Closely linked genetic loci required for swarm cell differentiation and multicellular migration by Proteus mirabilis

Overview of Proteus mirabilis pathogenicity and virulence. Insights into the role of metals

Proteus mirabilis is a Gram-negative bacterium with exclusive molecular and biological features. It is a versatile pathogen acclaimed for its distinct urease production, swarming behavior, and rapid multicellular activity. Clinically, P. mirabilis is a frequent pathogen of the human urinary system where it causes urinary tract infections (UTIs) and catheter-associated urinary tract infections (CAUTIs). This review explores the epidemiology, risk factors, clinical manifestations, and treatment of P. mirabilis infections, emphasizing its association with UTIs. The bacterium's genome analysis revealed the presence of resistance genes against commonly used antibiotics, an antibiotic-resistant phenotype that poses a serious clinical challenge. Particularly, the emergence of extended-spectrum β-lactamases (ESBLs) and carbapenemases resistant P. mirabilis strains. On a molecular level, P. mirabilis possesses a wide array of virulence factors including the production of fimbriae, urease, hemolysins, metallophores, and biofilm formation. This review thoroughly tackles a substantial gap in understanding the role of metallophores in shaping the virulence factors of P. mirabilis virulence. Siderophores, iron metal chelating and transporting metallophores, particularly contribute to the complex pathogenic strategies, displaying a potential target for therapeutic intervention.

Inhibition of swarming motility using in vitro hyperthermia

Hyperthermia is a therapeutic technique in which body tissue is exposed to temperatures in the region of 40-45 °C to induce a physiological or biological effect. Swarming motility is an important virulence factor for Proteus mirabilis and Pseudomonas aeruginosa and swarming phenomenon is a coordinated multicellular movement of differentiated bacterial population over semi-solid surfaces. In this study, we aimed to investigate the inhibitory effect of hyperthermia on bacterial swarming motility using a modified thermobiogram method and show the potential of this thermal method to treat bacterial infections. Ten P. mirabilis and 10 P. aeruginosa clinical isolates were included in the study. Sheep blood agar (SBA) plates were prepared and inoculated with bacterial suspensions of clinical isolates. Inoculated SBA plates were incubated inside 2 different incubators; at 37 °C and 45 °C for 20 h. The diameter of bacterial growing zones (swarming diameters) were measured every 2 h and noted. Finally, Gram stains of the isolates were prepared for microscopic examination. Wilcoxon signed-rank test was used to compare the swarming inhibition rates of the isolates incubated at 37 °C and 45 °C. Regarding P. mirabilis species, a significant difference was found between two different temperatures (P = 0.0078). So, a temperature at the level of hyperthermia significantly inhibited the swarming motility of P. mirabilis isolates. In addition, transformation to coccus form was observed at 45 °C. We speculate that these findings might be useful for employing thermal therapies including hyperthermia method to treat infectious diseases caused by swarming bacterial pathogens in the future.

In vitro activity of hyperthermia on swarming motility and antimicrobial susceptibility profiles of Proteus mirabilis isolates

AIM

Swarming motility is a virulence factor for Proteus mirabilis and is a coordinated multicellular movement of bacteria. In this study, we investigated the inhibitory effect of hyperthermia on bacterial swarming motility and antimicrobial resistance.

METHODS

Thirty-one P. mirabilis isolates were included in the study. Seven inoculated agar plates were incubated inside incubators with increasing temperature levels: at 36 °C (control) and 40-45 °C. On the next day, inhibition of swarming was evaluated and minimum paralyzing temperature (MPT) values were determined. An antimicrobial susceptibility test (antibiogram) is performed by exposing bacteria to increasing concentrations of antibiotics, in vitro. Thus, we used the Kirby-Bauer disk diffusion test as a screening method to analyze the antibiogram profiles of the isolates at 36 °C and 42 °C. Finally, a time-kill assay was performed to analyze the killing effect of hyperthermia (42 °C) on planktonic bacteria, in combination with the antibiotic meropenem at the first and third hours. A Wilcoxon signed-rank test was used to compare the killing effects of meropenem, hyperthermia and their combinations.

RESULTS

The median MPT value was determined as 44 °C. In the disk diffusion assay, susceptibility development was observed in 94% of isolates for at least one antibiotic. In the time-kill assay, we observed a significant killing effect of hyperthermia in combination with meropenem. Under the microscope, we observed the formation of spherical cells by the effect of heat.

CONCLUSION

We conclude that these findings might be useful when employing the hyperthermia method to treat infectious diseases caused by P. mirabilis in the future.

Proteus mirabilis and Urinary Tract Infections

Proteus mirabilis is a Gram-negative bacterium and is well known for its ability to robustly swarm across surfaces in a striking bulls'-eye pattern. Clinically, this organism is most frequently a pathogen of the urinary tract, particularly in patients undergoing long-term catheterization. This review covers P. mirabilis with a focus on urinary tract infections (UTI), including disease models, vaccine development efforts, and clinical perspectives. Flagella-mediated motility, both swimming and swarming, is a central facet of this organism. The regulation of this complex process and its contribution to virulence is discussed, along with the type VI-secretion system-dependent intra-strain competition, which occurs during swarming. P. mirabilis uses a diverse set of virulence factors to access and colonize the host urinary tract, including urease and stone formation, fimbriae and other adhesins, iron and zinc acquisition, proteases and toxins, biofilm formation, and regulation of pathogenesis. While significant advances in this field have been made, challenges remain to combatting complicated UTI and deciphering P. mirabilis pathogenesis.

A multi-scale approach of the mechanisms underlying exopolysaccharide auto-organization in the Proteus mirabilis extracellular matrix

Supramolecular organization within the extracellular matrix triggers the swarming/consolidation alternation in response to a periodic variation of water activity.

Flagellum density regulates Proteus mirabilis swarmer cell motility in viscous environments

Proteus mirabilis is an opportunistic pathogen that is frequently associated with urinary tract infections. In the lab, P. mirabilis cells become long and multinucleate and increase their number of flagella as they colonize agar surfaces during swarming. Swarming has been implicated in pathogenesis; however, it is unclear how energetically costly changes in P. mirabilis cell morphology translate into an advantage for adapting to environmental changes. We investigated two morphological changes that occur during swarming--increases in cell length and flagellum density--and discovered that an increase in the surface density of flagella enabled cells to translate rapidly through fluids of increasing viscosity; in contrast, cell length had a small effect on motility. We found that swarm cells had a surface density of flagella that was ∼5 times larger than that of vegetative cells and were motile in fluids with a viscosity that inhibits vegetative cell motility. To test the relationship between flagellum density and velocity, we overexpressed FlhD(4)C(2), the master regulator of the flagellar operon, in vegetative cells of P. mirabilis and found that increased flagellum density produced an increase in cell velocity. Our results establish a relationship between P. mirabilis flagellum density and cell motility in viscous environments that may be relevant to its adaptation during the infection of mammalian urinary tracts and movement in contact with indwelling catheters.

Transcriptome of swarming Proteus mirabilis

Swarming motility by the urinary tract pathogen Proteus mirabilis has been a long-studied but little understood phenomenon. On agar, a P. mirabilis colony grows outward in a bull's-eye pattern formed by consecutive waves of rapid swarming followed by consolidation into shorter cells. To examine differential gene expression in these growth phases, a microarray was constructed based on the completed genome sequence and annotation. RNA was extracted from broth-cultured, swarming, and consolidation-phase cells to assess transcription during each of these growth states. A total of 587 genes were differentially expressed in broth-cultured cells versus swarming cells, and 527 genes were differentially expressed in broth-cultured cells versus consolidation-phase cells (consolidate). Flagellar genes were highly upregulated in both swarming cells and consolidation-phase cells. Fimbriae were downregulated in swarming cells, while genes involved in cell division and anaerobic growth were upregulated in broth-cultured cells. Direct comparison of swarming cells to consolidation-phase cells found that 541 genes were upregulated in consolidate, but only nine genes were upregulated in swarm cells. Genes involved in flagellar biosynthesis, oligopeptide transport, amino acid import and metabolism, cell division, and phage were upregulated in consolidate. Mutation of dppA, oppB, and cysJ, upregulated during consolidation compared to during swarming, revealed that although these genes play a minor role in swarming, dppA and cysJ are required during ascending urinary tract infection. Swarming on agar to which chloramphenicol had been added suggested that protein synthesis is not required for swarming. These data suggest that the consolidation phase is a state in which P. mirabilis prepares for the next wave of swarming.

Salmonella typhimurium flhE, a conserved flagellar regulon gene required for swarming

The Salmonella typhimurium gene flhE is located at the end of a large flagellar locus in at least 10 peritrichously flagellated Gram-negative bacterial genera, but it shares no significant similarity with other genes. This study shows that flhE is transcribed as part of an flhBAE flagellar operon, under the control of the flagellar master regulator FlhD(2)C(2). Deletion of the chromosomal flhE gene did not affect swimming motility, but it abolished swarming motility across solid agar. Swarming was restored to the DeltaflhE mutant by the 130 aa putative envelope protein FlhE, but not by a truncated version lacking the N-terminal signal peptidase I recognition sequence. The DeltaflhE mutant was indistinguishable from the wild-type parent in number and distribution of flagella, secretion of flagellin subunits, and flagellar gene expression, and there were no obvious differences in cell-surface LPS and extracellular polysaccharide. The DeltaflhE mutant was able to swarm when non-ionic surfactant was included in agar medium, and it showed differences to the wild-type in binding calcofluor and Congo red dyes, and in biofilm production. The data show that the flhE gene is part of the flagella regulon but that it has no role in flagella biogenesis. It appears, nevertheless, to act at the cell envelope to influence flagella-dependent swarming.

Inhibition of swarming and virulence factor expression in Proteus mirabilis by resveratrol

Resveratrol (3,5,4-trihydroxy-trans-stilbene) is a phytoalexin compound with anti-inflammatory and antioxidant activities. The effect of resveratrol on swarming and virulence factor expression of Proteus mirabilis, an important pathogen infecting the urinary tract, was determined on swarming agar plates with and without the compound. Bacteria harvested at different times were assayed for cell length and the production of flagella, haemolysin and urease. Resveratrol inhibited P. mirabilis swarming and virulence factor expression in a dose-dependent manner. Resveratrol significantly inhibited swarming at 15 microg ml(-1), and completely inhibited swarming at 60 microg ml(-1). Inhibition of swarming and virulence factor expression was mediated through RsbA, a His-containing phosphotransmitter of the bacterial two-component signalling system possibly involved in quorum sensing. Complementation of an rsbA-defective mutant with the rsbA gene restored its responsiveness to resveratrol. The compound also inhibited the ability of P. mirabilis to invade human urothelial cells. These findings suggest that resveratrol has potential to be developed as an antimicrobial agent against P. mirabilis infection.

Mapping bacterial surface population physiology in real-time: infrared spectroscopy of Proteus mirabilis swarm colonies

We mapped the space-time distribution of stationary and swarmer cells within a growing Proteus mirabilis colony by infrared (IR) microspectroscopy. Colony mapping was performed at different positions between the inoculum and the periphery with a discrete microscope-mounted IR sensor, while continuous monitoring at a fixed location over time used an optical fiber based IR-attenuated total reflection (ATR) sensor, or "optrode." Phenotypes within a single P. mirabilis population relied on identification of functional determinants (producing unique spectral signals) that reflect differences in macromolecular composition associated with cell differentiation. Inner swarm colony domains are spectrally homogeneous, having patterns similar to those produced by the inoculum. Outer domains composed of active swarmer cells exhibit spectra distinguishable at multiple wavelengths dominated by polysaccharides. Our real-time observations agree with and extend earlier reports indicating that motile swarmer cells are restricted to a narrow (approximately 3 mm) annulus at the colony edge. This study thus validates the use of an IR optrode for real-time and noninvasive monitoring of biofilms and other bacterial surface populations.

Binding and transcriptional activation of non-flagellar genes by the Escherichia coli flagellar master regulator FlhD2C2

The gene hierarchy directing biogenesis of peritrichous flagella on the surface of Escherichia coli and other enterobacteria is controlled by the heterotetrameric master transcriptional regulator FlhD(2)C(2). To assess the extent to which FlhD(2)C(2) directly activates promoters of a wider regulon, a computational screen of the E. coli genome was used to search for gene-proximal DNA sequences similar to the 42-44 bp inverted repeat FlhD(2)C(2) binding consensus. This identified the binding sequences upstream of all eight flagella class II operons, and also putative novel FlhD(2)C(2) binding sites in the promoter regions of 39 non-flagellar genes. Nine representative non-flagellar promoter regions were all bound in vitro by active reconstituted FlhD(2)C(2) over the K(D) range 38-356 nM, and of the nine corresponding chromosomal promoter-lacZ fusions, those of the four genes b1904, b2446, wzz(fepE) and gltI showed up to 50-fold dependence on FlhD(2)C(2) in vivo. In comparison, four representative flagella class II promoters bound FlhD(2)C(2) in the K(D) range 12-43 nM and were upregulated in vivo 30- to 990-fold. The FlhD(2)C(2)-binding sites of the four regulated non-flagellar genes overlap by 1 or 2 bp the predicted -35 motif of the FlhD(2)C(2)-activated sigma(70) promoters, as is the case with FlhD(2)C(2)-dependent class II flagellar promoters. The data indicate a wider FlhD(2)C(2) regulon, in which non-flagellar genes are bound and activated directly, albeit less strongly, by the same mechanism as that regulating the flagella gene hierarchy.

Characterization of the dapA-nlpB genetic locus involved in regulation of swarming motility, cell envelope architecture, hemolysin production, and cell attachment ability in Serratia marcescens

Swarming migration of Serratia marcescens requires both flagellar motility and cellular differentiation and is a population-density-dependent behavior. While the flhDC and quorum-sensing systems have been characterized as important factors regulating S. marcescens swarming, the underlying molecular mechanisms are currently far from being understood. Serratia swarming is thermoregulated and is characterized by continuous surface migration on rich swarming agar surfaces at 30 degrees C but not at 37 degrees C. To further elucidate the mechanisms, identification of specific and conserved regulators that govern the initiation of swarming is essential. We performed transposon mutagenesis to screen for S. marcescens strain CH-1 mutants that swarmed at 37 degrees C. Analysis of a "precocious-swarming" mutant revealed that the defect in a conserved dapA(Sm)-nlpB(Sm) genetic locus which is closely related to the synthesis of bacterial cell wall peptidoglycan is responsible for the aberrant swarming phenotype. Further complementation and gene knockout studies showed that nlpB(Sm), which encodes a membrane lipoprotein, NlpB(Sm), but not dapA(Sm), is specifically involved in swarming regulation. On the other hand, dapA(Sm) but not nlpB(Sm) is responsible for the determination of cell envelope architecture, regulation of hemolysin production, and cellular attachment capability. While the nlpB(Sm) mutant showed similar cytotoxicity to its parent strain, the dapA(Sm) mutant significantly increased in cytotoxicity. We present evidence that DapA(Sm) is involved in the determination of cell-envelope-associated phenotypes and that NlpB(Sm) is involved in the regulation of swarming motility.

Modulation of swarming and virulence by fatty acids through the RsbA protein in Proteus mirabilis

After sensing external signals, Proteus mirabilis undergoes a multicellular behavior called swarming which is coordinately regulated with the expression of virulence factors. Here we report that exogenously added fatty acids could act as signals to regulate swarming in P. mirabilis. Specifically, while oleic acid enhanced swarming, some saturated fatty acids, such as lauric acid, myristic acid, palmitic acid, and stearic acid, inhibited swarming. We also found that expression of hemolysin, which has been shown to be coordinately regulated with swarming, was also inhibited by the above saturated fatty acids. Previously we identified a gene, rsbA, which may encode a histidine-containing phosphotransmitter of the bacterial two-component signaling system and act as a repressor of swarming and virulence factor expression in P. mirabilis. We found that while myristic acid, lauric acid, and palmitic acid exerted their inhibitory effect on swarming and hemolysin expression through an RsbA-dependent pathway, the inhibition by stearic acid was mediated through an RsbA-independent pathway. Biofilm formation and extracellular polysaccharide (EPS) production play an important role in P. mirabilis infection. We found that RsbA may act as a positive regulator of biofilm formation and EPS production. Myristic acid was found to slightly stimulate biofilm formation and EPS production, and this stimulation was mediated through an RsbA-dependent pathway. Together, these data suggest that fatty acids may act as environmental cues to regulate swarming and virulence in P. mirabilis and that RsbA may play an important role in this process.

Visualization of Proteus mirabilis morphotypes in the urinary tract: the elongated swarmer cell is rarely observed in ascending urinary tract infection

Proteus mirabilis, a common cause of nosocomial and catheter-associated urinary tract infection, colonizes the bladder and ascends the ureters to the proximal tubules of the kidneys, leading to the development of acute pyelonephritis. P. mirabilis is capable of swarming, a form of multicellular behavior in which bacteria differentiate from the short rod typical of members of the family Enterobacteriaceae, termed the swimmer cell, into hyperflagellated elongated bacteria capable of rapid and coordinated population migration across surfaces, called the swarmer cell. There has been considerable debate as to which morphotype predominates during urinary tract infection. P. mirabilis(pBAC001), which expresses green fluorescent protein in both swimming and swarming morphotypes, was constructed to quantify the prevalence of each morphotype in ascending urinary tract infection. Transurethral inoculation of P. mirabilis(pBAC001) resulted in ascending urinary tract infection and kidney pathology in mice examined at both 2 and 4 days postinoculation. Using confocal microscopy, we were able to investigate the morphotypes of the bacteria in the urinary tract. Of 5,087 bacteria measured in bladders, ureters, and kidneys, only 7 (0.14%) were identified as swarmers. MR/P fimbria expression, which correlates with the swimmer phenotype, is prevalent on bacteria in the ureters and bladder. We conclude that, by far, the predominant morphotype present in the urinary tract during ascending infection is the short rod-the swimmer cell.

Swarming-coupled expression of the Proteus mirabilis hpmBA haemolysin operon

The HpmA haemolysin toxin of Proteus mirabilis is encoded by the hpmBA locus and its production is upregulated co-ordinately with the synthesis and assembly of flagella during differentiation into hyperflagellated swarm cells. Primer extension identified a sigma(70) promoter upstream of hpmB that was upregulated during swarming. Northern blotting indicated that this promoter region was also required for concomitant transcription of the immediately distal hpmA gene, and that the unstable hpmBA transcript generated a stable hpmA mRNA and an unstable hpmB mRNA. Transcriptional luxAB fusions to the DNA regions 5' of the hpmB and hpmA genes confirmed that hpmB sigma(70) promoter activity increased in swarm cells, and that there was no independent hpmA promoter. Increased transcription of the hpmBA operon in swarm cells was dependent upon a 125 bp sequence 5' of the sigma(70) promoter -35 hexamer. This sequence spans multiple putative binding sites for the leucine-responsive regulatory protein (Lrp), and band-shift assays with purified Lrp confirmed the presence of at least two such sites. The influence on hpmBA expression of the key swarming positive regulators FlhD(2)C(2) (encoded by the flagellar master operon), Lrp, and the membrane-located upregulator of the master operon, UmoB, was examined. Overexpression of each of these regulators moderately increased hpmBA transcription in wild-type P. mirabilis, and the hpmBA operon was not expressed in any of the flhDC, lrp or umoB mutants. Expression in the mutants was not recovered by cross-complementation, i.e. by overexpression of FlhD(2)C(2), Lrp or UmoB. Expression of the zapA protease virulence gene, which like hpmBA is also upregulated in swarm cells, did not require Lrp, but like flhDC it was upregulated by UmoB. The results indicate intersecting pathways of control linking virulence gene expression and swarm cell differentiation.

Dynamic aspects of the structured cell population in a swarming colony of Proteus mirabilis

Proteus mirabilis forms a concentric-ring colony by undergoing periodic swarming. A colony in the process of such synchronized expansion was examined for its internal population structure. In alternating phases, i.e., swarming (active migration) and consolidation (growth without colony perimeter expansion), phase-specific distribution of cells differing in length, in situ mobility, and migration ability on an agar medium were recognized. In the consolidation phase, the distribution of mobile cells was restricted to the inner part of a new ring and a previous terrace. Cells composing the outer part of the ring were immobile in spite of their ordinary swimming ability in a viscous solution. A sectorial cell population having such an internal structure was replica printed on fresh agar medium. After printing, a transplant which was in the swarming phase continued its ongoing swarming while a transplanted consolidation front continued its scheduled consolidation. This shows that cessation of migration during the consolidation phase was not due to substances present in the underlying agar medium. The ongoing swarming schedule was modifiable by separative cutting of the swarming front or disruption of the ring pattern by random mixing of the pattern-forming cell population. The structured cell population seemed to play a role in characteristic colony growth. However, separation of a narrow consolidation front from a backward area did not induce disturbance in the ongoing swarming schedule. Thus, cells at the frontal part of consolidation area were independent of the internal cell population and destined to exert consolidation and swarming with the ongoing ordinary schedule.

The structure of the colony migration factor from pathogenic Proteus mirabilis. A capsular polysaccharide that facilitates swarming

Swarming by Proteus mirabilis is characterized by cycles of rapid and coordinated population migration across surfaces following differentiation of vegetative cells into elongated hyperflagellated swarm cells. It has been shown that surface colony expansion by the swarm cell population is facilitated by a colony migration factor (Cmf), a capsular polysaccharide (CPS) that also contributes to the uropathogenicity of P. mirabilis (Gygi, D., Rahman, M. M., Lai, H.-C., Carlson, R., Guard-Petter, J., and Hughes, C. (1995) Mol. Microbiol. 17, 1167-1175). In this report, the Cmf-CPS was extracted with hot water, precipitated with ethanol, and further purified by gel permeation chromatography. Its structure was established by glycosyl composition and linkage analyses, and by one- and two-dimensional NMR spectroscopy. The Cmf-CPS is composed of the following tetrasaccharide repeating unit. [see text]

Substrate-specific binding of hook-associated proteins by FlgN and FliT, putative chaperones for flagellum assembly

During flagellum assembly by motile enterobacteria, flagellar axial proteins destined for polymerization into the cell surface structure are thought to be exported through the 25-30 A flagellum central channel as partially unfolded monomers. How are premature folding and oligomerization in the cytosol prevented? We have shown previously using hyperflagellated Proteus mirabilis and a motile but non-swarming flgN transposon mutant that the apparently cytosolic 16. 5 kDa flagellar protein FlgN facilitates efficient flagellum filament assembly. Here, we investigate further whether FlgN, predicted to contain a C-terminal amphipathic helix typical of type III export chaperones, acts as a chaperone for axial proteins. Incubation of soluble radiolabelled FlgN from Salmonella typhimurium with nitrocellulose-immobilized cell lysates of wild-type S. typhimurium and a non-flagellate class 1 flhDC mutant indicated that FlgN binds to flagellar proteins. Identical affinity blot analysis of culture supernatants from the wild-type and flhDC, flgI, flgK, flgL, fliC or fliD flagellar mutants showed that FlgN binds to the flagellar hook-associated proteins (HAPs) FlgK and FlgL. This was confirmed by blotting artificially expressed individual HAPs in Escherichia coli. Analysis of axial proteins secreted into the culture medium by the original P. mirabilis flgN mutant demonstrated that export of FlgK and FlgL was specifically reduced, with concomitant increased release of unpolymerized flagellin (FliC), the immediately distal component of the flagellum. These data suggest that FlgN functions as an export chaperone for FlgK and FlgL. Parallel experiments showed that FliT, a similarly small (14 kDa), potentially helical flagellar protein, binds specifically to the flagellar filament cap protein, FliD (HAP2), indicating that it too might be an export chaperone. Flagellar axial proteins all contain amphipathic helices at their termini. Removal of the HAP C-terminal helical domains abolished binding by FlgN and FliT in each case, and polypeptides comprising each of the HAP C-termini were specifically bound by FlgN and FliT. We suggest that FlgN and FliT are substrate-specific flagellar chaperones that prevent oligomerization of the HAPs by binding to their helical domains before export.

Differential expression of nonagglutinating fimbriae and MR/P pili in swarming colonies of Proteus mirabilis

The expression of nonagglutinating fimbriae (NAF) and mannose-resistant/Proteus-like (MR/P) pili in swarming colonies of Proteus mirabilis was investigated. Elongated swarmer cells do not express pili, and the relative number of bacteria expressing NAF during swarming and early consolidation phases was very low (<5%). Relative expression of NAF in a terrace increased to approximately 30% at 48 h. We also determined the expression of NAF and MR/P pili in two phenotypically distinguishable regions of each terrace. The expression of both NAF and MR/P pili was always higher in the region closer (proximal) to the middle of the colony than in the distal region of the terrace. The relative numbers of bacteria expressing NAF or MR/P pili in the proximal region were between 39.1 and 63% and between 5.9 and 7.7%, respectively. In the distal region, expression levels were between 20.8 and 27.3% and between 3.7 and 5. 6%, respectively. A time course experiment testing NAF expression in both the proximal and distal regions of a terrace indicated that NAF expression in the proximal regions was always higher than in the distal regions and increased to a plateau 40 to 50 h after the start of the swarming phase for any given terrace. These results indicate that expression of NAF or MR/P pili in swarming colonies of P. mirabilis is highly organized, spatially and temporally. The significance of this controlled differentiation remains to be uncovered.

The Yersinia enterocolitica motility master regulatory operon, flhDC, is required for flagellin production, swimming motility, and swarming motility

The ability to move over and colonize surface substrata has been linked to the formation of biofilms and to the virulence of some bacterial pathogens. Results from this study show that the gastrointestinal pathogen Yersinia enterocolitica can migrate over and colonize surfaces by swarming motility, a form of cooperative multicellular behavior. Immunoblot analysis and electron microscopy indicated that swarming motility is dependent on the same flagellum organelle that is required for swimming motility, which occurs in fluid environments. Furthermore, motility genes such as flgEF, flgMN, flhBA, and fliA, known to be required for the production of flagella, are essential for swarming motility. To begin to investigate how environmental signals are processed and integrated by Y. enterocolitica to stimulate the production of flagella and regulate these two forms of cell migration, the motility master regulatory operon, flhDC, was cloned. Mutations within flhDC completely abolished swimming motility, swarming motility, and flagellin production. DNA sequence analysis revealed that this locus is similar to motility master regulatory operons of other gram-negative bacteria. Genetic complementation and functional analysis of flhDC indicated that it is required for the production of flagella. When flhDC was expressed from an inducible ptac promoter, flagellin production was shown to be dependent on levels of flhDC expression. Phenotypically, induction of the ptac-flhDC fusion also corresponded to increased levels of both swimming and swarming motility.

A novel membrane protein influencing cell shape and multicellular swarming of Proteus mirabilis

Swarming in Proteus mirabilis is characterized by the coordinated surface migration of multicellular rafts of highly elongated, hyperflagellated swarm cells. We describe a transposon mutant, MNS185, that was unable to swarm even though vegetative cells retained normal motility and the ability to differentiate into swarm cells. However, these elongated cells were irregularly curved and had variable diameters, suggesting that the migration defect results from the inability of these deformed swarm cells to align into multicellular rafts. The transposon was inserted at codon 196 of a 228-codon gene that lacks recognizable homologs. Multiple copies of the wild-type gene, called ccmA, for curved cell morphology, restored swarming to the mutant. The 25-kDa CcmA protein is predicted to span the inner membrane twice, with its C-terminal major domain being present in the cytoplasm. Membrane localization was confirmed both by immunoblotting and by electron microscopy of immunogold-labelled sections. Two forms of CcmA were identified for wild-type P. mirabilis; they were full-length integral membrane CcmA1 and N-terminally truncated peripheral membrane CcmA2, both present at approximately 20-fold higher concentrations in swarm cells. Differentiated MNS185 mutant cells contained wild-type levels of the C-terminally truncated versions of both proteins. Elongated cells of a ccmA null mutant were less misshapen than those of MNS185 and were able to swarm, albeit more slowly than wild-type cells. The truncated CcmA proteins may therefore interfere with normal morphogenesis, while the wild-type proteins, which are not essential for swarming, may enhance migration by maintaining the linearity of highly elongated cells. Consistent with this view, overexpression of the ccmA gene caused cells of both Escherichia coli and P. mirabilis to become enlarged and ellipsoidal.

The diverse and dynamic structure of bacterial genomes

Bacterial genome sizes, which range from 500 to 10,000 kbp, are within the current scope of operation of large-scale nucleotide sequence determination facilities. To date, 8 complete bacterial genomes have been sequenced, and at least 40 more will be completed in the near future. Such projects give wonderfully detailed information concerning the structure of the organism's genes and the overall organization of the sequenced genomes. It will be very important to put this incredible wealth of detail into a larger biological picture: How does this information apply to the genomes of related genera, related species, or even other individuals from the same species? Recent advances in pulsed-field gel electrophoretic technology have facilitated the construction of complete and accurate physical maps of bacterial chromosomes, and the many maps constructed in the past decade have revealed unexpected and substantial differences in genome size and organization even among closely related bacteria. This review focuses on this recently appreciated plasticity in structure of bacterial genomes, and diversity in genome size, replicon geometry, and chromosome number are discussed at inter- and intraspecies levels.

N-Acyl-L-homoserine lactone autoinducers control production of an extracellular lipopeptide biosurfactant required for swarming motility of Serratia liquefaciens MG1

A nonswarming Serratia liquefaciens mutant deficient in serrawettin W2 production was constructed by transposon mutagenesis. Sequence homology indicated that insertion had occurred in gene swrA, which encodes a putative peptide synthetase. Expression of swrA is controlled by quorum sensing.

A swarming-defective mutant of Proteus mirabilis lacking a putative cation-transporting membrane P-type ATPase

The motile TnphoA mutant IC24 of Proteus mirabilis U6450 generates an aberrant swarming colony, and was shown to be impaired in swarm cell differentiation, i.e. cell elongation and hyperflagellation, causing delayed and slower population migration across a solid growth medium. Levels of transcript from the flagellin filament gene fliC, the flagellar master operon flhDC, and the leucine-responsive regulatory protein gene lrp, a regulator of swarming differentiation, were reduced in IC24 mutant swarm cells. The transposon had inserted into a gene encoding a putative P-type ATPase closely related to those transporting cations across bacterial membranes. This ppa gene (Proteus P-type ATPase) was maximally expressed in differentiated swarm cells. The data suggest an effect of ion homeostasis on swarm cell differentiation, possibly mediated via the lrp-flhDC pathway.

Isolation of Rhodospirillum centenum mutants defective in phototactic colony motility by transposon mutagenesis

The purple photosynthetic bacterium Rhodospirillum centenum is capable of forming swarm colonies that rapidly migrate toward or away from light, depending on the wavelength of excitation. To identify components specific for photoperception, we conducted mini-Tn5-mediated mutagenesis and screened approximately 23,000 transposition events for mutants that failed to respond to either continuous illumination or to a step down in light intensity. A majority of the ca. 250 mutants identified lost the ability to form motile swarm cells on an agar surface. These cells appeared to contain defects in the synthesis or assembly of surface-induced lateral flagella. Another large fraction of mutants that were unresponsive to light were shown to be defective in the formation of a functional photosynthetic apparatus. Several photosensory mutants also were obtained with defects in the perception and transmission of light signals. Twelve mutants in this class were shown to contain disruptions in a chemotaxis operon, and five mutants contained disruptions of components unique to photoperception. It was shown that screening for photosensory defective R. centenum swarm colonies is an effective method for genetic dissection of the mechanism of light sensing in eubacteria.

A nonswarming mutant of Proteus mirabilis lacks the Lrp global transcriptional regulator

Proteus swarming is the rapid cyclical population migration across surfaces by elongated cells that hyperexpress flagellar and virulence genes. The mini-Tn5 transposon mutant mns2 was isolated as a tight nonswarming mutant that did not elongate or upregulate flagellar and hemolysin genes. Individual cell motility was retained but was reduced. The transposon had inserted in the gene encoding the global transcriptional regulator Lrp (leucine-responsive regulatory protein), expression of which was upregulated in differentiating swarm cells. Swarming was restored to the lrp mutant by artificial overexpression of the flhDC flagellar regulatory master operon. Lrp may be a key component in generating or relaying signals that are required for flagellation and swarming, possibly acting through the flhDC operon.

Periodic phenomena in Proteus mirabilis swarm colony development

Proteus mirabilis colonies exhibit striking geometric regularity. Basic microbiological methods and imaging techniques were used to measure periodic macroscopic events in swarm colony morphogenesis. We distinguished three initial phases (lag phase, first swarming phase, and first consolidation phase) followed by repeating cycles of subsequent swarming plus consolidation phases. Each Proteus swarm colony terrace corresponds to one swarming-plus-consolidation cycle. The duration of the lag phase was dependent upon inoculation density in a way that indicated the operation of both cooperative and inhibitory multicellular effects. On our standard medium, the second and subsequent swarm phases displayed structure in the form of internal waves visible with reflected and dark-field illumination. These internal waves resulted from organization of the migrating bacteria into successively thicker cohorts of swarmer cells. Bacterial growth and motility were independently modified by altering the composition of the growth medium. By varying the glucose concentration in the substrate, it was possible to alter biomass production without greatly affecting the kinetics of colony surface area expansion. By varying the agar concentration in the substrate, initial bacterial biomass production was unaffected but colony expansion dynamics were significantly altered. Higher agar concentrations led to slower, shorter swarm phases and longer consolidation phases. Thus, colony growth was restricted by higher agar concentrations but the overall timing of the swarming-plus-consolidation cycles remained constant. None of a variety of factors which had significant effects on colony expansion altered terracing frequencies at 32 degrees C, but the length of the swarming-plus-consolidation cycle was affected by temperature and medium enrichment. Some clinical isolates displayed significant differences in terracing frequencies at 32 degrees C. Our results defined a number of readily quantifiable parameters in swarm colony development. The data showed no connection between nutrient (glucose) depletion and the onset of different phases in swarm colony morphogenesis. Several observations point to the operation of density-dependent thresholds in controlling the transitions between distinct phases.

Requirement for FlhA in flagella assembly and swarm-cell differentiation by Proteus mirabilis

Swarming by Proteus mirabilis is characterized by cycles of rapid population migration across surfaces, following differentiation of typical rods into long, aseptate swarm cells that overexpress flagella and virulence factors, particularly haemolysin. A non-swarming Tn5phoA mutant was unable to synthesize flagella, to fully elongate or to induce high levels of the toxin. The mutation lay within a 2091 bp gene encoding a homologue of the Escherichia coli FlhA belonging to a family of proteins that are required for assembly of flagella or virulence proteins and that are suggested to act either directly in membrane translocation and/or in regulating synthesis of the export apparatus. In trans expression of multicopy flhA restored cell elongation and migration and generated differentiation-specific hyperexpression of flagellin and toxin genes to levels above those seen in the wild-type strain. Transcription of flhA was strongly induced during differentiation, from its own putative sigma 28 promoter. The results suggest a mechanistic coupling of flagella assembly and swarm-cell differentiation.

Cell differentiation of Proteus mirabilis is initiated by glutamine, a specific chemoattractant for swarming cells

Swarming by Proteus mirabilis involves differentiation of typical short vegetative rods into filamentous hyperflagellated swarm cells which undergo cycles of rapid and co-ordinated population migration across surfaces and exhibit high levels of virulence gene expression. By supplementing a minimal growth medium (MGM) unable to support swarming migration we identified a single amino acid, glutamine, as sufficient to signal initiation of cell differentiation and migration. Bacteria isolated from the migrating edge of colonies grown for 8 h with glutamine as the only amino acid were filamentous and synthesized the characteristic high levels of flagellin and haemolysin. In contrast, addition of the other 19 common amino acids (excluding glutamine) individually or in combination did not initiate differentiation even after 24 h, cells remaining typical vegetative rods with basal levels of haemolysin and flagellin. The glutamine analogue gamma-glutamyl hydroxamate (GH) inhibited swarming but not growth of P. mirabilis on glutamine MGM and transposon mutants defective in glutamine uptake retained their response to glutamine signalling and its inhibition by GH, suggesting that differentiation signalling by glutamine may be transduced independently of the cellular glutamine transport system. Levels of mRNA transcribed from the haemolysin (hpmA) and flagellin (fliC) genes were low in vegetative cells grown on MGM without glutamine or with glutamine and GH, but were specifically increased c. 40-fold during glutamine-dependent differentiation. In liquid glutamine-MGM cultures, differentiation to filamentous hyper-flagellated hyper-haemolytic swarm cells occurred early in the exponential phase of growth, and increased concomitantly with the concentration of glutamine from a 0.1 mM threshold up to 10 mM.(ABSTRACT TRUNCATED AT 250 WORDS)

Fractal morphogenesis by a bacterial cell population

Many species of bacteria have been found to form fractal colonies. Environmental (physicochemical) and biological factors for this fractal morphogenesis have been examined for their roles in the genesis of fractal and pattern diversity. Morphology of a bacterial colony on a solid agar medium depends on the nutrient diffusion field (two-dimensional). When concentrations of nutrients are low, point-inoculated bacteria (e.g., Bacillus subtilis) exert diffusion-limited growth. A self-similar fractal colony formed slowly under such a condition has the same morphology as one made by the diffusion-limited aggregation (DLA) model. The value of fractal dimensions (ca. 1.72) and the appearance of specific phenomena (screening and repulsion effects) are consistent with computer simulations of the DLA fractal model. On the other hand, a round colony recognized on an agar-rich medium was considered to be the product of reaction-limited growth and was simulated by the Eden model. When motile bacteria are point inoculated onto semi-solid agar media, bacterial spreading behavior also is morphogenic. Branching patterns with various morphologies (e.g., dense-branching morphology) have been recognized and examined for factors responsible for pattern changes. By microscopic inspection of the extending branch, multicellular behavior of bacteria has been observed in the structured cell distribution. Besides cell division and translocation activities, wetting agents produced by some species of bacteria (e.g., serrawettins produced by Serratia marcescens) are considered to be important microbial factors for efficient space occupation and specific cell transpositions in various surface environments.

Ability of Proteus mirabilis to invade human urothelial cells is coupled to motility and swarming differentiation

Proteus mirabilis causes serious kidney infections which can involve invasion of host urothelial cells. We present data showing that the ability to invade host urothelial cells is closely coupled to swarming, a form of cyclical multicellular behavior in which vegetative bacteria differentiate into hyperflagellated, filamentous swarm cells capable of coordinated and rapid population migration. Entry into the human urothelial cell line EJ/28 by P. mirabilis U6450 isolated at different stages throughout the swarming cycle was measured by the antibiotic protection assay method and confirmed by electron microscopy. Differentiated filaments entered urothelial cells within 30 min and were 15-fold more invasive (ca. 0.18% entry in 2 h) than an equivalent dry weight of vegetative cells isolated before differentiation, which attained only ca. 0.012% entry in the 2-h assay. The invasive ability of P. mirabilis was modulated in parallel with flagellin levels throughout two cycles of swarming. Septation and division of intracellular swarm cells produced between 50 and 300 vegetative bacteria per human cell, compared with 4 to 12 intracellular bacteria after incubation with vegetative cells. Transposon (Tn5) mutants of P. mirabilis with specific defects in motility and multicellular behavior were compared with the wild-type for the ability to invade. Mutants which lacked flagella (nonmotile nonswarming) were entirely noninvasive, and those which were motile but defective in swarm cell formation (motile nonswarming) were 25-fold less invasive than wild-type vegetative cells. Mutants with defects in the coordination of multicellular migration and the temporal control of consolidation (cyclical reversion of swarm cells to vegetative cells) were reduced ca. 3- to 12-fold in the ability to enter urothelial cells. In contrast, a nonhemolytic transposon mutant which swarmed normally retained over 80% of wild-type invasive ability. Swarm cells and early consolidation cells were at least 10-fold more cytolytic than vegetative cells as a result of their high-level production of hemolysin.

Co-ordinate expression of virulence genes during swarm-cell differentiation and population migration of Proteus mirabilis

The uropathogenic Gram-negative bacterium Proteus mirabilis exhibits a form of multicellular behaviour termed swarming, which involves cyclical differentiation of typical vegetative cells into filamentous, multinucleate, hyperflagellate swarm cells capable of rapid and co-ordinated population migration across surfaces. We observed that differentiation into swarm cells was accompanied by substantial increases in the activities of intracellular urease and extracellular haemolysin and metalloprotease, which are believed to be central to the pathogenicity of P. mirabilis. In addition, the ability of P. mirabilis to invade human urothelial cells in vitro was primarily a characteristic of differentiated swarm cells, not vegetative cells. These virulence factor activities fell back as the cells underwent cyclical reversion to the vegetative form (consolidation), in parallel with the diagnostic modulation of flagellin levels on the cell surface. Control cellular alkaline phosphatase activities did not increase during differentiation or consolidation. Non-flagellated, nonmotile transposon insertion mutants were unable to invade urothelial cells and they generated only low-level activities of haemolysin, urease and protease (0-10% of wild type). Motile mutants unable to differentiate into swarm cells were comparably reduced in their haemolytic, ureolytic and invasive phenotypes and generated threefold less protease activity. Mutants that were able to form swarm cells but exhibited various aberrant patterns of swarming migration produced wild-type activities of haemolysin, urease and protease, but their ability to enter urothelial cells was three- to 10-fold lower.(ABSTRACT TRUNCATED AT 250 WORDS)