Multicellular and aggregative behaviour of Salmonella typhimurium strains is controlled by mutations in the agfD promoter

Regulation of the Escherichia coli csgD promoter: interplay between five transcription factors

Under stressful conditions in nature, Escherichia coli forms biofilms for long-term survival. Curli fimbriae are an essential architecture for cell–cell contacts within biofilms. Structural components and assembly factors of curli are encoded by two operons, csgBA and csgDEFG. The csgD gene product controls transcription of both operons. Reflecting the response of csgD expression to external stresses, a number of transcription factors participate in the regulation of the csgD promoter. Analysis of the csgD mRNA obtained from E. coli mutants in different transcription factors indicated that CpxR and H-NS act as repressors while OmpR, RstA and IHF act as activators. An acid-stress response regulator, RstA, activates csgD only under acidic conditions. These five factors bind within a narrow region of about 200 bp upstream of the csgD promoter. After pair-wise promoter-binding assays, the increase in csgD transcription in the stationary phase was suggested to be due, at least in part, to the increase in IHF level cancelling the silencing effect of H-NS. In addition, we propose a novel regulation model of this complex csgD promoter through cooperation between the two positive factors (OmpR–IHF and RstA–IHF) and also between the two negative factors (CpxR–H-NS).

Survival of Salmonella on a polypropylene surface under dry conditions in relation to biofilm-formation capability

This study was conducted to gain insights into the survival of Salmonella on a polypropylene surface in relation to the ability of these bacteria to form a biofilm. We selected Salmonella strains known for the relative ease or difficulty with which they formed biofilms based on microtiter plate assays and studied the survival of these strains on polypropylene discs in a desiccation chamber by sequentially counting CFUs. The biofilm-forming strains survived longer on the plastic disc surface than did biofilm-deficient strains. The biofilm-forming strains remained at over 10(4) CFU per plate until day 175, whereas the biofilm-deficient strains decreased to below 10(2) CFU per plate on day 20 or below 10(4) CFU per plate on day 108. Extracellular materials on the polypropylene surface were observed by scanning electron microscopy and crystal violet staining for the biofilm-forming strains but not for the biofilm-deficient strains. The extracellular polymeric materials on the polypropylene surface may have protected the bacterial cells from dryness, although the possibility of some inherent resistance to environmental stresses linked to biofilm formation could not be excluded. These results indicate that Salmonella strains with high biofilm productivity may be a greater risk to human health via food contamination by surviving for longer periods compared with strains with low biofilm productivity.

A global metabolic shift is linked to Salmonella multicellular development

Bacteria can elaborate complex patterns of development that are dictated by temporally ordered patterns of gene expression, typically under the control of a master regulatory pathway. For some processes, such as biofilm development, regulators that initiate the process have been identified but subsequent phenotypic changes such as stress tolerance do not seem to be under the control of these same regulators. A hallmark feature of biofilms is growth within a self-produced extracellular matrix. In this study we used metabolomics to compare Salmonella cells in rdar colony biofilms to isogenic csgD deletion mutants that do not produce an extracellular matrix. The two populations show distinct metabolite profiles. Even though CsgD controls only extracellular matrix production, metabolite signatures associated with cellular adaptations associated with stress tolerances were present in the wild type but not the mutant cells. To further explore these differences we examine the temporal gene expression of genes implicated in biofilm development and stress adaptations. In wild type cells, genes involved in a metabolic shift to gluconeogenesis and various stress-resistance pathways exhibited an ordered expression profile timed with multicellular development even though they are not CsgD regulated. In csgD mutant cells, the ordered expression was lost. We conclude that the induction of these pathways results from production of, and growth within, a self produced matrix rather than elaboration of a defined genetic program. These results predict that common physiological properties of biofilms are induced independently of regulatory pathways that initiate biofilm formation.

Functional amyloid in Pseudomonas

Amyloids are highly abundant in many microbial biofilms and may play an important role in their architecture. Nevertheless, little is known of the amyloid proteins. We report the discovery of a novel functional amyloid expressed by a Pseudomonas strain of the P. fluorescens group. The amyloid protein was purified and the amyloid-like structure verified. Partial sequencing by MS/MS combined with full genomic sequencing of the Pseudomonas strain identified the gene coding for the major subunit of the amyloid fibril, termed fapC. FapC contains a thrice repeated motif that differs from those previously found in curli fimbrins and prion proteins. The lack of aromatic residues in the repeat shows that aromatic side chains are not needed for efficient amyloid formation. In contrast, glutamine and asparagine residues seem to play a major role in amyloid formation as these are highly conserved in curli, prion proteins and FapC. fapC is conserved in many Pseudomonas strains including the opportunistic pathogen P. aeruginosa and is situated in a conserved operon containing six genes, of which one encodes a fapC homologue. Heterologous expression of the fapA-F operon in Escherichia coli BL21(DE3) resulted in a highly aggregative phenotype, showing that the operon is involved in biofilm formation.

Cyclic di-GMP signaling regulates invasion by Ehrlichia chaffeensis of human monocytes

Cyclic di-GMP (c-di-GMP) is a bacterial second messenger produced by GGDEF domain-containing proteins. The genome of Ehrlichia chaffeensis, an obligatory intracellular bacterium that causes human monocytic ehrlichiosis, encodes a single protein that contains a GGDEF domain, called PleD. In this study, we investigated the effects of c-di-GMP signaling on E. chaffeensis infection of the human monocytic cell line THP-1. Recombinant E. chaffeensis PleD showed diguanylate cyclase activity as it generated c-di-GMP in vitro. Because c-di-GMP is not cell permeable, the c-di-GMP hydrophobic analog 2'-O-di(tert-butyldimethylsilyl)-c-di-GMP (CDGA) was used to examine intracellular c-di-GMP signaling. CDGA activity was first tested with Salmonella enterica serovar Typhimurium. CDGA inhibited well-defined c-di-GMP-regulated phenomena, including cellulose synthesis, clumping, and upregulation of csgD and adrA mRNA, indicating that CDGA acts as an antagonist in c-di-GMP signaling. [(32)P]c-di-GMP bound several E. chaffeensis native proteins and two E. chaffeensis recombinant I-site proteins, and this binding was blocked by CDGA. Although pretreatment of E. chaffeensis with CDGA did not reduce bacterial binding to THP-1 cells, bacterial internalization was reduced. CDGA facilitated protease-dependent degradation of particular, but not all, bacterial surface-exposed proteins, including TRP120, which is associated with bacterial internalization. Indeed, the serine protease HtrA was detected on the surface of E. chaffeensis, and TRP120 was degraded by treatment of E. chaffeensis with recombinant E. chaffeensis HtrA. Furthermore, anti-HtrA inhibited CDGA-induced TRP120 degradation. Our results suggest that E. chaffeensis invasion is regulated by c-di-GMP signaling, which stabilizes some bacterial surface-exposed proteins against proteases.

Unphosphorylated CsgD controls biofilm formation in Salmonella enterica serovar Typhimurium

The transcriptional regulator CsgD of Salmonella enterica serovar Typhimurium (S. Typhimurium) is a major regulator of biofilm formation required for the expression of csgBA, which encodes curli fimbriae, and adrA, coding for a diguanylate cyclase. CsgD is a response regulator with an N-terminal receiver domain with a conserved aspartate (D59) as a putative target site for phosphorylation and a C-terminal LuxR-like helix-turn-helix DNA binding motif, but the mechanisms of target gene activation remained unclear. To study the DNA-binding properties of CsgD we used electrophoretic mobility shift assays and DNase I footprint analysis to show that unphosphorylated CsgD-His(6) binds specifically to the csgBA and adrA promoter regions. In vitro transcription analysis revealed that CsgD-His(6) is crucial for the expression of csgBA and adrA. CsgD-His(6) is phosphorylated by acetyl phosphate in vitro, which decreases its DNA-binding properties. The functional impact of D59 in vivo was demonstrated as S. Typhimurium strains expressing modified CsgD protein (D59E and D59N) were dramatically reduced in biofilm formation due to decreased protein stability and DNA-binding properties in the case of D59E. In summary, our findings suggest that the response regulator CsgD functions in its unphosphorylated form under the conditions of biofilm formation investigated in this study.

Amino Acids Enhance Adaptive Behaviour ofPseudomonas Aeruginosain the Cystic Fibrosis Lung Environment

Sputum of cystic fibrosis (CF) patients is a nutrient-rich environment. Higher amino acid content of CF sputum compared to normal sputum plays a major role in the CF-specific phenotype of P. aeruginosa. Presence of amino acids in the sputum-like environment influenced P. aeruginosa quorum-sensing activity and the formation of an unknown exopolysaccharide in the biofilm. Lipopolysaccharides isolated from P. aeruginosa grown in the presence of amino acids enhanced the release of cytokine IL-8 by human kidney and lung epithelial cells. The results of this study provide additional evidence on the role of amino acids towards adaptation of P. aeruginosa to the CF lung environment.

Genes involved in Cronobacter sakazakii biofilm formation

Cronobacter spp. are opportunistic food-borne pathogens that can cause severe and sometimes lethal infections in neonates. In some outbreaks, the sources of infection were traced to contaminated powdered infant formula (PIF) or contaminated utensils used for PIF reconstitution. In this study, we investigated biofilm formation in Cronobacter sakazakii strain ES5. To investigate the genetic basis of biofilm formation in Cronobacter on abiotic surfaces, we screened a library of random transposon mutants of strain ES5 for reduced biofilm formation using a polystyrene microtiter assay. Genetic characterization of the mutants led to identification of genes that are associated with cellulose biosynthesis and flagellar structure and biosynthesis and genes involved in basic cellular processes and virulence, as well as several genes whose functions are currently unknown. In two of the mutants, hypothetical proteins ESA_00281 and ESA_00282 had a strong impact on flow cell biofilm architecture, and their contribution to biofilm formation was confirmed by genetic complementation. In addition, adhesion of selected biofilm formation mutants to Caco-2 intestinal epithelial cells was investigated. Our findings suggest that flagella and hypothetical proteins ESA_00281 and ESA_00282, but not cellulose, contribute to adhesion of Cronobacter to this biotic surface.

Biofilm formation by and multicellular behavior of Escherichia coli O55:H7, an atypical enteropathogenic strain

Enteropathogenic Escherichia coli (EPEC) is an important causal agent of diarrheal illness throughout the world. Nevertheless, researchers have only recently begun to explore its capacity to form biofilms. Strain O55:H7 (DMS9) is a clinical isolate belonging to the atypical EPEC (aEPEC) group, which displays a high degree of genetic relatedness to enterohemorrhagic E. coli. Strain DMS9 formed a robust biofilm on an abiotic surface at 26 degrees C, but not at 37 degrees C. It also formed a dense pellicle at the air-liquid interface and developed a red, rough, and dry (RDAR) morphotype on Congo red agar. Unlike a previously described E. coli O157:H7 strain, the aEPEC strain seems to express cellulose. Transposon mutagenesis was used to identify biofilm-deficient mutants. One of the mutants was inactivated in the csgFG genes, required for assembly and secretion of curli fimbriae, while a second mutant had a mutation in crl, a thermosensitive global regulator that modulates sigma(S) activity and downstream expression of curli and cellulose. The two mutants were deficient in their biofilm formation capabilities and did not form a pellicle at the air-liquid interface. Unlike in Salmonella, the csgFG mutant in aEPEC completely lost the RDAR phenotype, while the crl mutant displayed a unique RDAR "pizza"-like morphotype. Genetic complementation of the two mutants resulted in restoration of the wild-type phenotype. This report is the first to describe and analyze a multicellular behavior in aEPEC and support a major role for curli and the crl regulator in biofilm development at low temperatures corresponding to the nonmammalian host environment.

The transcriptional programme of Salmonella enterica serovar Typhimurium reveals a key role for tryptophan metabolism in biofilms

BACKGROUND

Biofilm formation enhances the capacity of pathogenic Salmonella bacteria to survive stresses that are commonly encountered within food processing and during host infection. The persistence of Salmonella within the food chain has become a major health concern, as biofilms can serve as a reservoir for the contamination of food products. While the molecular mechanisms required for the survival of bacteria on surfaces are not fully understood, transcriptional studies of other bacteria have demonstrated that biofilm growth triggers the expression of specific sets of genes, compared with planktonic cells. Until now, most gene expression studies of Salmonella have focused on the effect of infection-relevant stressors on virulence or the comparison of mutant and wild-type bacteria. However little is known about the physiological responses taking place inside a Salmonella biofilm.

RESULTS

We have determined the transcriptomic and proteomic profiles of biofilms of Salmonella enterica serovar Typhimurium. We discovered that 124 detectable proteins were differentially expressed in the biofilm compared with planktonic cells, and that 10% of the S. Typhimurium genome (433 genes) showed a 2-fold or more change in the biofilm compared with planktonic cells. The genes that were significantly up-regulated implicated certain cellular processes in biofilm development including amino acid metabolism, cell motility, global regulation and tolerance to stress. We found that the most highly down-regulated genes in the biofilm were located on Salmonella Pathogenicity Island 2 (SPI2), and that a functional SPI2 secretion system regulator (ssrA) was required for S. Typhimurium biofilm formation. We identified STM0341 as a gene of unknown function that was needed for biofilm growth. Genes involved in tryptophan (trp) biosynthesis and transport were up-regulated in the biofilm. Deletion of trpE led to decreased bacterial attachment and this biofilm defect was restored by exogenous tryptophan or indole.

CONCLUSIONS

Biofilm growth of S. Typhimurium causes distinct changes in gene and protein expression. Our results show that aromatic amino acids make an important contribution to biofilm formation and reveal a link between SPI2 expression and surface-associated growth in S. Typhimurium.

Survival potential of wild type cellulose deficient Salmonella from the feed industry

BACKGROUND

Biofilm has been shown to be one way for Salmonella to persist in the feed factory environment. Matrix components, such as fimbriae and cellulose, have been suggested to play an important role in the survival of Salmonella in the environment. Multicellular behaviour by Salmonella is often categorized according to colony morphology into rdar (red, dry and rough) expressing curli fimbriae and cellulose, bdar (brown, dry and rough) expressing curli fimbriae and pdar (pink, dry and rough) expressing cellulose. The aim of the study was to look into the distribution of morphotypes among feed and fish meal factory strains of Salmonella, with emphasis on potential differences between morphotypes with regards to survival in the feed factory environment.

RESULTS

When screening a total of 148 Salmonella ser. Agona, Salmonella ser. Montevideo, Salmonella ser. Senftenberg and Salmonella ser. Typhimurium strains of feed factory, human clinical and reference collection origin, as many as 99% were able to express rough morphology (rdar or bdar). The dominant morphotype was rdar (74%), however as many as 55% of Salmonella ser. Agona and 19% of Salmonella ser. Senftenberg displayed the bdar morphology. Inconsistency in Calcofluor binding, indicating expression of cellulose, was found among 25% of all the strains tested, however Salmonella ser. Agona showed to be highly consistent in Calcofluor binding (98%). In biofilm, Salmonella ser. Agona strains with bdar mophology was found to be equally tolerant to disinfection treatment as strains with rdar morphotype. However, rdar morphology appeared to be favourable in long term survival in biofilm in a very dry environment. Chemical analysis showed no major differences in polysaccharide content between bdar and rdar strains. Our results indicate that cellulose is not a major component of the Salmonella biofilm matrix.

CONCLUSION

The bdar morphotype is common among Salmonella ser. Agona strains isolated from the factory environment. The rdar and the bdar strains were found to be equally tolerant to disinfectants, while the rdar strain was found to be more tolerant to long-term desiccation and nutrient depletion in biofilm than the bdar strain. Cellulose does not appear to be a major component of the Salmonella biofilm matrix.

Complex regulatory network encompassing the Csr, c-di-GMP and motility systems of Salmonella Typhimurium

Bacterial survival depends on the ability to switch between sessile and motile lifestyles in response to changing environmental conditions. In many species, this switch is governed by (3'-5')-cyclic-diguanosine monophosphate (c-di-GMP), a signalling molecule, which is metabolized by proteins containing GGDEF and/or EAL domains. Salmonella Typhimurium contains 20 such proteins. Here, we show that the RNA-binding protein CsrA regulates the expression of eight genes encoding GGDEF, GGDEF-EAL and EAL domain proteins. CsrA bound directly to the mRNA leaders of five of these genes, suggesting that it may regulate these genes post-transcriptionally. The c-di-GMP-specific phosphodiesterase STM3611, which reciprocally controls flagella function and production of biofilm matrix components, was regulated by CsrA binding to the mRNA, but was also indirectly regulated by CsrA through the FlhDC/FliA flagella cascade and STM1344. STM1344 is an unconventional (c-di-GMP-inactive) EAL domain protein, recently identified as a negative regulator of flagella gene expression. Here, we demonstrate that CsrA directly downregulates expression of STM1344, which in turn regulates STM3611 through fliA and thus reciprocally controls motility and biofilm factors. Altogether, our data reveal that the concerted and complex regulation of several genes encoding GGDEF/EAL domain proteins allows CsrA to control the motility-sessility switch in S. Typhimurium at multiple levels.

Bistable expression of CsgD in biofilm development of Salmonella enterica serovar typhimurium

Bacterial persistence in the environment and in the infected host is often aided by the formation of exopolymer-enclosed communities known as biofilms. Heterogeneous gene expression takes place in microcompartments formed within the complex biofilm structure. This study describes cell differentiation within an isogenic bacterial cell population based on the example of biofilm formation by Salmonella enterica serovar Typhimurium. We analyzed the expression of the major biofilm regulator CsgD at the single-cell level with a chromosomal CsgD-green fluorescent protein (GFP) translational fusion. In individual cells, CsgD-GFP expression is mostly found in the cytoplasm. Quantitative expression analysis and results from three different models of S. Typhimurium biofilms demonstrated that CsgD is expressed in a bistable manner during biofilm development. CsgD expression is, however, monomodal when CsgD is expressed in larger amounts due to a promoter mutation or elevated levels of the secondary signaling molecule c-di-GMP. High levels of CsgD-GFP are associated with cellular aggregation in all three biofilm models. Furthermore, the subpopulation of cells expressing large amounts of CsgD is engaged in cellulose production during red, dry, and rough (rdar) morphotype development and in microcolony formation under conditions of continuous flow. Consequently, bistability at the level of CsgD expression leads to a corresponding pattern of task distribution in S. Typhimurium biofilms.

Determinants for the activation and autoinhibition of the diguanylate cyclase response regulator WspR

The bacterial second messenger bis-(3'-5')-cyclic dimeric guanosine monophosphate (c-di-GMP) controls secretion, cell adhesion, and motility, leading to biofilm formation and increased cytotoxicity. Diguanylate cyclases containing GGDEF and phosphodiesterases containing EAL or HD-GYP domains have been identified as the enzymes controlling cellular c-di-GMP levels, yet less is known regarding the molecular mechanisms governing regulation and signaling specificity. We recently determined a product-inhibition pathway for the diguanylate cyclase response regulator WspR from Pseudomonas, a potent molecular switch that controls biofilm formation. In WspR, catalytic activity is modulated by a helical stalk motif that connects its phospho-receiver and GGDEF domains. The stalks facilitate the formation of distinct oligomeric states that contribute to both activation and autoinhibition. Here, we provide novel insights into the regulation of diguanylate cyclase activity in WspR based on the crystal structures of full-length WspR, the isolated GGDEF domain, and an artificially dimerized catalytic domain. The structures highlight that inhibition is achieved by restricting the mobility of rigid GGDEF domains, mediated by c-di-GMP binding to an inhibitory site at the GGDEF domain. Kinetic measurements and biochemical characterization corroborate a model in which the activation of WspR requires the formation of a tetrameric species. Tetramerization occurs spontaneously at high protein concentration or upon addition of the phosphomimetic compound beryllium fluoride. Our analyses elucidate common and WspR-specific mechanisms for the fine-tuning of diguanylate cyclase activity.

Influence of culture conditions and medium composition on the production of cellulose by shiga toxin-producing Escherichia coli cells

Culture conditions favoring cellulose production by Shiga toxin-producing Escherichia coli included a 28 degrees C incubation temperature, an aerobic atmosphere, and the presence of 2% ethanol in Luria-Bertani no-salt agar with pH 6.0 and a water activity of 0.99. These findings will assist in formulating microbiological media useful for cellulose and biofilm research.

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.

Long-term survival of Salmonella enterica serovar Typhimurium reveals an infectious state that is underrepresented on laboratory media containing bile salts

Cells in desiccated Salmonella enterica serovar Typhimurium rdar (red, dry, and rough) morphotype colonies were examined for culturability and infectivity after 30 months. Culturability decreased only 10-fold; however, cells were underrepresented on Salmonella selective media containing bile salts. These cells were mildly attenuated compared to the infectivity of freshly grown cells but still able to cause systemic infections in mice.

Biofilm forming abilities of Salmonella are correlated with persistence in fish meal- and feed factories

BACKGROUND

Feed contaminated with Salmonella spp. constitutes a risk of Salmonella infections in animals, and subsequently in the consumers of animal products. Salmonella are occasionally isolated from the feed factory environment and some clones of Salmonella persist in the factory environment for several years. One hypothesis is that biofilm formation facilitates persistence by protecting bacteria against environmental stress, e.g. disinfection. The aim of this study was to investigate the biofilm forming potential of Salmonella strains from feed- and fishmeal factories. The study included 111 Salmonella strains isolated from Norwegian feed and fish meal factories in the period 1991-2006 of serovar Agona, serovar Montevideo, serovar Senftenberg and serovar Typhimurium.

RESULTS

Significant differences were found between serovars regarding the abilities to form biofilm on polystyrene (microtiter plate assay) and in the air-liquid interface of nutrient broth (pellicle assay). Strains of serovar Agona and serovar Montevideo were good biofilm producers. In Norwegian factories, clones of these serovars have been observed to persist for several years. Most serovar Senftenberg clones appear to persist for a shorter period, and strains of this serovar were medium biofilm producers in our test systems. Strains of the serovar Typhimurium were relatively poor biofilm producers. Salmonella ser. Typhimurium clones have not been observed to persist even though this serovar is resident in Norwegian wild life. When classifying strains according to persistence or presumed non-persistence, persistent strains produced more biofilm than presumed non-persisting strains.

CONCLUSION

The results indicate a correlation between persistence and biofilm formation which suggests that biofilm forming ability may be an important factor for persistence of Salmonella in the factory environment.

Regulation of c-di-GMP metabolism in biofilms

Cyclic (5 to 3 )-diguanosine monophosphate (c-di-GMP) is a small molecule that regulates the transition between the sessile and motile lifestyle, an integrative part of biofilm formation and other multicellular behavior, in many bacteria. The recognition of c-di-GMP as a novel secondary messenger soon raised the question about the specificity of the signaling system, as individual bacterial genomes frequently encode numerous c-di-GMP metabolizing proteins. Recent work has demonstrated that several global regulators concertedly modify the expression of selected panels of c-di-GMP metabolizing proteins, which act on targets with physiological functions. Within complex feed-forward arrangements, the global regulators commonly combine the control of c-di-GMP metabolism with the direct regulation of proteins with functions in motility or biofilm formation, leading to precise and fine-tuned output responses that determine bacterial behavior. c-di-GMP metabolizing proteins are also controlled at the post-translational level by mechanisms including phosphorylation, localization, protein-protein interactions or protein stability. A detailed understanding of such complex regulatory mechanisms will not only help to explain the specificity in c-di-GMP signaling systems, but will also be necessary to understand the high phenotypic diversity within bacterial biofilms at the single cell level.

Percolation and survival of Escherichia coli O157:H7 and Salmonella enterica serovar Typhimurium in soil amended with contaminated dairy manure or slurry

The effect of cattle manure and slurry application on percolation and survival of Escherichia coli O157:H7 and Salmonella enterica serovar Typhimurium was investigated for different soil depths after the addition of water. Four treatments were chosen for the first set of experiments: (i) addition of inoculated farmyard manure on the soil surface, (ii) mixing of inoculated farmyard manure with the top 10 cm of soil, (iii) addition of inoculated slurry on the soil surface, and (iv) injection of inoculated slurry into the top 10 cm of the soil. Homogeneity of water distribution in the soil profile was confirmed by a nondestructive nuclear magnetic resonance method. Survival data were fitted to a modified logistic model, and estimated survival times were compared. In the second set of experiments, pathogen-inoculated farmyard manure or slurry was applied to soil columns with 1-month-old lettuce plants. More pathogen cells percolated to greater depths after slurry than after manure application. Survival of E. coli O157:H7 was significantly longer in soil with slurry than in that with manure, while survival of Salmonella serovar Typhimurium was equally high with manure and slurry. The densities of the pathogens were not different in the rhizosphere compared to the bulk soil with manure, while the densities were higher by 0.88 +/- 0.11 and 0.71 +/- 0.23 log CFU per g (dry weight), respectively, in the rhizosphere than in bulk soil after slurry application. Our results suggest that surface application of manure may decrease the risk of contamination of groundwater and lettuce roots compared to injection of slurry.

A role for the EAL-like protein STM1344 in regulation of CsgD expression and motility in Salmonella enterica serovar Typhimurium

The bacterial second messenger cyclic di-GMP (c-di-GMP) regulates the transition between sessility and motility. In Salmonella enterica serovar Typhimurium, the expression of CsgD, the regulator of multicellular rdar morphotype behavior, is a major target of c-di-GMP signaling. CsgD expression is positively regulated by at least two diguanylate cyclases, GGDEF domain proteins, and negatively regulated by at least four phosphodiesterases, EAL domain proteins. Here, we show that in contrast to EAL domain proteins acting as phosphodiesterases, the EAL-like protein STM1344 regulated CsgD expression positively and motility negatively. STM1344, however, did not have a role in c-di-GMP turnover and also did not bind the nucleotide. STM1344 acted upstream of the phosphodiesterases STM1703 and STM3611, previously identified to participate in CsgD downregulation, where it repressed their expression. Consequently, although STM1344 has not retained a direct role in c-di-GMP metabolism, it still participates in the regulation of c-di-GMP turnover and has a role in the transition between sessility and motility.

Cross-talk mechanisms in biofilm formation and responses to environmental and physiological stress in Escherichia coli

Switching from single-cell (planktonic) to biofilm growth (and vice versa) is regulated by a variety of environmental and physiological cues. Signals leading to activation of stress responses often lead to biofilm formation which, in turn, can trigger induction of stress response mechanisms, suggesting direct cross-talk between the two cellular processes. Regulatory mechanisms of this process include two-component regulatory systems, master regulators such as the rpoS gene and signal molecules such as cyclic-di-GMP, in a tight and complex interplay.

Transfer of Salmonella enterica serovar Typhimurium from contaminated irrigation water to parsley is dependent on curli and cellulose, the biofilm matrix components

Enteric pathogens can contaminate fresh produce, and this contaminated produce can be a significant potential source of human illness. The objective of this study was to determine a possible mode of transfer of Salmonella Typhimurium from contaminated irrigation water to mature parsley plants and to investigate the role of bacterial cellulose and curli. Parsley plants were drip irrigated with water containing green fluorescent protein-labeled Salmonella Typhimurium. Stems and leaves were harvested 1 day after the third irrigation and examined for the presence of Salmonella Typhimurium. Three weeks after harvesting, the presence of Salmonella was again confirmed in the regrown plants. During this period, bacterial numbers on leaves declined from 4.1 (+/- 0.3) to 2.3 (+/- 0.1) log CFU g(-1) (P < 0.05). Numbers in the soil were constant (5 log CFU g(-1)). Results demonstrated the ability of Salmonella Typhimurium to transfer from irrigation water to the edible parts of the plants. Confocal laser scanning microscopic images revealed that Salmonella Typhimurium formed aggregates at a depth of 8 to 32 microm beneath the leaf surface. Penetration might be achieved through the roots or the phyllosphere. The importance of the bacterial cellulose and curli was determined by comparing the wild-type strain with its mutants, which lack the ability to synthesize cellulose and curli. Counts of the double mutant were 2-log higher in the soil but 1-log lower in the leaves (P < 0.05). Deletion of the agfBA gene (for curli) was more effective than deletion of bcsA (for cellulose). Thus, curli and cellulose play a role in the transfer or survival of Salmonella Typhimurium in the plant, as they do for plant pathogens.

Characterization of cellulose production in Escherichia coli Nissle 1917 and its biological consequences

Bacterial species of the Enterobacteriaceae family produce cellulose and curli fimbriae as extracellular matrix components, and their synthesis is positively regulated by the transcriptional activator CsgD. In this group of bacteria, cellulose biosynthesis is commonly regulated by CsgD via the GGDEF domain protein AdrA, a diguanylate cyclase that produces cyclic-diguanylic acid (c-di-GMP), an allosteric activator of cellulose synthase. In the probiotic Escherichia coli strain Nissle 1917 and its recent clonal isolates, CsgD activates the production of curli fimbriae at 28 degrees C, but neither CsgD nor AdrA is required for the c-di-GMP-dependent biosynthesis of cellulose at 28 degrees C and 37 degrees C. In these strains, the GGDEF domain protein YedQ, a diguanylate cyclase that activates cellulose biosynthesis in certain E. coli strains, is not required for cellulose biosynthesis and it has in fact evolved into a novel protein. Cellulose production in Nissle 1917 is required for adhesion of bacteria to the gastrointestinal epithelial cell line HT-29, to the mouse epithelium in vivo, and for enhanced cytokine production. The role of cellulose in this strain is in contrast to the role of cellulose in the commensal strain E. coli TOB1. Consequently, the role of cellulose in bacterial-host interaction is dependent on the E. coli strain background.

Quantitative determination of cyclic diguanosine monophosphate concentrations in nucleotide extracts of bacteria by matrix-assisted laser desorption/ionization-time-of-flight mass spectrometry

The physiological response to small molecules (secondary messengers) is the outcome of a delicate equilibrium between biosynthesis and degradation of the signal. Cyclic diguanosine monophosphate (c-di-GMP) is a novel secondary messenger present in many bacteria. It has a complex cellular metabolism whereby usually more than one enzyme synthesizing and degrading c-di-GMP is encoded by a bacterial genome. To assess the in vivo conditions of c-di-GMP signaling, we developed a high-performance liquid chromatography (HPLC)-mass spectrometry-based method to detect c-di-GMP with high sensitivity and to quantify the c-di-GMP concentration in the bacterial cell as described here in detail. We successfully used the methodology to determine and compare the c-di-GMP concentrations in bacterial species such as Salmonella typhimurium, Escherichia coli, Pseudomonas aeruginosa, and Vibrio cholerae. We describe the use of the methodology to assess the change in c-di-GMP concentration during the growth phase and the contribution of a point mutation in S. typhimurium to the overall cellular c-di-GMP concentration.

Two dissimilar N-acyl-homoserine lactone acylases of Pseudomonas syringae influence colony and biofilm morphology

Plant aerial surfaces comprise a complex habitat for microorganisms, and many plant-associated bacteria, such as the pathogen Pseudomonas syringae, exhibit density-dependent survival on leaves by utilizing quorum sensing (QS). QS is often mediated by diffusible signals called N-acyl-homoserine lactones (AHLs), and P. syringae utilizes N-3-oxo-hexanoyl-dl-homoserine lactone (3OC6HSL) to control traits influencing epiphytic fitness and virulence. The P. syringae pathovar syringae B728a genome sequence revealed two putative AHL acylases, termed HacA (Psyr_1971) and HacB (Psyr_4858), which are N-terminal nucleophile hydrolases that inactivate AHLs by cleaving their amide bonds. HacA is a secreted AHL acylase that degrades only long-chain (C > or = 8) AHLs, while HacB is not secreted and degrades all tested AHLs. Targeted disruptions of hacA, hacB, and hacA and hacB together do not alter endogenous 3OC6HSL levels under the tested conditions. Surprisingly, targeted disruptions of hacA alone and hacA and hacB together confer complementable phenotypes that are very similar to autoaggregative phenotypes seen in other species. While AHL acylases might enable P. syringae B728a to degrade signals of competing species and block expression of their QS-dependent traits, these enzymes also play fundamental roles in biofilm formation.

Cellulose biosynthesis by the beta-proteobacterium, Chromobacterium violaceum

The Chromobacterium violaceum ATCC 12472 genome was sequenced by The Brazilian National Genome Project Consortium. Previous annotation reported the presence of cellulose biosynthesis genes in that genome. Analysis of these genes showed that, as observed in other bacteria, they are organized in two operons. In the present work, experimental evidences of the presence of cellulose in the extracellular matrix of the biofilm produced by C. violaceum in static cultures are shown. Biofilm samples were enzymatically digested by cellulase, releasing glucose units, suggesting the presence of cellulose as an extracellular matrix component. Fluorescence microscopy observations showed that C. violaceum produces a cellulase-sensitive extracellular matrix composed of fibers able to bind calcofluor. C. violaceum grows on medium containing Congo red, forming brown-red colonies. Together, these results suggest that cellulase-susceptible matrix material is cellulose. Scanning electronic microscopy analysis showed that the extracellular matrix exhibited a network of microfibrils, typical of bacterial cellulose. Although cellulose production is widely distributed between several bacterial species, including at least the groups of Gram-negative proteobacteria alpha and gamma, we give for the first time experimental evidence for cellulose production in beta-proteobacteria.

Cellulose modulates biofilm formation by counteracting curli-mediated colonization of solid surfaces in Escherichia coli

In enterobacteria, the CsgD protein activates production of two extracellular structures: thin aggregative fimbriae (curli) and cellulose. While curli fibres promote biofilm formation and cell aggregation, the evidence for a direct role of cellulose as an additional determinant for biofilm formation is not as straightforward. The MG1655 laboratory strain of Escherichia coli only produces limited amounts of curli and cellulose; however, ectopic csgD expression results in strong stimulation of curli and cellulose production. We show that, in a csgD-overexpressing derivative of MG1655, cellulose production negatively affects curli-mediated surface adhesion and cell aggregation, thus acting as a negative determinant for biofilm formation. Consistent with this observation, deletion of the bcsA gene, necessary for cellulose production, resulted in a significant increase in curli-dependent adhesion. We found that cellulose production increased tolerance to desiccation, suggesting that the function of cellulose might be related to resistance to environmental stresses rather than to biofilm formation. Production of the curli/cellulose network in enterobacteria typically takes place at low growth temperature (<32 degrees C), but not at 37 degrees C. We show that CsgD overexpression can overcome temperature-dependent control of the curli-encoding csgBA operon, but not of the cellulose-related adrA gene, suggesting very tight temperature control of cellulose production in E. coli MG1655.

Identification of a bile-induced exopolysaccharide required for Salmonella biofilm formation on gallstone surfaces

Salmonella enterica serovar Typhi can establish a chronic, asymptomatic infection of the human gallbladder, suggesting that this bacterium utilizes novel mechanisms to mediate enhanced colonization and persistence in a bile-rich environment. Gallstones are one of the most important risk factors for developing carriage, and we have previously demonstrated that salmonellae form biofilms on human gallstones in vitro. Thus, we hypothesize that bile-induced biofilms on gallstone surfaces promote gallbladder colonization and maintenance of the carrier state. A colanic acid/cellulose S. enterica serovar Typhimurium double mutant formed a mature biofilm on gallstones in a test tube assay and in a new, gallstone-independent assay using cholesterol-coated Eppendorf tubes. These data suggest the presence of an unidentified exopolysaccharide necessary for mature biofilm development and demonstrate specific binding affinity between salmonellae and cholesterol. Our experiments indicate that the Salmonella O-antigen capsule (yihU-yshA and yihV-yihW) is a crucial determinant in gallstone and cholesterol biofilms but that expression of this exopolysaccharide is not necessary for binding to glass or plastic. Real-time PCR revealed that growth in bile resulted in upregulation of the O-antigen capsule-encoding operon in an agfD-independent manner. Thus, the O-antigen capsule genes are bile induced, and the capsule produced by the enzymes of this operon is specifically required for biofilm formation on cholesterol gallstones. These studies provide new therapeutic targets for preventing asymptomatic serovar Typhi gallbladder carriage.

Application of the rpoS gene for the detection of Vibrio anguillarum in flounder and prawn by polymerase chain reaction

Vibrio anguillarum, an opportunistic fish pathogen, is the main species responsible for vibriosis, a disease that affects feral and farmed fish and shellfish, and causes considerable economic losses in marine aquaculture. In this study, we used polymerase chain reaction (PCR) to detect V. anguillarum. PCR specificity was evaluated by amplifying the rpoS gene, a general stress regulator, in six strains of V. anguillarum and 36 other bacterial species. PCR amplified a species-specific fragment (689 bp) from V. anguillarum. Furthermore, the PCR assay was sensitive enough to detect rpoS expression from 3 pg of genomic DNA, or from six colony-forming units (CFU) mL(-1) of cultured V. anguillarum. However, the assay was less sensitive when genomic DNA from the infected flounder and prawn was used (limit of detection, 50 ng and 10 ng g(-1) tissue, respectively). These data demonstrate that PCR amplification of the rpoS gene is a sensitive and species-specific method to detect V. anguillarum in practical situations.

Amyloids: friend or foe?

Amyloidogenesis is the aggregation of soluble proteins into structurally conserved fibers. Amyloid fibers are distinguished by their resistance to proteinase K, tinctorial properties and beta-sheet-rich secondary structure. Amyloid formation is a hallmark of many human diseases including Alzheimer's, Huntington's and the prion diseases. Therefore, understanding amyloidogenesis will provide insights into the development of therapeutics that target these debilitating diseases. A new class of ;functional' amyloids promises a unique glimpse at how nature has harnessed the amyloid fiber to accomplish important physiological tasks. Functional amyloids are produced by organisms spanning all aspects of cellular life. Herein we review amyloidogenesis, with special attention focused on the similarities and differences between the best characterized disease-associated amyloidogenic protein amyloid-beta and the formation of several functional amyloids. The implications of studying functional amyloidogenesis and the strategies organisms employ to limit exposure to toxic intermediates will also be discussed.

Phosphorylation-independent regulation of the diguanylate cyclase WspR

Environmental signals that trigger bacterial pathogenesis and biofilm formation are mediated by changes in the level of cyclic dimeric guanosine monophosphate (c-di-GMP), a unique eubacterial second messenger. Tight regulation of cellular c-di-GMP concentration is governed by diguanylate cyclases and phosphodiesterases, which are responsible for its production and degradation, respectively. Here, we present the crystal structure of the diguanylate cyclase WspR, a conserved GGDEF domain-containing response regulator in Gram-negative bacteria, bound to c-di-GMP at an inhibitory site. Biochemical analyses revealed that feedback regulation involves the formation of at least three distinct oligomeric states. By switching from an active to a product-inhibited dimer via a tetrameric assembly, WspR utilizes a novel mechanism for modulation of its activity through oligomerization. Moreover, our data suggest that these enzymes can be activated by phosphodiesterases. Thus, in addition to the canonical pathways via phosphorylation of the regulatory domains, both product and enzyme concentration contribute to the coordination of c-di-GMP signaling. A structural comparison reveals resemblance of the oligomeric states to assemblies of GAF domains, widely used regulatory domains in signaling molecules conserved from archaea to mammals, suggesting a similar mechanism of regulation.

Bacteria can switch from a single-cell, free-floating behavioral mode to a community life-form via colonization of surfaces and the secretion of an extracellular matrix. This process, called biofilm formation, has been attributed to a majority of chronic infections, including the lungs, as occurs in patients with cystic fibrosis. Recently, a small intracellular signaling molecule, the nucleotide cyclic dimeric guanosine monophosphate (c-di-GMP), and enzymes for its production and degradation have been discovered that relay environmental cues to changes in secretion, cell adhesion and ultimately, biofilm formation and virulence. We have studied the molecular mechanism and mode of regulation of WspR, an enzyme from Pseudomonas and related pathogenic bacteria responsible for the generation of c-di-GMP and biofilm formation. On the basis of its crystal structure and functional assays, we elucidated a sophisticated regulatory mechanism in WspR that is controlled by feedback inhibition mediated by c-di-GMP. We hypothesize that WspR is primed for the (re)activation by enzymatic degradation of the inhibitory nucleotide. In addition, we identified mutations at the inhibitory site of WspR in a subset of bacteria that are frequently found in cystic fibrosis patients, suggesting that altered c-di-GMP signaling, mediated by modified WspR, may contribute to the pathogenicity of these strains. Furthermore, we present a structural comparison with GAF domains, which are widely used conserved regulatory signaling domains, suggesting a similar mechanism of regulation.

We present a model for the regulation of a conserved diguanyate cyclase fromPseudomonas that is responsible for cyclic di-GMP production and biofilm formation, providing insight into the molecular mechani7sm controlling cell signaling and virulence.

Amino acids important for DNA recognition by the response regulator OmpR

Response regulators undergo regulated phosphorylation and dephosphorylation at conserved aspartic acid residues in bacterial signal transduction systems. OmpR is a winged helix-turnhelix DNA-binding protein that functions as a global regulator in bacteria and is also important in pathogenesis. A detailed mechanistic picture of how OmpR binds to DNA and activates transcription is lacking. We used NMR spectroscopy to solve the solution structure of the C-terminal domain of OmpR (OmpR(C)) and to analyze the chemical shift changes that occur upon DNA binding. There is little overlap in the interaction surface with residues of PhoB that were reportedly involved in protein/protein interactions in its head-to-tail dimer. Multiple factors account for the lack of overlap. One is that the spacing between the OmpR half-sites is shorter than observed with PhoB, requiring the arrangement of the two OmpR molecules to be different from that of the PhoB dimer on DNA. A second is the demonstration herein that OmpR can bind to its high affinity site as a monomer. As a result, OmpR(C) appears to be capable of adopting alternative orientations depending on the precise base composition of the binding site, which also contributes to the lack of overlap. In the presence of DNA, chemical shift changes occur in OmpR in the recognition alpha-helix 3, the loop between beta-strand 4 and alpha-helix 1, and the loop between beta-strands 5 and 6. DNA contact residues are Val(203) (T), Arg(207) (G), and Arg(209) (phosphate backbone). Our results suggest that OmpR binds to DNA as a monomer and then forms a symmetric or asymmetric dimer, depending on the binding site. We propose that during activation OmpR binds to DNA and undergoes a conformational change that promotes phosphorylation of the N-terminal receiver domain, the receiver domains dimerize, and then the second monomer binds to DNA. The flexible linker of OmpR enables the second monomer to bind in multiple orientations (head-to-tail and head-to-head), depending on the specific DNA contacts.

Evolutionary loss of the rdar morphotype in Salmonella as a result of high mutation rates during laboratory passage

Rapid evolution of microbes under laboratory conditions can lead to domestication of environmental or clinical strains. In this work, we show that domestication due to laboratory passage in rich medium is extremely rapid. Passaging of wild-type Salmonella in rich medium led to diversification of genotypes contributing to the loss of a spatial phenotype, called the rdar morphotype, within days. Gene expression analysis of the rdar regulatory network demonstrated that mutations were primarily within rpoS, indicating that the selection pressure for scavenging during stationary phase had the secondary effect of impairing this highly conserved phenotype. If stationary phase was omitted from the experiment, radiation of genotypes and loss of the rdar morphotype was also demonstrated, but due to mutations within the cellulose biosynthesis pathway and also in an unknown upstream regulator. Thus regardless of the selection pressure, rapid regulatory changes can be observed on laboratory timescales. The speed of accumulation of rpoS mutations during daily passaging could not be explained by measured fitness and mutation rates. A model of mutation accumulation suggests that to generate the observed accumulation of sigma 38 mutations, this locus must experience a mutation rate of approximately 10(-4) mutations/gene/generation. Sequencing and gene expression of population isolates indicated that there were a wide variety of sigma 38 phenotypes within each population. This suggests that the rpoS locus is highly mutable by an unknown pathway, and that these mutations accumulate rapidly under common laboratory conditions.

Aggregation via the red, dry, and rough morphotype is not a virulence adaptation in Salmonella enterica serovar Typhimurium

The Salmonella rdar (red, dry, and rough) morphotype is an aggregative and resistant physiology that has been linked to survival in nutrient-limited environments. Growth of Salmonella enterica serovar Typhimurium was analyzed in a variety of nutrient-limiting conditions to determine whether aggregation would occur at low cell densities and whether the rdar morphotype was involved in this process. The resulting cultures consisted of two populations of cells, aggregated and nonaggregated, with the aggregated cells preferentially displaying rdar morphotype gene expression. The two groups of cells could be separated based on the principle that aggregated cells were producing greater amounts of thin aggregative fimbriae (Tafi or curli). In addition, the aggregated cells retained some physiological characteristics of the rdar morphotype, such as increased resistance to sodium hypochlorite. Competitive infection experiments in mice showed that nonaggregative DeltaagfA cells outcompeted rdar-positive wild-type cells in all tissues analyzed, indicating that aggregation via the rdar morphotype was not a virulence adaptation in Salmonella enterica serovar Typhimurium. Furthermore, in vivo imaging experiments showed that Tafi genes were not expressed during infection but were expressed once Salmonella was passed out of the mice into the feces. We hypothesize that the primary role of the rdar morphotype is to enhance Salmonella survival outside the host, thereby aiding in transmission.

Amyloid-like adhesins produced by floc-forming and filamentous bacteria in activated sludge

Amyloid proteins (fimbriae or other microbial surface-associated structures) are expressed by many types of bacteria, not yet identified, in biofilms from various habitats, where they likely are of key importance to biofilm formation and biofilm properties. As these amyloids are potentially of great importance to the floc properties in activated sludge wastewater treatment plants (WWTP), the abundance of amyloid adhesins in activated sludge flocs from different WWTP and the identity of bacteria producing these were investigated. Amyloid adhesins were quantified using a combination of conformationally specific antibodies targeting amyloid fibrils, propidium iodide to target all fixed bacterial cells, confocal laser scanning microscopy, and digital image analysis. The biovolume fraction containing amyloid adhesins ranged from 10 to 40% in activated sludge from 10 different WWTP. The identity of bacteria producing amyloid adhesins was determined using fluorescence in situ hybridization with oligonucleotide probes in combination with antibodies or thioflavin T staining. Among the microcolony-forming bacteria, amyloids were primarily detected among Alpha- and Betaproteobacteria and Actinobacteria. A more detailed analysis revealed that many denitrifiers (from Thauera, Azoarcus, Zoogloea, and Aquaspirillum-related organisms) and Actinobacteria-related polyphosphate-accumulating organisms most likely produced amyloid adhesins, whereas nitrifiers did not. Many filamentous bacteria also expressed amyloid adhesins, including several Alphaproteobacteria (e.g., Meganema perideroedes), some Betaproteobacteria (e.g., Aquaspirillum-related filaments), Gammaproteobacteria (Thiothrix), Bacteroidetes, Chloroflexi (e.g., Eikelboom type 1851), and some foam-forming Actinobacteria (e.g., Gordonia amarae). The results show that amyloid adhesins were an abundant component of activated sludge extracellular polymeric substances and seem to have unexpected, divers functions.

Biofilm formation in field strains of Salmonella enterica serovar Typhimurium: identification of a new colony morphology type and the role of SGI1 in biofilm formation

In this study we examined the extent of biofilm formation in field strains of Salmonella enterica serovar Typhimurium (S. Typhimurium), an important foodborne pathogen. Ninety-four field strains of S. Typhimurium were tested for their ability to form biofilm and components contributing to its formation. Most S. Typhimurium strains were highly capable of biofilm formation except for strains of phage type DT2 originating from pigeons. The most efficient biofilm forming strains were those of phage type DT104 positive for Salmonella genomic island 1 (SGI1). A comparison of SGI1 positive and negative strains indicated that the increased biofilm formation of SGI1 positive strains was associated with the presence of this genomic island. Finally, in five strains we found an alternative strategy of biofilm formation independent of curli fimbriae and cellulose production but solely dependent on an overproduction of capsular polysaccharide. Due to a mucoid and brown appearance on Congo Red agar we designated these strains as belonging to the SBAM (smooth brown and mucoid) morphotype.

Attachment and biofilm formation by various serotypes of Salmonella as influenced by cellulose production and thin aggregative fimbriae biosynthesis

This study was undertaken to quantify thin aggregative fimbriae and cellulose produced by Salmonella and to evaluate their roles in attachment and biofilm formation on polystyrene and glass surfaces. Thin aggregative fimbriae and cellulose produced by four wild-type and two pairs of Salmonella, representing four different colony morphotypes (rdar: red, dry, and rough; pdar: pink, dry, and rough; bdar: brown, dry, and rough; and saw: smooth and white), were quantified. The ability of the Salmonella cells to attach and form biofilms on the selected surfaces was evaluated in Luria-Bertani (LB) broth with or without salt (0.5%) or glucose (2%) at 28 degrees C during a 7-day period. The cells expressing the rdar or pdar colony morphotypes produced significantly greater amounts of thin aggregative fimbriae and cellulose on LB no salt agar, respectively. The cells expressing the rdar colony morphotype attached in higher numbers and formed more biofilm than did the cells expressing the pdar colony morphotype. The members of the pairs expressing the bdar colony morphotype attached more efficiently and formed more biofilm on the tested surfaces than did their counterparts expressing the saw colony morphotype. These results indicated that thin aggregative fimbriae impart attachment ability to Salmonella and, upon coexpression with cellulose, enhance biofilm formation on certain abiotic surfaces. The knowledge acquired in the study may help develop better cleaning strategies for food processing equipment.

Thinking about Bacillus subtilis as a multicellular organism

Initial attempts to use colony morphogenesis as a tool to investigate bacterial multicellularity were limited by the fact that laboratory strains often have lost many of their developmental properties. Recent advances in elucidating the molecular mechanisms underlying colony morphogenesis have been made possible through the use of undomesticated strains. In particular, Bacillus subtilis has proven to be a remarkable model system to study colony morphogenesis because of its well-characterized developmental features. Genetic screens that analyze mutants defective in colony morphology have led to the discovery of an intricate regulatory network that controls the production of an extracellular matrix. This matrix is essential for the development of complex colony architecture characterized by aerial projections that serve as preferential sites for sporulation. While much progress has been made, the challenge for future studies will be to determine the underlying mechanisms that regulate development such that differentiation occurs in a spatially and temporally organized manner.

Biofilm formation-gene expression relay system in Escherichia coli: modulation of sigmaS-dependent gene expression by the CsgD regulatory protein via sigmaS protein stabilization

Bacteria can switch from a single-cell (planktonic) mode to a multicellular community (biofilm) mode via production of cell-cell aggregation and surface adhesion factors. In this report, we present evidence that the CsgD protein, a transcription regulator involved in biofilm formation in Escherichia coli, modulates the expression of the rpoS (sigma(S)) regulon. Protein pattern analysis of E. coli cells in stationary phase shows that CsgD affects the expression of several proteins encoded by sigma(S)-dependent genes. CsgD regulation of sigma(S)-dependent genes takes place at gene transcription level, does not bypass the need for rpoS, and is abolished in an rpoS-null mutant. Consistent with these results, we find that CsgD expression leads to an increase in sigma(S) intracellular concentration. Increase in sigma(S) cellular amount is mediated by CsgD-dependent transcription activation of iraP, encoding a factor involved in sigma(S) protein stabilization. Our results strongly suggest that the CsgD regulatory protein plays a major role as a relay between adhesion factors production and sigma(S)-dependent gene expression via sigma(S) protein stabilization. Direct coordination between biofilm formation and expression of the rpoS regulon could positively impact important biological processes, such as host colonization or response to environmental stresses.

The role of cellulose and O-antigen capsule in the colonization of plants by Salmonella enterica

Numerous salmonellosis outbreaks have been associated with vegetables, in particular sprouted seed. Thin aggregative fimbriae (Tafi), a component of the extracellular matrix responsible for multicellular behavior, are important for Salmonella enterica attachment and colonization of plants. Here, we demonstrate that the other surface polymers composing the extracellular matrix, cellulose, and O-antigen capsule also play a role in colonization of plants. Mutations in bacterial cellulose synthesis (bcsA) and O-antigen capsule assembly and translocation (yihO) reduced the ability to attach to and colonize alfalfa sprouts. A colanic acid mutant was unaffected in plant attachment or colonization. Tafi, cellulose synthesis, and O-antigen capsule, all of which contribute to attachment and colonization of plants, are regulated by AgfD, suggesting that AgfD is a key regulator for survival outside of hosts of Salmonella spp. The cellulose biosynthesis regulator adrA mutant was not affected in the ability to attach to or colonize plants; however, promoter probe assays revealed expression by cells attached to alfalfa sprouts. Furthermore, quantitative reverse-transcriptase polymerase chain reaction revealed differential expression of agfD and adrA between planktonic and plant-attached cells. In addition, there was no correlation among mutants between biofilm formation in culture and attachment to plants. Outside of animal hosts, S. enterica appears to rely on an arsenal of adhesins to persist on plants, which can act as vectors and perpetuate public health concerns.

The symbiosis regulator rscS controls the syp gene locus, biofilm formation and symbiotic aggregation by Vibrio fischeri

Successful colonization of a eukaryotic host by a microbe involves complex microbe-microbe and microbe-host interactions. Previously, we identified in Vibrio fischeri a putative sensor kinase, RscS, required for initiating symbiotic colonization of its squid host Euprymna scolopes. Here, we analysed the role of rscS by isolating an allele, rscS1, with increased activity. Multicopy rscS1 activated transcription of genes within the recently identified symbiosis polysaccharide (syp) cluster. Wild-type cells carrying rscS1 induced aggregation phenotypes in culture, including the formation of pellicles and wrinkled colonies, in a syp-dependent manner. Colonies formed by rscSl-expressing cells produced a matrix not found in control colonies and largely lost in an rscSl-expressing sypN mutant. Finally, multicopy rscS1 provided a colonization advantage over control cells and substantially enhanced the ability of wild-type cells to aggregate on the surface of the symbiotic organ of E. scolopes; this latter phenotype similarly depended upon an intact syp locus. These results suggest that transcription induced by RscS-mediated signal transduction plays, a key role in colonization at the aggregation stage by modifying the cell surface and increasing the ability of the cells to adhere to one another and/or to squid-secreted mucus.