Negative control of flagellum synthesis in Pseudomonas aeruginosa is modulated by the alternative sigma factor AlgT (AlgU)

The immune system vs. Pseudomonas aeruginosa biofilms

Ilya Metchnikoff and Paul Ehrlich were awarded the Nobel price in 1908. Since then, numerous studies have unraveled a multitude of mechanistically different immune responses to intruding microorganisms. However, in the vast majority of these studies, the underlying infectious agents have appeared in the planktonic state. Accordingly, much less is known about the immune responses to the presence of biofilm-based infections (which is probably also due to the relatively short period of time in which the immune response to biofilms has been studied). Nevertheless, more recent in vivo and in vitro studies have revealed both innate as well as adaptive immune responses to biofilms. On the other hand, measures launched by biofilm bacteria to achieve protection against the various immune responses have also been demonstrated. Whether particular immune responses to biofilm infections exist remains to be firmly established. However, because biofilm infections are often persistent (or chronic), an odd situation appears with the simultaneous activation of both arms of the host immune response, neither of which can eliminate the biofilm pathogen, but instead, in synergy, causes collateral tissue damage. Although the present review on the immune system vs. biofilm bacteria is focused on Pseudomonas aeruginosa (mainly because this is the most thoroughly studied), many of the same mechanisms are also seen with biofilm infections generated by other microorganisms.

Pseudomonas aeruginosa biofilm infections in cystic fibrosis: insights into pathogenic processes and treatment strategies

IMPORTANCE OF THE FIELD

CF airway mucus can be infected by opportunistic microorganisms, notably Pseudomonas aeruginosa. Once organisms are established as biofilms, even the most potent antibiotics have little effect on their viability, especially during late-stage chronic infections. Better understanding of the mechanisms used by P. aeruginosa to circumvent host defenses and therapeutic intervention strategies is critical for advancing novel treatment strategies.

AREAS COVERED IN THIS REVIEW

Inflammatory injury in CF lung, role of neutrophils in pathogenesis, P. aeruginosa biofilms, mucoidy and its relationship with poor airway oxygenation, mechanisms by which P. aeruginosa biofilms in the CF airway can be killed.

WHAT THE READER WILL GAIN

An understanding of the processes that P. aeruginosa undergoes during CF airway disease and clues to better treat such infections in future.

TAKE HOME MESSAGE

The course of CF airway disease is a process involving host and microbial factors that often dictate frequency of pulmonary exacerbations, thus affecting the overall course. In the past decade significant discoveries have been made regarding the pathogenic processes used by P. aeruginosa to bypass the immune system. Many new and exciting features of P. aeruginosa now illuminate weaknesses in the organism that may render it susceptible to inexpensive compounds that force its own destruction.

Genetic analysis of the regulation of type IV pilus function by the Chp chemosensory system of Pseudomonas aeruginosa

The virulence of the opportunistic pathogen Pseudomonas aeruginosa involves the coordinate expression of many virulence factors, including type IV pili, which are required for colonization of host tissues and for twitching motility. Type IV pilus function is controlled in part by the Chp chemosensory system, which includes a histidine kinase, ChpA, and two CheY-like response regulators, PilG and PilH. How the Chp components interface with the type IV pilus motor proteins PilB, PilT, and PilU is unknown. We present genetic evidence confirming the role of ChpA, PilG, and PilB in the regulation of pilus extension and the role of PilH and PilT in regulating pilus retraction. Using informative double and triple mutants, we show that (i) ChpA, PilG, and PilB function upstream of PilH, PilT, and PilU; (ii) that PilH enhances PilT function; and (iii) that PilT and PilB retain some activity in the absence of signaling input from components of the Chp system. By site-directed mutagenesis, we demonstrate that the histidine kinase domain of ChpA and the phosphoacceptor sites of both PilG and PilH are required for type IV pilus function, suggesting that they form a phosphorelay system important in the regulation of pilus extension and retraction. Finally, we present evidence suggesting that pilA transcription is regulated by intracellular PilA levels. We show that PilA is a negative regulator of pilA transcription in P. aeruginosa and that the Chp system functionally regulates pilA transcription by controlling PilA import and export.

Outer membrane machinery and alginate synthesis regulators control membrane vesicle production in Pseudomonas aeruginosa

The opportunistic human bacterial pathogen Pseudomonas aeruginosa produces membrane vesicles (MVs) in its surrounding environment. Several features of the P. aeruginosa MV production mechanism are still unknown. We previously observed that depletion of Opr86, which has a role in outer membrane protein (OMP) assembly, resulted in hypervesiculation. In this study, we showed that the outer membrane machinery and alginate synthesis regulatory machinery are closely related to MV production in P. aeruginosa. Depletion of Opr86 resulted in increased expression of the periplasmic serine protease MucD, suggesting that the accumulation of misfolded OMPs in the periplasm is related to MV production. Indeed, the mucD mutant showed a mucoid phenotype and the mucD mutation caused increased MV production. Strains with the gene encoding alginate synthetic regulator AlgU, MucA, or MucB deleted also caused altered MV production. Overexpression of either MucD or AlgW serine proteases resulted in decreased MV production, suggesting that proteases localized in the periplasm repress MV production in P. aeruginosa. Deletion of mucD resulted in increased MV proteins, even in strains with mutations in the Pseudomonas quinolone signal (PQS), which serves as a positive regulator of MV production. This study suggests that misfolded OMPs may be important for MV production, in addition to PQS, and that these regulators act in independent pathways.

Immunization of young African green monkeys with OprF epitope 8-OprI-type A- and B-flagellin fusion proteins promotes the production of protective antibodies against nonmucoid Pseudomonas aeruginosa

There is currently no approved vaccine against Pseudomonas aeruginosa, the major cause of morbidity and mortality in cystic fibrosis (CF) patients and a major pathogen in ventilated and burn patients. In a previous study, we demonstrated the immunization of mice with OprF(311-341)-OprI-type A- and B-flagellin fusion proteins dramatically enhanced clearance of nonmucoid P. aeruginosa. The goal of the current study was to evaluate the ability of OprF(311-341)-OprI-flagellins to elicit the production of protective IgG in young (4-6 months old) African green monkeys. Intramuscular immunization of African green monkeys with 1, 3, 10, or 30mug of OprF(311-341)-OprI-flagellins generated robust antigen-specific IgG responses. In addition, immunization with OprF(311-341)-OprI-flagellins elicited high-affinity anti-flagellins, OprI, and OprF IgG that individually promoted extensive deposition of complement component C3 on P. aeruginosa and synergized to facilitate maximal C3 deposition. Passive immunization of mice with plasma from OprF(311-341)-OprI-flagellins immunized monkeys significantly reduced lung bacterial burden three days post-challenge compared to mice that received pre-immunization plasma. Based on our results, OprF(311-341)-OprI-A- and B-flagellin fusion proteins are highly effective in mice and nonhuman primates and thus merit additional development as a potential vaccine for use in humans.

Signals, regulatory networks, and materials that build and break bacterial biofilms

Biofilms are communities of microorganisms that live attached to surfaces. Biofilm formation has received much attention in the last decade, as it has become clear that virtually all types of bacteria can form biofilms and that this may be the preferred mode of bacterial existence in nature. Our current understanding of biofilm formation is based on numerous studies of myriad bacterial species. Here, we review a portion of this large body of work including the environmental signals and signaling pathways that regulate biofilm formation, the components of the biofilm matrix, and the mechanisms and regulation of biofilm dispersal.

flhDC, but not fleQ, regulates flagella biogenesis in Azotobacter vinelandii, and is under AlgU and CydR negative control

Azotobacter vinelandii is a nitrogen-fixing soil bacterium that undergoes differentiation to form cysts resistant to desiccation. Upon encystment, this bacterium becomes non-motile. As in enteric bacteria, motility in A. vinelandii occurs through the use of peritrichous flagella. Pseudomonas aeruginosa, a phylogenetically close relative of A. vinelandii, possesses a single polar flagellum. The FlhDC proteins are the master regulators of flagella and motility in enterobacteria, whereas FleQ is the master regulator in P. aeruginosa, and it is under AlgU (sigmaE) negative control. At present, nothing is known about the organization and expression of flagella genes in A. vinelandii. Here, we identified the flagella gene cluster of this bacterium. Homologues of the master regulatory genes flhDC and fleQ are present in A. vinelandii. Inactivation of flhDC, but not fleQ, impaired flagella biogenesis and motility. We present evidence indicating that a negative effect of the AlgU sigma factor on flhDC expression causes loss of motility in A. vinelandii, and that CydR (a homologue of Fnr) is under AlgU control and has a negative effect on flhDC expression. Taken together, these results suggest the existence of a cascade consisting of AlgU and CydR that negatively controls expression of flhDC; the results also suggest that the block in flagella synthesis under encystment conditions centres on flhDC repression by the AlgU–CydR cascade.

Ralstonia eutropha H16 flagellation changes according to nutrient supply and state of poly(3-hydroxybutyrate) accumulation

Two-dimensional polyacrylamide gel electrophoresis (2D PAGE), in combination with matrix-assisted laser desorption ionization-time of flight analysis, and the recently revealed genome sequence of Ralstonia eutropha H16 were employed to detect and identify proteins that are differentially expressed during different phases of poly(3-hydroxybutyric acid) (PHB) metabolism. For this, a modified protein extraction protocol applicable to PHB-harboring cells was developed to enable 2D PAGE-based proteome analysis of such cells. Subsequently, samples from (i) the exponential growth phase, (ii) the stationary growth phase permissive for PHB biosynthesis, and (iii) a phase permissive for PHB mobilization were analyzed. Among several proteins exhibiting quantitative changes during the time course of a cultivation experiment, flagellin, which is the main protein of bacterial flagella, was identified. Initial investigations that report on changes of flagellation for R. eutropha were done, but 2D PAGE and electron microscopic examinations of cells revealed clear evidence that R. eutropha exhibited further significant changes in flagellation depending on the life cycle, nutritional supply, and, in particular, PHB metabolism. The results of our study suggest that R. eutropha is strongly flagellated in the exponential growth phase and loses a certain number of flagella in transition to the stationary phase. In the stationary phase under conditions permissive for PHB biosynthesis, flagellation of cells admittedly stagnated. However, under conditions permissive for intracellular PHB mobilization after a nitrogen source was added to cells that are carbon deprived but with full PHB accumulation, flagella are lost. This might be due to a degradation of flagella; at least, the cells stopped flagellin synthesis while normal degradation continued. In contrast, under nutrient limitation or the loss of phasins, cells retained their flagella.

Pseudomonas aeruginosa infections are prevented in cystic fibrosis patients by avian antibodies binding Pseudomonas aeruginosa flagellin

Pseudomonas aeruginosa (PA) is the main cause of morbidity and mortality in cystic fibrosis (CF) patients. CF patients with chronic PA infections have a more rapid deterioration of their lung function and the bacteria become impossible to eradicate from the lungs. Antibiotic resistance among PA strains in CF patients is steadily increasing. Specific chicken (IgY) antibodies against PA have been shown to have potential to prevent PA infections in CF. Anti-Pseudomonas IgY reduces PA adhesion to epithelia, but the mechanism has not been fully elucidated. To gain further insight into the prophylactic effect of these antibodies, the immunoreactivity was investigated by 2D electrophoresis of PA strains, immunoblotting and MALDI-TOF-MS. To confirm the identity of the proteins, the tryptic peptides were analyzed by MALDI-TOF-MS to accurately measure their monoisotopic masses as well as determine their amino acid sequences. In order to facilitate fragmentation of the peptides they were N-terminally or C-terminally labeled. Several strains were investigated and anti-Pseudomonas IgY was immunoreactive against all of these strains, which strengthens its potential as a prophylactic treatment against PA. Flagellin was identified as the major antigen. Flagellin is the main protein of the flagella and is crucial for establishing infections in hosts as well as being involved in PA chemotaxis, motility, adhesion and inflammation. Furthermore, secreted flagellin elicits an inflammatory response. In conclusion, anti-Pseudomonas IgY binds flagellin, which may prevent PA infections in CF patients by hindering host invasion.

Inverse regulation of biofilm formation and swarming motility by Pseudomonas aeruginosa PA14

We previously reported that SadB, a protein of unknown function, is required for an early step in biofilm formation by the opportunistic pathogen Pseudomonas aeruginosa. Here we report that a mutation in sadB also results in increased swarming compared to the wild-type strain. Our data are consistent with a model in which SadB inversely regulates biofilm formation and swarming motility via its ability both to modulate flagellar reversals in a viscosity-dependent fashion and to influence the production of the Pel exopolysaccharide. We also show that SadB is required to properly modulate flagellar reversal rates via chemotaxis cluster IV (CheIV cluster). Mutational analyses of two components of the CheIV cluster, the methyl-accepting chemotaxis protein PilJ and the PilJ demethylase ChpB, support a model wherein this chemotaxis cluster participates in the inverse regulation of biofilm formation and swarming motility. Epistasis analysis indicates that SadB functions upstream of the CheIV cluster. We propose that P. aeruginosa utilizes a SadB-dependent, chemotaxis-like regulatory pathway to inversely regulate two key surface behaviors, biofilm formation and swarming motility.

Anaerobic culture conditions favor biofilm-like phenotypes inPseudomonas aeruginosaisolates from patients with cystic fibrosis

Pseudomonas aeruginosa causes chronic infections in the lungs of cystic fibrosis (CF) individuals and remains the leading cause of morbidity and mortality associated with the disease. Biofilm growth and phenotypic diversification are factors thought to contribute to this organism's persistence. Most studies have focused on laboratory isolates such as strain PAO1, and there are relatively few reports characterizing the properties of CF strains, especially under decreased oxygen conditions such as occur in the CF lung. This study compared the phenotypic and functional properties of P. aeruginosa from chronically infected CF adults with those of strain PAO1 and other clinical non-CF isolates under aerobic and anaerobic culture conditions. The CF isolates overall displayed a reduced ability to form biofilms in standard in vitro short-term models. They also grew more slowly in culture, and exhibited decreased adherence to glass and decreased motilities (swimming, swarming and twitching). All of these characteristics were markedly accentuated by anaerobic growth conditions. Moreover, the CF strain phenotypes were not readily reversed by culture manipulations designed to encourage planktonic growth. The CF strains were thus inherently different from strain PAO1 and most of the other non-CF clinical P. aeruginosa isolates tested. In vitro models used to research CF isolate biofilm growth need to take the above properties of these strains into account.

Infection in ventricular assist devices: the role of biofilm

Ventricular assist devices improve hemodynamics in patients with heart failure, but like most implantable medical devices, they are prone to infection; organisms that are adept at forming biofilm cause most of these. Biofilm confers many advantages to the organisms, including protecting them against natural host defenses and antimicrobial therapies. This review will focus on the mechanisms of biofilm formation, including quorum sensing and subsequent changes in microbial gene and protein expression. Novel therapies targeting these processes, as well as improvements in device design and clinical management, have begun to emerge and will aid in the management of these recalcitrant infections.

The impact of quorum sensing and swarming motility on Pseudomonas aeruginosa biofilm formation is nutritionally conditional

The role of quorum sensing in Pseudomonas aeruginosa biofilm formation is unclear. Some researchers have shown that quorum sensing is important for biofilm development, while others have indicated it has little or no role. In this study, the contribution of quorum sensing to biofilm development was found to depend upon the nutritional environment. Depending upon the carbon source, quorum-sensing mutant strains (lasIrhlI and lasRrhlR) either exhibited a pronounced defect early in biofilm formation or formed biofilms identical to the wild-type strain. Quorum sensing was then shown to exert its nutritionally conditional control of biofilm development through regulation of swarming motility. Examination of pilA and fliM mutant strains further supported the role of swarming motility in biofilm formation. These data led to a model proposing that the prevailing nutritional conditions dictate the contributions of quorum sensing and swarming motility at a key juncture early in biofilm development.

Proteomic analysis of Escherichia coli biofilms reveals the overexpression of the outer membrane protein OmpA

Bacterial colonisation and biofilm formation on the surface of urinary catheters is a common cause of nosocomial infection, and as such is a major impediment to their long-term use. Understanding the mechanisms of biofilm formation on urinary catheters is critical to their control and will aid the future development of materials used in their manufacture. In this report we have used proteomic analysis coupled with immunoassays to show that the major outer membrane protein (OmpA) of Escherichia coli is overexpressed during biofilm formation. A series of synthetic hydrogels being developed for potential use as catheter coatings were used as the substrata and OmpA expression was increased in biofilms on all these surfaces, as well as being a feature of both a laboratory and a clinical strain of E. coli. Up-regulation of OmpA may, therefore, be a common feature of E. coli biofilms. These findings present OmpA as a potential target for biofilm inhibition and may contribute to the rational design of biofilm inhibiting hydrogel coatings for urinary catheters.

The AlgT-dependent transcriptional regulator AmrZ (AlgZ) inhibits flagellum biosynthesis in mucoid, nonmotile Pseudomonas aeruginosa cystic fibrosis isolates

Pseudomonas aeruginosa is a microorganism associated with the disease cystic fibrosis. While environmental P. aeruginosa strains are generally nonmucoid and motile, isolates recovered from the cystic fibrosis lung frequently display a mucoid, nonmotile phenotype. This phenotypic conversion is mediated by the alternative sigma factor AlgT. Previous work has shown that repression of fleQ by AlgT accounts for the loss of flagellum biosynthesis in these strains. Here, we elucidate the mechanism involved in the AlgT-mediated control of fleQ. Electrophoretic mobility shift assays using purified AlgT and extracts derived from isogenic AlgT(+) and AlgT(-) strains revealed that AlgT inhibits fleQ indirectly. We observed that the AlgT-dependent transcriptional regulator AmrZ interacts directly with the fleQ promoter. To determine whether AmrZ functions as a repressor of fleQ, we mutated amrZ in the mucoid, nonmotile P. aeruginosa strain FRD1. Unlike the parental strain, the amrZ mutant was nonmucoid and motile. Complementation of the mutant with amrZ restored the mucoid, nonmotile phenotype. Thus, our data show that AlgT inhibits flagellum biosynthesis in mucoid, nonmotile P. aeruginosa cystic fibrosis isolates by promoting expression of AmrZ, which subsequently represses fleQ. Since fleQ directly or indirectly controls the expression of almost all flagellar genes, its repression ultimately leads to the loss of flagellum biosynthesis.

Acetyl phosphate-sensitive regulation of flagellar biogenesis and capsular biosynthesis depends on the Rcs phosphorelay

As part of our attempt to map the impact of acetyl phosphate (acetyl approximately P) on the entire network of two-component signal transduction pathways in Escherichia coli, we asked whether the influence of acetyl approximately P on capsular biosynthesis and flagellar biogenesis depends on the Rcs phosphorelay. To do so, we performed a series of epistasis experiments: mutations in the components of the pathway that controls acetyl approximately P levels were combined with mutations in components of the Rcs phosphorelay. Cells that did not synthesize acetyl approximately P produced no capsule under normally permissive conditions, while those that accumulated acetyl approximately P synthesized capsule under conditions previously considered to be non-permissive. Acetyl approximately P-dependent capsular biosynthesis required both RcsB and RcsA, while the lack of RcsC restored capsular biosynthesis to acetyl approximately P-deficient cells. Similarly, acetyl approximately P-sensitive repression of flagellar biogenesis was suppressed by the loss of RcsB (but not of RcsA), while it was enhanced by the lack of RcsC. Taken together, these results show that both acetyl approximately P-sensitive activation of capsular biosynthesis and acetyl approximately P-sensitive repression of flagellar biogenesis require the Rcs phosphorelay. Moreover, they provide strong genetic support for the hypothesis that RcsC can function as either a kinase or a phosphatase dependent on environmental conditions. Finally, we learned that RcsB and RcsC inversely regulated the timing of flagellar biogenesis: rcsB mutants elaborated flagella prematurely, while rcsC mutants delayed their display of flagella. Temporal control of flagella biogenesis implicates the Rcs phosphorelay (and, by extension, acetyl approximately P) in the transition of motile, planktonic individuals into sessile biofilm communities.

Anaerobic killing of mucoid Pseudomonas aeruginosa by acidified nitrite derivatives under cystic fibrosis airway conditions

Mucoid, mucA mutant Pseudomonas aeruginosa cause chronic lung infections in cystic fibrosis (CF) patients and are refractory to phagocytosis and antibiotics. Here we show that mucoid bacteria perish during anaerobic exposure to 15 mM nitrite (NO2) at pH 6.5, which mimics CF airway mucus. Killing required a pH lower than 7, implicating formation of nitrous acid (HNO2) and NO, that adds NO equivalents to cellular molecules. Eighty-seven percent of CF isolates possessed mucA mutations and were killed by HNO2 (3-log reduction in 4 days). Furthermore, antibiotic-resistant strains determined were also equally sensitive to HNO2. More importantly, HNO2 killed mucoid bacteria (a) in anaerobic biofilms; (b) in vitro in ultrasupernatants of airway secretions derived from explanted CF patient lungs; and (c) in mouse lungs in vivo in a pH-dependent fashion, with no organisms remaining after daily exposure to HNO2 for 16 days. HNO2 at these levels of acidity and NO2 also had no adverse effects on cultured human airway epithelia in vitro. In summary, selective killing by HNO2 may provide novel insights into the important clinical goal of eradicating mucoid P. aeruginosa from the CF airways.

The alternative sigma factor AlgT represses Pseudomonas aeruginosa flagellum biosynthesis by inhibiting expression of fleQ

Pseudomonas aeruginosa poses a serious risk in individuals suffering from cystic fibrosis (CF). Strains colonizing the CF lung are generally motile but frequently convert to a nonmotile phenotype as the disease progresses. In many cases, this is coordinately regulated with the overproduction of the exopolysaccharide alginate. Both the expression of alginate (mucoidy) and the loss of flagellum synthesis may provide the bacterium with a selective advantage in the CF lung. Previously published data showed that the regulation of alginate production and flagellum biosynthesis in the CF isolate FRD1 is inversely controlled by the alternative sigma factor AlgT. In this study, we observed that in CF isolates, the mucoid and the nonmotile phenotypes occur predominantly together. Using microarrays, we compared the transcriptomes of isogenic AlgT(+) and AlgT(-) P. aeruginosa and discovered that AlgT significantly downregulated the majority of flagellar genes. A pronounced inhibitory effect was observed in several genes essential for proper flagellum expression, including fleQ, which encodes an essential flagellar regulator. The microarray data were confirmed by reverse transcriptase PCR analysis and promoter fusion assays in isogenic AlgT(+) and AlgT(-) strains. Transmission electron microscopy, motility assays, and Western blots showed that ectopic expression of FleQ in mucoid, nonmotile CF isolates restored flagellum biosynthesis and motility. Together, these data show that AlgT mediates the negative control of flagellum expression by inhibiting the expression of the flagellar regulator fleQ.

Fast exopolysaccharide secretion of Pseudomonas aeruginosa on polar polymer surfaces

The influence of the surface chemical structure and related physicochemical properties on the adhesion of P. aeruginosa has been studied for moderately hydrophobic polymers and for hydrophilic surfaces obtained by O2-plasma treatments and 50 keV Ar+ beam irradiation of poly(hydroxymethylsiloxane) and poly(ethyleneterephthalate). The surface chemical structure has been obtained by X-ray photoelectron spectroscopy, the roughness was measured by atomic force microscopy, and the surface free energy was evaluated from contact angle measurements for all the polymer substrates before and after the irradiation treatments. It is shown that a massive and unusually fast secretion of exopolysaccharides onto highly polar surfaces, corresponding to the formation of complex three-dimensional multilayers (i.e., biofilm-like structures), occurs already after 2 h of incubation. It is suggested that such highly polar surfaces can operate either by promoting, by means of a still unknown biomolecular mechanism, an early gene expression process or by mimicking the P. aeruginosa cellular walls.

Understanding the control of Pseudomonas aeruginosa alginate synthesis and the prospects for management of chronic infections in cystic fibrosis

Decades of research have been dedicated to the study of the opportunistic pathogen Pseudomonas aeruginosa, a Gram-negative, environmental bacterium that secretes the exopolysaccharide alginate during chronic lung infection of cystic fibrosis (CF) patients. Although P. aeruginosa utilizes a variety of factors to establish a successful infection in the lungs of CF patients, alginate has stood out as one of the best-studied prognostic indicators of chronic lung infection. While the genetics, biosynthesis and regulation of alginate are well understood, questions still remain concerning its role in biofilm development and its potential as a therapeutic target. The purpose of this review is to provide a brief summary of alginate biosynthesis and regulation, and to highlight recent discoveries in the areas of alginate production, biofilm formation and vaccine design. This information is placed in context with a proposed P. aeruginosa infectious pathway, highlighting avenues for the use of existing therapies as well as the potential for novel agents to reduce or eliminate chronic infections in CF patients.

Dynamics and control of biofilms of the oligotrophic bacterium Caulobacter crescentus

Caulobacter crescentus is an oligotrophic alpha-proteobacterium with a complex cell cycle involving sessile-stalked and piliated, flagellated swarmer cells. Because the natural lifestyle of C. crescentus intrinsically involves a surface-associated, sessile state, we investigated the dynamics and control of C. crescentus biofilms developing on glass surfaces in a hydrodynamic system. In contrast to biofilms of the well-studied Pseudomonas aeruginosa, Escherichia coli, and Vibrio cholerae, C. crescentus CB15 cells form biphasic biofilms, consisting predominantly of a cell monolayer biofilm and a biofilm containing densely packed, mushroom-shaped structures. Based on comparisons between the C. crescentus strain CB15 wild type and its holdfast (hfsA; DeltaCC0095), pili (DeltapilA-cpaF::Omegaaac3), motility (motA), flagellum (flgH) mutants, and a double mutant lacking holdfast and flagellum (hfsA; flgH), a model for biofilm formation in C. crescentus is proposed. For both biofilm forms, the holdfast structure at the tip of a stalked cell is crucial for mediating the initial attachment. Swimming motility by means of the single polar flagellum enhances initial attachment and enables progeny swarmer cells to escape from the monolayer biofilm. The flagellum structure also contributes to maintaining the mushroom structure. Type IV pili enhance but are not absolutely required for the initial adhesion phase. However, pili are essential for forming and maintaining the well-defined three-dimensional mushroom-shaped biofilm. The involvement of pili in mushroom architecture is a novel function for type IV pili in C. crescentus. These unique biofilm features demonstrate a spatial diversification of the C. crescentus population into a sessile, "stem cell"-like subpopulation (monolayer biofilm), which generates progeny cells capable of exploring the aqueous, oligotrophic environment by swimming motility and a subpopulation accumulating in large mushroom structures.

Pseudomonas aeruginosaFlagellin and Alginate Elicit Very Distinct Gene Expression Patterns in Airway Epithelial Cells: Implications for Cystic Fibrosis Disease

Infection with the opportunistic pathogen Pseudomonas aeruginosa remains a major health concern. Two P. aeruginosa phenotypes relevant in human disease include motility and mucoidy. Motility is characterized by the presence of flagella and is essential in the establishment of acute infections, while mucoidy, defined by the production of the exopolysaccharide alginate, is critical in the development of chronic infections, such as the infections seen in cystic fibrosis patients. Indeed, chronic infection of the lung by mucoid P. aeruginosa is a major cause of morbidity and mortality in cystic fibrosis patients. We have used Calu-3 human airway epithelial cells to investigate global responses to infection with motile and mucoid P. aeruginosa. The response of airway epithelial cells to exposure to P. aeruginosa motile strains is characterized by a specific increase in gene expression in pathways controlling inflammation and host defense. By contrast, the response of airway epithelia to the stimuli presented by mucoid P. aeruginosa is not proinflammatory and, hence, may not be conducive to the effective elimination of the pathogen. The pattern of gene expression directed by flagellin, but not alginate, includes innate host defense genes, proinflammatory cytokines, and chemokines. By contrast, infection with alginate-producing P. aeruginosa results in an overall attenuation of host responses and an antiapoptotic effect.

The complex flagellar torque generator of Pseudomonas aeruginosa

Flagella act as semirigid helical propellers that are powered by reversible rotary motors. Two membrane proteins, MotA and MotB, function as a complex that acts as the stator and generates the torque that drives rotation. The genome sequence of Pseudomonas aeruginosa PAO1 contains dual sets of motA and motB genes, PA1460-PA1461 (motAB) and PA4954-PA4953 (motCD), as well as another gene, motY (PA3526), which is known to be required for motor function in some bacteria. Here, we show that these five genes contribute to motility. Loss of function of either motAB-like locus was dispensable for translocation in aqueous environments. However, swimming could be entirely eliminated by introduction of combinations of mutations in the two motAB-encoding regions. Mutation of both genes encoding the MotA homologs or MotB homologs was sufficient to abolish motility. Mutants carrying double mutations in nonequivalent genes (i.e., motA motD or motB motC) retained motility, indicating that noncognate components can function together. motY appears to be required for motAB function. The combination of motY and motCD mutations rendered the cells nonmotile. Loss of function of motAB, motY, or motAB motY produced similar phenotypes; although the swimming speed was only reduced to approximately 85% of the wild-type speed, translocation in semisolid motility agar and swarming on the surface of solidified agar were severely impeded. Thus, the flagellar motor of P. aeruginosa represents a more complex configuration than the configuration that has been studied in other bacteria, and it enables efficient movement under different circumstances.

Genetic evidence that the Vibrio cholerae monolayer is a distinct stage in biofilm development

Biofilm development is conceived as a developmental process in which free swimming cells attach to a surface, first transiently and then permanently, as a single layer. This monolayer of immobilized cells gives rise to larger cell clusters that eventually develop into the biofilm, a three-dimensional structure consisting of large pillars of bacteria interspersed with water channels. Previous studies have shown that efficient development of the Vibrio cholerae biofilm requires a combination of pili, flagella and exopolysaccharide. Little is known, however, regarding the requirements for monolayer formation by wild-type V. cholerae. In this work, we have isolated the wild-type V. cholerae monolayer and demonstrated that the environmental signals, bacterial structures, and transcription profiles that induce and stabilize the monolayer state are unique. Cells in a monolayer are specialized to maintain their attachment to a surface. The surface itself activates mannose-sensitive haemagglutinin type IV pilus (MSHA)-mediated attachment, which is accompanied by repression of flagellar gene transcription. In contrast, cells in a biofilm are specialized to maintain intercellular contacts. Progression to this stage occurs when exopolysaccharide synthesis is induced by environmental monosaccharides. We propose a model for biofilm development in natural environments in which cells form a stable monolayer on a surface. As biotic surfaces are degraded with subsequent release of carbohydrates, the monolayer develops into a biofilm.

Alginate production affects Pseudomonas aeruginosa biofilm development and architecture, but is not essential for biofilm formation

Extracellular polymers can facilitate the non-specific attachment of bacteria to surfaces and hold together developing biofilms. This study was undertaken to qualitatively and quantitatively compare the architecture of biofilms produced byPseudomonas aeruginosastrain PAO1 and its alginate-overproducing (mucA22) and alginate-defective (algD) variants in order to discern the role of alginate in biofilm formation. These strains, PAO1, Alg+PAOmucA22and Alg−PAOalgD, tagged with green fluorescent protein, were grown in a continuous flow cell system to characterize the developmental cycles of their biofilm formation using confocal laser scanning microscopy. Biofilm Image Processing (bip) and Community Statistics (comstat) software programs were used to provide quantitative measurements of the two-dimensional biofilm images. All three strains formed distinguishable biofilm architectures, indicating that the production of alginate is not critical for biofilm formation. Observation over a period of 5 days indicated a three-stage development pattern consisting of initiation, establishment and maturation. Furthermore, this study showed that phenotypically distinguishable biofilms can be quantitatively differentiated.

Pseudomonas aeruginosa biofilms exposed to imipenem exhibit changes in global gene expression and beta-lactamase and alginate production

The lungs of cystic fibrosis (CF) patients are commonly colonized with Pseudomonas aeruginosa biofilms. Chronic endobronchial P. aeruginosa infections are impossible to eradicate with antibiotics, but intensive suppressive antibiotic therapy is essential to maintain the lung function of CF patients. The treatment often includes beta-lactam antibiotics. How these antibiotics influence gene expression in the surviving biofilm population of P. aeruginosa is not clear. Thus, we used the microarray technology to study the effects of subinhibitory concentrations of a beta-lactam antibiotic, imipenem, on gene expression in biofilm populations. Many genes showed small but statistically significant differential expression in response to imipenem. We identified 34 genes that were induced or repressed in biofilms exposed to imipenem more than fivefold compared to the levels of induction or repression for the controls. As expected, the most strongly induced gene was ampC, which codes for chromosomal beta-lactamase. We also found that genes coding for alginate biosynthesis were induced by exposure to imipenem. Alginate production is correlated to the development of impaired lung function, and P. aeruginosa strains isolated from chronically colonized lungs of CF patients are nearly always mucoid due to the overproduction of alginate. Exposure to subinhibitory concentrations of imipenem caused structural changes in the biofilm, e.g., an increased biofilm volume. Increased levels of alginate production may be an unintended adverse consequence of imipenem treatment in CF patients.

Significant differences in type IV pilin allele distribution among Pseudomonas aeruginosa isolates from cystic fibrosis (CF) versus non-CF patients

Type IV pili (TFP) are important colonization factors of the opportunistic pathogenPseudomonas aeruginosa, involved in biofilm formation and attachment to host cells. This study undertook a comprehensive analysis of TFP alleles in more than 290 environmental, clinical, rectal and cystic fibrosis (CF) isolates ofP. aeruginosa. Based on the results, a new system of nomenclature is proposed, in whichP. aeruginosaTFP are divided into five distinct phylogenetic groups. Each pilin allele is stringently associated with characteristic, distinct accessory genes that allow the identification of the allele by specific PCR. The invariant association of the pilin and accessory genes implies horizontal transfer of the entire locus. Analysis of pilin allele distribution among isolates from various sources revealed a striking bias in the prevalence of isolates with group I pilin genes from CF compared with non-CF human sources (P<0·0001), suggesting this particular pilin type, which can be post-translationally modified by glycosylation via the action of TfpO (PilO), may confer a colonization or persistence advantage in the CF host. This allele was also predominant in paediatric CF isolates (29 of 43; 67·4 %), showing that this bias is apparent early in colonization. Group I pilins were also the most common type found in environmental isolates tested. To the authors' knowledge, this is the first example of aP. aeruginosavirulence factor allele that is strongly associated with CF isolates.

Antimicrobial resistance of Pseudomonas aeruginosa biofilms

Resistance to antimicrobial agents is the most important feature of biofilm infections. As a result, infections caused by bacterial biofilms are persistent and very difficult to eradicate. Although several mechanisms have been postulated to explain reduced susceptibility to antimicrobials in bacterial biofilms, it is becoming evident that biofilm resistance is multifactorial. The contribution of each of the different mechanisms involved in biofilm resistance is now beginning to emerge.

Pseudomonas aeruginosa alginate is refractory to Th1 immune response and impedes host immune clearance in a mouse model of acute lung infection

Pseudomonas aeruginosa is an opportunistic respiratory pathogen that accounts for most of the morbidity and mortality in cystic fibrosis (CF) patients. In CF-affected lungs, the bacteria undergo conversion from a non-mucoid to a non-tractable mucoid phenotype, due to overproduction of alginate. The effect of alginate production on pathogenicity was investigated by using an acute lung infection mouse model that compared a non-mucoid P. aeruginosa strain, PAO1, to its constitutive alginate-overproducing derivative, Alg(+) PAOmucA22, and an alginate-defective strain, Alg(-) PAOalgD. Bacterial suspensions were instilled into the left bronchus and examined 24 and 48 h post-infection. The highest bacterial loads and the most severe lung pathology were observed with strain Alg(-) PAOalgD at 24 h post-infection, which may have been due to an increase in expression of bacterial elastase by the mutant. Significantly lower lung and spleen bacterial loads were found in the two non-mucoid (PAO1 and Alg(-) PAOalgD) groups, compared to the mucoid Alg(+) PAOmucA22 group, between 24 and 48 h post-infection. The positive correlation between lung bacteriology and lung macroscopic pathology in the Alg(+) PAOmucA22 group suggests that alginate production not only impedes pulmonary clearing, but also results in severe lung damage. Positive correlations between IL12 levels and lung macroscopic pathology, and between IL12 and IFN-gamma levels in the Alg(+) PAOmucA22 group, suggested a possible contribution of these pro-inflammatory cytokines to tissue damage. No significant differences were found between the three groups in lung cytokine responses at 24 or 48 h post-infection. However, on comparison within each group at 24 and 48 h post-infection, a significant increase in the pro-inflammatory cytokine IFN-gamma was observed. Higher ratios of IFN-gamma/IL4 and IFN-gamma/IL10, but lower IL10 levels, were also found in all three groups. These results indicate a Th1-predominated immune response in these animals. Such cytokine responses could have aided the clearance of non-mucoid P. aeruginosa, but were not sufficient to alleviate infection by the mucoid variants. Alginate production may promote survival and persistence of this pathogenic micro-organism in the lung.

Highly adherent small-colony variants of Pseudomonas aeruginosa in cystic fibrosis lung infection

Pseudomonas aeruginosa, an opportunistic human pathogen and ubiquitous environmental bacterium, is capable of forming specialized bacterial communities, referred to as biofilm. The results of this study demonstrate that the unique environment of the cystic fibrosis (CF) lung seems to select for a subgroup of autoaggregative and hyperpiliated P. aeruginosa small-colony variants (SCVs). These morphotypes showed increased fitness under stationary growth conditions in comparison with clonal wild-types and fast-growing revertants isolated from the SCV population in vitro. In accordance with the SCVs being hyperpiliated, they exhibited increased twitching motility and capacity for biofilm formation. In addition, the SCVs attached strongly to the pneumocytic cell line A549. The emergence of these highly adherent SCVs within the CF lung might play a key role in the pathogenesis of P. aeruginosa lung infection, where a biofilm mode of growth is thought to be responsible for persistent infection.

[Biofilm: set-up and organization of a bacterial community]

Bacterial attachment on various surfaces mostly takes place in the form of specialised bacterial communities, referred to as biofilm. The biofilm is formed through series of interactions between cells and adherence to surface, resulting in an organised structure. In this review we have been using Pseudomonas aeruginosa as a model microorganism to describe the series of events that occurred during this developmental process. P. aeruginosa is an opportunistic pathogen that has a wide variety of hosts and infectious sites. In addition to biofilm formation in certain tissues, inert surfaces, such as catheters, are also target for bacterial biofilm development. The use of convenient genetic screens has made possible the identification of numerous biofilm-defective mutants, which have been characterised further. These studies have allowed the proposal for a global model, in which key events are described for the different stages of biofilm formation. Briefly, flagellar mobility is crucial for approaching the surface, whereas type IV pili motility is preponderant for surface colonisation and microcolonies formation. These microcolonies are finally packed together and buried in an exopolysaccharide matrix to form the differentiated bio-film. It is obvious that the different stages of biofilm formation also involved perception of environmental stimuli. These stimuli, and their associated complex regulatory networks, have still to be fully characterised to understand the bacterial strategy, which initiates biofilm formation. One such regulatory system, called Quorum sensing, is one of the key player in the initial differentiation of biofilm. Finally, a better understanding, at the molecular level, of biofilm establishment and persistence should help for the design of antimicrobials that prevent bacterial infections.

Detection of multiple extracytoplasmic function (ECF) sigma factors in the genome of Pseudomonas putida KT2440 and their counterparts in Pseudomonas aeruginosa PA01

Pseudomonas putida KT2440 is highly successful in colonizing a variety habitats, including aquatic and edaphic niches. In accordance with this ability and with the need to adapt to changing environmental conditions, P. putida has developed sophisticated mechanisms of transcriptional regulation. We analysed, at the genome level, the repertoire of sigma factors in P. putida KT2440 and identified 24 sigma factors, 19 of which corresponded to the subfamily of extracytoplasmic function (ECF) sigma factors. We detected 13 ECF sigma factors that showed similarity to the Escherichia coli FecI sigma factor, which is involved in iron acquisition. In 11 cases, a fecR-like gene was found adjacent to the fecI-like gene and, in 10 cases, a gene encoding an iron receptor lies in the vicinity of the fecI/fecR cluster. This may explain the ability of P. putida KT2440 to grow under low iron availability conditions. Five fecI/fecR/iron receptor gene clusters from P. putida were also identified in the human pathogen Pseudomonas aeruginosa.

Regulation of capsule synthesis and cell motility in Salmonella enterica by the essential gene igaA

Mutants of Salmonella enterica carrying the igaA1 allele, selected as able to overgrow within fibroblast cells in culture, are mucoid and show reduced motility. Mucoidy is caused by derepression of wca genes (necessary for capsule synthesis); these genes are regulated by the RcsC/YojN/RcsB phosphorelay system and by the RcsA coregulator. The induction of wca expression in an igaA1 mutant is suppressed by mutations in rcsA and rcsC. Reduced motility is caused by lowered expression of the flagellar master operon, flhDC, and is suppressed by mutations in rcsB or rcsC, suggesting that mutations in the igaA gene reduce motility by activating the RcsB/C system. A null igaA allele can be maintained only in an igaA(+)/igaA merodiploid, indicating that igaA is an essential gene. Lethality is suppressed by mutations in rcsB, rcsC, and yojN, but not in rcsA, suggesting that the viability defect of an igaA null mutant is mediated by the RcsB/RcsC system, independently of RcsA (and therefore of the wca genes). Because all the defects associated with igaA mutations are suppressed by mutations that block the RcsB/RcsC system, we propose a functional interaction between the igaA gene product and either the Rcs regulatory network or one of its regulated products.

Static growth of mucoid Pseudomonas aeruginosa selects for non-mucoid variants that have acquired flagellum-dependent motility

When mucoid (alginate-producing) Pseudomonas aeruginosa FRD1 is grown under low oxygen conditions in liquid culture (static), non-mucoid variants appear and eventually predominate. This conversion is not readily observed in aerobic, shaken cultures or static cultures containing the alternative electron acceptor nitrate. In this study, it is shown that the non-mucoid variants that arise under static growth conditions are almost exclusively algT mutants. It has been shown that AlgT not only positively regulates alginate biosynthesis, but also directly or indirectly negatively regulates flagellum synthesis. Indeed, during static growth, conversion to the non-mucoid phenotype is accompanied by the acquisition of flagellum-mediated motility. Surprisingly, by using a reporter gene fusion with the fliC promoter (pfliC::xylE), it was found that fliC expression begins within hours of static growth and is reversible after returning the culture to shaking conditions. The ability of the strain to produce alginate seems to be irrelevant to this phenomenon, as an AlgT(+) deltaalgD strain showed identical results. Thus, it is suggested that the first effect of static growth is to induce motility as an adaptive measure in the presence of wild-type algT. This may afford P. aeruginosa the ability to swim towards areas of higher oxygen concentrations. Subsequent to this, algT mutations are likely to secure the motile phenotype.

Vibrio cholerae CytR is a repressor of biofilm development

Vibrio cholerae is both a human pathogen and a natural inhabitant of aquatic environments. In the aquatic environment, microorganisms are found attached to surfaces in structures known as biofilms. We have identified a transcriptional repressor in V. cholerae that inhibits exopolysaccharide synthesis and biofilm development. Our studies show that this repressor is the V. cholerae homologue of Escherichia coli CytR, a protein that represses nucleoside uptake and catabolism when nucleosides are scarce. We propose that the role of CytR in V. cholerae biofilm development is to co-ordinate bacterial biofilm accumulation with the presence of nucleosides. Thus, nucleosides may be a signal to planktonic cells to join the biofilm.

Microbiology: breaking down biofilms

Biofilms--adherent bacterial communities embedded in a polymer matrix--are common in nature and can cause persistent human infections that are highly resistant to antibiotics. Recent work provides insight into how these communities develop and function, and offers clues to combating them.

Genetic analysis of Salmonella enteritidis biofilm formation: critical role of cellulose

We report here a new screening method based on the fluorescence of colonies on calcofluor agar plates to identify transposon insertion mutants of Salmonella enteritidis that are defective in biofilm development. The results not only confirmed the requirement of genes already described for the modulation of multicellular behaviour in Salmonella typhimurium and other species, but also revealed new aspects of the biofilm formation process, such as two new genetic elements, named as bcsABZC and bcsEFG operons, required for the synthesis of an exopolysaccharide, digestible with cellulase. Non-polar mutations of bcsC and bcsE genes and complementation experiments demonstrated that both operons are responsible for cellulose biosynthesis in both S. enteritidis and S. typhimurium. Using two different growth media, ATM and LB, we showed that the biofilm produced by S. enteritidis is made of different constituents, suggesting that biofilm composition and regulation depends on environmental conditions. Bacterial adherence and invasion assays of eukaryotic cells and in vivo virulence studies of cellulose-deficient mutants indicated that, at least under our experimental conditions, the production of cellulose is not involved in the virulence of S. enteritidis. However, cellulose-deficient mutants were more sensitive to chlorine treatments, suggesting that cellulose production and biofilm formation may be an important factor for the survival of S. enteritidis on surface environments.

Relationship between genotype and phenotype of flagellin C in Salmonella

AIM: To discover the relationship between the genotype and antigen serotype of flagellin C among Salmonella strains.

METHODS: Fragment of Salmonella flagellin C in plasmid pLS408 was cloned, sequenced and compared with the corresponding sequence in other strains. Salmonella strains including two typhi strains, one paratyphoid strain, one enteritidis and one typhimurium strain were isolated from outpatients. Genome DNA was purified respectively from these clinical isolates, then the corresponding flagellin C fragment was amplified by polymerase chain reaction, and the amplification products were analyzed by agarose gel electrophoresis.

RESULTS: The cloned fragment includes 582 nucleotides encoding the variable region and partial conservative region of Salmonella flagellin C in plasmid pLS408. With comparison to the corresponding sequences reported previously, there is only a little difference from other strains with the same flagellar serotype in both nucleotide and amino acid level. Specific PCR products were amplified in Salmonella strains with flagellar serotype H-1-d including S. muenchen, typhi and typhimurium, but not in S. paratyphoid C or S. enteritidis strains.

CONCLUSION: In this experiment, the specificity of nucleotide sequence could be found in flagellin C central variable regions as it exists in flagellar serotypes in Salmonella. It may be helpful to developing a rapid, sensitive, accurate and PCR-based method to detect Salmonella strains with serotype H-1-d.

Characterization of phenotypic changes in Pseudomonas putida in response to surface-associated growth

The formation of complex bacterial communities known as biofilms begins with the interaction of planktonic cells with a surface. A switch between planktonic and sessile growth is believed to result in a phenotypic change in bacteria. In this study, a global analysis of physiological changes of the plant saprophyte Pseudomonas putida following 6 h of attachment to a silicone surface was carried out by analysis of protein profiles and by mRNA expression patterns. Two-dimensional (2-D) gel electrophoresis revealed 15 proteins that were up-regulated following bacterial adhesion and 30 proteins that were down-regulated. N-terminal sequence analyses of 11 of the down-regulated proteins identified a protein with homology to the ABC transporter, PotF; an outer membrane lipoprotein, NlpD; and five proteins that were homologous to proteins involved in amino acid metabolism. cDNA subtractive hybridization revealed 40 genes that were differentially expressed following initial attachment of P. putida. Twenty-eight of these genes had known homologs. As with the 2-D gel analysis, NlpD and genes involved in amino acid metabolism were identified by subtractive hybridization and found to be down-regulated following surface-associated growth. The gene for PotB was up-regulated, suggesting differential expression of ABC transporters following attachment to this surface. Other genes that showed differential regulation were structural components of flagella and type IV pili, as well as genes involved in polysaccharide biosynthesis. Immunoblot analysis of PilA and FliC confirmed the presence of flagella in planktonic cultures but not in 12- or 24-h biofilms. In contrast, PilA was observed in 12-h biofilms but not in planktonic culture. Recent evidence suggests that quorum sensing by bacterial homoserine lactones (HSLs) may play a regulatory role in biofilm development. To determine if similar protein profiles occurred during quorum sensing and during early biofilm formation, HSLs extracted from P. putida and pure C(12)-HSL were added to 6-h planktonic cultures of P. putida, and cell extracts were analyzed by 2-D gel profiles. Differential expression of 16 proteins was observed following addition of HSLs. One protein, PotF, was found to be down-regulated by both surface-associated growth and by HSL addition. The other 15 proteins did not correspond to proteins differentially expressed by surface-associated growth. The results presented here demonstrate that P. putida undergoes a global change in gene expression following initial attachment to a surface. Quorum sensing may play a role in the initial attachment process, but other sensory processes must also be involved in these phenotypic changes.

Alginate overproduction affects Pseudomonas aeruginosa biofilm structure and function

During the course of chronic cystic fibrosis (CF) infections, Pseudomonas aeruginosa undergoes a conversion to a mucoid phenotype, which is characterized by overproduction of the exopolysaccharide alginate. Chronic P. aeruginosa infections involve surface-attached, highly antibiotic-resistant communities of microorganisms organized in biofilms. Although biofilm formation and the conversion to mucoidy are both important aspects of CF pathogenesis, the relationship between them is at the present unclear. In this study, we report that the overproduction of alginate affects biofilm development on an abiotic surface. Biofilms formed by an alginate-overproducing strain exhibit a highly structured architecture and are significantly more resistant to the antibiotic tobramycin than a biofilm formed by an isogenic nonmucoid strain. These results suggest that an important consequence of the conversion to mucoidy is an altered biofilm architecture that shows increasing resistance to antimicrobial treatments.

Biofilm formation as microbial development

Biofilms can be defined as communities of microorganisms attached to a surface. It is clear that microorganisms undergo profound changes during their transition from planktonic (free-swimming) organisms to cells that are part of a complex, surface-attached community. These changes are reflected in the new phenotypic characteristics developed by biofilm bacteria and occur in response to a variety of environmental signals. Recent genetic and molecular approaches used to study bacterial and fungal biofilms have identified genes and regulatory circuits important for initial cell-surface interactions, biofilm maturation, and the return of biofilm microorganisms to a planktonic mode of growth. Studies to date suggest that the planktonic-biofilm transition is a complex and highly regulated process. The results reviewed in this article indicate that the formation of biofilms serves as a new model system for the study of microbial development.

Initiation of biofilm formation by Pseudomonas aeruginosa 57RP correlates with emergence of hyperpiliated and highly adherent phenotypic variants deficient in swimming, swarming, and twitching motilities

Pseudomonas aeruginosa is a ubiquitous environmental bacterium capable of forming biofilms on surfaces as a survival strategy. It exhibits a large variety of competition/virulence factors, such as three types of motilities: flagellum-mediated swimming, flagellum-mediated swarming, and type IV pilus-mediated twitching. A strategy frequently used by bacteria to survive changing environmental conditions is to create a phenotypically heterogeneous population by a mechanism called phase variation. In this report, we describe the characterization of phenotypic variants forming small, rough colonies that spontaneously emerged when P. aeruginosa 57RP was cultivated as a biofilm or in static liquid cultures. These small-colony (S) variants produced abundant type IV fimbriae, displayed defective swimming, swarming, and twitching motilities, and were impaired in chemotaxis. They also autoaggregated in liquid cultures and rapidly initiated the formation of strongly adherent biofilms. In contrast, the large-colony variant (parent form) was poorly adherent, homogeneously dispersed in liquid cultures, and produced scant polar fimbriae. Further analysis of the S variants demonstrated differences in a variety of other phenotypic traits, including increased production of pyocyanin and pyoverdine and reduced elastase activity. Under appropriate growth conditions, cells of each phenotype switched to the other phenotype at a fairly high frequency. We conclude that these S variants resulted from phase variation and were selectively enriched when P. aeruginosa 57RP was grown as a biofilm or in static liquid cultures. We propose that phase variation ensures the prior presence of phenotypic forms well adapted to initiate the formation of a biofilm as soon as environmental conditions are favorable.

Bacterial biofilms and human disease

The term biofilm is used to denote a polymer-encased community of microbes which accumulates at a surface. Biofilms are responsible for a number of diseases of man and, because of the intrinsic resistance of these structures to antibiotics and host defence systems, such diseases are very difficult to treat effectively. The application of new microscopic and molecular techniques to biofilms has revolutionised our understanding of their structure, composition, organisation and activities. This review will describe the role that biofilms play in human disease and will outline our new millennial view of these complex and fascinating bacterial communities.

The absence of a flagellum leads to altered colony morphology, biofilm development and virulence in Vibrio cholerae O139

Throughout most of history, epidemic and pandemic cholera was caused by Vibrio cholerae of the serogroup O1. In 1992, however, a V. cholerae strain of the serogroup O139 emerged as a new agent of epidemic cholera. Interestingly, V. cholerae O139 forms biofilms on abiotic surfaces more rapidly than V. cholerae O1 biotype El Tor, perhaps because regulation of exopolysaccharide synthesis in V. cholerae O139 differs from that in O1 El Tor. Here, we show that all flagellar mutants of V. cholerae O139 have a rugose colony morphology that is dependent on the vps genes. This suggests that the absence of the flagellar structure constitutes a signal to increase exopolysaccharide synthesis. Furthermore, although exopolysaccharide production is required for the development of a three-dimensional biofilm, inappropriate exopolysaccharide production leads to inefficient colonization of the infant mouse intestinal epithelium by flagellar mutants. Thus, precise regulation of exopolysaccharide synthesis is an important factor in the survival of V. cholerae O139 in both aquatic environments and the mammalian intestine.

Microbial biofilms: from ecology to molecular genetics

Biofilms are complex communities of microorganisms attached to surfaces or associated with interfaces. Despite the focus of modern microbiology research on pure culture, planktonic (free-swimming) bacteria, it is now widely recognized that most bacteria found in natural, clinical, and industrial settings persist in association with surfaces. Furthermore, these microbial communities are often composed of multiple species that interact with each other and their environment. The determination of biofilm architecture, particularly the spatial arrangement of microcolonies (clusters of cells) relative to one another, has profound implications for the function of these complex communities. Numerous new experimental approaches and methodologies have been developed in order to explore metabolic interactions, phylogenetic groupings, and competition among members of the biofilm. To complement this broad view of biofilm ecology, individual organisms have been studied using molecular genetics in order to identify the genes required for biofilm development and to dissect the regulatory pathways that control the plankton-to-biofilm transition. These molecular genetic studies have led to the emergence of the concept of biofilm formation as a novel system for the study of bacterial development. The recent explosion in the field of biofilm research has led to exciting progress in the development of new technologies for studying these communities, advanced our understanding of the ecological significance of surface-attached bacteria, and provided new insights into the molecular genetic basis of biofilm development.