helps the pathogen to survive in different environments.

Regulation of lipopolysaccharide O antigen expression in Pseudomonas aeruginosa

Pseudomonas aeruginosa is a Gram-negative bacterium that is ubiquitously found in the environment. It is an important opportunistic pathogen in immunocompromised patients and causes life-threatening lung infections in individuals with cystic fibrosis. A prominent virulence factor for many Gram-negative bacteria, including P. aeruginosa, is lipopolysaccharide (LPS), which is an immunodominant antigen located in the outer portion of the outer membrane. P. aeruginosa produces two O antigens that are attached to lipid A + core: a B-band O antigen and an A-band O polysaccharide. The B-band O antigen-repeating unit of LPS is responsible for serotype specificity; strains lacking O antigen have been shown to be less virulent in animal models of infection. What is less well understood is how the O antigen chain length is regulated and why P. aeruginosa and some other bacteria show two preferred O antigen lengths. P. aeruginosa encodes two genes encoding O antigen chain length regulators. These genes, wzz1 and wzz2, influence the expression of the long and very long chain lengths, respectively. The long chain length appears more important for resistance to the action of sera and virulence in a mouse model of infection, while the very long chain length appears to be more sensitive to environmental stress conditions. Studies in other bacteria point to regulation at the level of transcription and complex formation as being involved in determining the O antigen chain length and may provide clues to the regulation in P. aeruginosa.

Rho GTPase-activating bacterial toxins: from bacterial virulence regulation to eukaryotic cell biology

Studies on the interactions of bacterial pathogens with their host have provided an invaluable source of information on the major functions of eukaryotic and prokaryotic cell biology. In addition, this expanding field of research, known as cellular microbiology, has revealed fascinating examples of trans-kingdom functional interplay. Bacterial factors actually exploit eukaryotic cell machineries using refined molecular strategies to promote invasion and proliferation within their host. Here, we review a family of bacterial toxins that modulate their activity in eukaryotic cells by activating Rho GTPases and exploiting the ubiquitin/proteasome machineries. This family, found in human and animal pathogenic Gram-negative bacteria, encompasses the cytotoxic necrotizing factors (CNFs) from Escherichia coli and Yersinia species as well as dermonecrotic toxins from Bordetella species. We survey the genetics, biochemistry, molecular and cellular biology of these bacterial factors from the standpoint of the CNF1 toxin, the paradigm of Rho GTPase-activating toxins produced by urinary tract infections causing pathogenic Escherichia coli. Because it reveals important connections between bacterial invasion and the host inflammatory response, the mode of action of CNF1 and its related Rho GTPase-targetting toxins addresses major issues of basic and medical research and constitutes a privileged experimental model for host-pathogen interaction.

The outer core lipopolysaccharide of Salmonella enterica serovar Typhi is required for bacterial entry into epithelial cells

Salmonella enterica serovar Typhi causes typhoid fever in humans. Central to the pathogenicity of serovar Typhi is its capacity to invade intestinal epithelial cells. The role of lipopolysaccharide (LPS) in the invasion process of serovar Typhi is unclear. In this work, we constructed a series of mutants with defined deletions in genes for the synthesis and polymerization of the O antigen (wbaP, wzy, and wzz) and the assembly of the outer core (waaK, waaJ, waaI, waaB, and waaG). The abilities of each mutant to associate with and enter HEp-2 cells and the importance of the O antigen in serum resistance of serovar Typhi were investigated. We demonstrate here that the presence and proper chain length distribution of the O-antigen polysaccharide are essential for serum resistance but not for invasion of epithelial cells. In contrast, the outer core oligosaccharide structure is required for serovar Typhi internalization in HEp-2 cells. We also show that the outer core terminal glucose residue (Glc II) is necessary for efficient entry of serovar Typhi into epithelial cells. The Glc I residue, when it becomes terminal due to a polar insertion in the waaB gene affecting the assembly of the remaining outer core residues, can partially substitute for Glc II to mediate bacterial entry into epithelial cells. Therefore, we conclude that a terminal glucose in the LPS core is a critical residue for bacterial recognition and internalization by epithelial cells.

The transcriptional antiterminator RfaH represses biofilm formation in Escherichia coli

We investigated the influence of regulatory and pathogenicity island-associated factors (Hha, RpoS, LuxS, EvgA, RfaH, and tRNA5Leu) on biofilm formation by uropathogenic Escherichia coli (UPEC) strain 536. Only inactivation of rfaH, which encodes a transcriptional antiterminator, resulted in increased initial adhesion and biofilm formation by E. coli 536. rfaH inactivation in nonpathogenic E. coli K-12 isolate MG1655 resulted in the same phenotype. Transcriptome analysis of wild-type strain 536 and an rfaH mutant of this strain revealed that deletion of rfaH correlated with increased expression of flu orthologs. flu encodes antigen 43 (Ag43), which mediates autoaggregation and biofilm formation. We confirmed that deletion of rfaH leads to increased levels of flu and flu-like transcripts in E. coli K-12 and UPEC. Supporting the hypothesis that RfaH represses biofilm formation through reduction of the Ag43 level, the increased-biofilm phenotype of E. coli MG1655rfaH was reversed upon inactivation of flu. Deletion of the two flu orthologs, however, did not modify the behavior of mutant 536rfaH. Our results demonstrate that the strong initial adhesion and biofilm formation capacities of strain MG1655rfaH are mediated by both increased steady-state production of Ag43 and likely increased Ag43 presentation due to null rfaH-dependent lipopolysaccharide depletion. Although the roles of rfaH in the biofilm phenotype are different in UPEC strain 536 and K-12 strain MG1655, this study shows that RfaH, in addition to affecting the expression of bacterial virulence factors, also negatively controls expression and surface presentation of Ag43 and possibly another Ag43-independent factor(s) that mediates cell-cell interactions and biofilm formation.

Comparative signature-tagged mutagenesis identifies Pseudomonas factors conferring resistance to the pulmonary collectin SP-A

The pulmonary collectin, surfactant protein A (SP-A), is a broad spectrum opsonin with microbicidal membrane permeabilization properties that plays a role in the innate immune response of the lung. However, the factors that govern SP-A's microbial specificity and the mechanisms by which it mediates membrane permeabilization and opsonization are not fully understood. In an effort to identify bacterial factors that confer susceptibility or resistance to SP-A, we used comparative signature-tagged mutagenesis to screen a library of 1,680 Pseudomonas aeruginosa mutants for evidence of differential pulmonary clearance in SP-A-sufficient (SP-A^+/+) and SP-A-deficient (SP-A^−/−) mice. Two SP-A-sensitive P. aeruginosa mutants harboring transposon insertions in genes required for salicylate biosynthesis (pch) and phosphoenolpyruvate-protein-phosphotransferase (ptsP) were recovered. The mutants were indistinguishable from the parental wild-type PA01 with regard to opsonization by SP-A, but they exhibited increased susceptibility to SP-A-mediated membrane permeabilization. These results suggest that bacterial gene functions that are required to maintain membrane integrity play crucial roles in resistance of P. aeruginosa to the permeabilizing effects of SP-A.

Everyday, normal breathing deposits numerous microorganisms on the surfactant membrane that lines the air-exchanging surfaces of the lung. Surfactant protein SP-A, a component of the surfactant membrane, helps to maintain the lung in a germ-free state by aggregating inhaled microorganisms and facilitating their ingestion by immune cells, and by increasing the permeability of their cell membranes. However, the bacterial pathogen Pseudomonas aeruginosa is resistant to SP-A-mediated membrane disruption.

Using a genetic tool called comparative signature-tagged mutagenesis, the authors have identified two P. aeruginosa genes, pch and ptsP, that are required to resist SP-A-mediated membrane permeabilization. Molecular analyses indicate that the pch gene encodes an enzyme that synthesizes salicylate, a compound utilized by bacteria to acquire essential metal ions. The ptsP gene encodes an enzyme called phosphoenolpyruvate-protein-phosphotransferase. The loss of salicylate and phosphoenolpyruvate-protein-phosphotransferase weakens the P. aeruginosa cell membrane, which allows SP-A to poke holes on the membrane and kill the bacteria.

This is the first known report of the roles played by salicylate and phosphoenolpyruvate-protein-phosphotransferase in maintenance of bacterial membrane, and consequently, protecting bacteria from killing by SP-A, through disruption of membrane integrity.

Hierarchical gene regulators adapt to its host milieus

The facultative intracellular pathogen Salmonella enterica serovar Typhimurium possesses an elaborate set of virulence genes that enables the bacterium successfully to move between and adapt to the environment, different host organisms and various micro-niches within a given host. Expression of virulence attributes is by no means constitutive. Rather, the regulation of virulence determinants is highly coordinated and integrated into normal bacterial physiological responses. By integrating discriminating virulence gene regulators with conserved housekeeping regulatory processes, the bacteria can sense alterations in the repertoire of environmental cues, and translate the sensing events into a pragmatic and coordinated expression of virulence genes. While the description of transmissible genetic elements that import global gene regulatory factors into a cell brings conceptual problems into the established regulatory network, the existence of mobile gene regulators may actually enable the bacteria to further modulate virulence expression.

Virulence regulators RfaH and YaeQ do not operate in the same pathway

The expression of virulence factors such as hemolysin and lipopolysaccharides in Proteobacteria is regulated by the transcription elongation factor RfaH. RfaH reduces pausing and termination at intergenic sites, and thus allows RNA polymerase to conclude transcription of the distal genes in long virulence operons. The yaeQ gene of Salmonella enterica sv. Typhimurium has been identified as a high-copy-number suppressor of the hemolytic defect in an rfaH deletion strain, leading to speculation regarding a direct role of YaeQ in the transcriptional control of bacterial virulence. In order to evaluate this hypothesis, yaeQ genes from Escherichia coli and S. enterica sv. Typhimurium were cloned and expressed. Their products, purified YaeQ proteins, displayed no antitermination effects in in-vitro transcription assays over a wide range of concentrations, neither by themselves nor in competition with RfaH. When overexpressed in vivo, plasmid-borne E. coli and S. enterica sv. Typhimurium yaeQ genes also failed to restore hemolytic activity in an rfaH deletion strain under conditions in which episomal E. coli rfaH and its orthologs exhibited full complementation of the genomic rfaH deletion. Taken together, our findings do not support the hypothesis of YaeQ involvement in RfaH-dependent regulation of virulence, even in stoichiometric excess in vitro or upon overexpression in vivo.