Switching aconitase B between catalytic and regulatory modes involves iron-dependent dimer formation

In addition to being the major citric acid cycle aconitase in Escherichia coli the aconitase B protein (AcnB) is also a post-transcriptional regulator of gene expression. The AcnB proteins represent a distinct branch of the aconitase superfamily that possess a HEAT-like domain (domain 5). The HEAT domains of other proteins are implicated in protein:protein interactions. Gel filtration analysis has now shown that cell-free extracts contain high-molecular-weight species of AcnB. Furthermore, in vitro and in vivo protein interaction experiments have shown that AcnB forms homodimers. Addition of the iron chelator bipyridyl to cultures inhibited the dimer-dependent readout from an AcnB bacterial two-hybrid system. A similar response was observed with a catalytically inactive AcnB variant, AcnB(C769S), suggesting that the monomer-dimer transition is not mediated by the state of the AcnB iron-sulphur cluster. The iron-responsive interacting unit was accordingly traced to the N-terminal region (domains 4 and 5) of the AcnB protein, and not to domain 3 that houses the iron-sulphur cluster. Thus, it was shown that a polypeptide containing AcnB N-terminal domains 5 and 4 (AcnB5-4) interacts with a second AcnB5-4 to form a homodimer. AcnB has recently been shown to initiate a regulatory cascade controlling flagella biosynthesis in Salmonella enterica by binding to the ftsH transcript and inhibiting the synthesis of the FtsH protease. A plasmid encoding AcnB5-4 complemented the flagella-deficient phenotype of a S. enterica acnB mutant, and the isolated AcnB5-4 polypeptide specifically recognized and bound to the ftsH transcript. Thus, the N-terminal region of AcnB is necessary and sufficient for promoting the formation of AcnB dimers and also for AcnB binding to target mRNA. Furthermore, the relative effects of iron on these processes provide a simple iron-mediated dimerization mechanism for switching the AcnB protein between catalytic and regulatory roles.

Biofilm formation, cellulose production, and curli biosynthesis by Salmonella originating from produce, animal, and clinical sources

The ability of 71 strains of Salmonella enterica originating from produce, meat, or clinical sources to form biofilms was investigated. A crystal violet binding assay demonstrated no significant differences in biofilm formation by isolates from any source when tested in any of the following three media: Luria-Bertani broth supplemented with 2% glucose, tryptic soy broth (TSB), or 1/20th-strength TSB. Incubation was overnight at 30 degrees C under static conditions. Curli production and cellulose production were monitored by assessing morphotypes on Luria-Bertani agar without salt containing Congo red and by assessing fluorescence on Luria-Bertani agar containing calcofluor, respectively. One hundred percent of the clinical isolates exhibited curli biosynthesis, and 73% demonstrated cellulose production. All meat-related isolates formed curli, and 84% produced cellulose. A total of 80% of produce-related isolates produced curli, but only 52% produced cellulose. Crystal violet binding was not statistically different between isolates representing the three morphotypes when grown in TSB; however, significant differences were observed when strains were cultured in the two other media tested. These data demonstrate that the ability to form biofilms is not dependent on the source of the test isolate and suggest a relationship between crystal violet binding and morphotype, with curli- and cellulose-deficient isolates being least effective in biofilm formation.

A novel mechanism for connecting bacterial two-component signal-transduction systems

Bacteria have many two-component signal-transduction systems (TCSs) that respond to specific environmental signals by altering the phosphorylated state of a response regulator. Although these systems are presumed to form an intricate signal network, the detailed mechanism of how they interact with each other remains largely unexplained. In a recent study of Salmonella, two TCSs have been discovered to be connected by a protein that protects a response regulator from dephosphorylation promoted by its cognate sensor kinase. This novel mechanism might provide an answer to some of the linkages found between other TCSs.

The Flag-2 locus, an ancestral gene cluster, is potentially associated with a novel flagellar system from Escherichia coli

Escherichia coli K-12 possesses two adjacent, divergent, promoterless flagellar genes, fhiA-mbhA, that are absent from Salmonella enterica. Through bioinformatics analysis, we found that these genes are remnants of an ancestral 44-gene cluster and are capable of encoding a novel flagellar system, Flag-2. In enteroaggregative E. coli strain 042, there is a frameshift in lfgC that is likely to have inactivated the system in this strain. Tiling path PCR studies showed that the Flag-2 cluster is present in 15 of 72 of the well-characterized ECOR strains. The Flag-2 system resembles the lateral flagellar systems of Aeromonas and Vibrio, particularly in its apparent dependence on RpoN. Unlike the conventional Flag-1 flagellin, the Flag-2 flagellin shows a remarkable lack of sequence polymorphism. The Flag-2 gene cluster encodes a flagellar type III secretion system (including a dedicated flagellar sigma-antisigma combination), thus raising the number of distinct type III secretion systems in Escherichia/Shigella to five. The presence of the Flag-2 cluster at identical sites in E. coli and its close relative Citrobacter rodentium, combined with its absence from S. enterica, suggests that it was acquired by horizontal gene transfer after the former two species diverged from Salmonella. The presence of Flag-2-like gene clusters in Yersinia pestis, Yersinia pseudotuberculosis, and Chromobacterium violaceum suggests that coexistence of two flagellar systems within the same species is more common than previously suspected. The fact that the Flag-2 gene cluster was not discovered in the first 10 Escherichia/Shigella genome sequences studied emphasizes the importance of maintaining an energetic program of genome sequencing for this important taxonomic group.

Identification of the outer-membrane protein PagC required for the serum resistance phenotype in Salmonella enterica serovar Choleraesuis

Serum resistance is a crucial virulence factor for the development of systemic infections, including bacteraemia, by many pathogenic bacteria. Salmonella enterica serovar Choleraesuis is an important enteric pathogen that causes serious systemic infections in swine and humans. Here, it was found that, when introduced into Escherichia coli, a recombinant plasmid carrying the pagC gene from a plasmid-based genomic library of S. enterica serovar Choleraesuis conferred a high-level resistance to the bactericidal activity of pooled normal swine serum. The resistance was equal to the level conferred by rck, a gene encoding a 17 kDa outer-membrane protein which promotes the serum resistance phenotype in S. enterica serovar Typhimurium. Insertional mutagenesis of the cloned pagC gene generated a mutation that resulted in the loss of the serum resistance phenotype in E. coli. When this mutation was introduced into the chromosome of S. enterica serovar Choleraesuis by homology recombination with the wild-type allele, the resulting strain could not produce PagC, and it showed a decreased level of resistance to complement-mediated killing. The mutation could be restored by introduction of the intact pagC gene on a plasmid, but not by introduction of the point-mutated pagC gene. In addition, PagC was able to promote serum resistance in the S. enterica serovar Choleraesuis LPS mutant strain, which is highly sensitive to serum killing. Although PagC is not thought to confer serum resistance directly, these results strongly suggest that PagC is an important outer-membrane protein that plays an important role in the serum resistance of S. enterica serovar Choleraesuis.

Regulation of enteric endophytic bacterial colonization by plant defenses

Bacterial endophytes reside within the interior of plants without causing disease or forming symbiotic structures. Some endophytes, such as Klebsiella pneumoniae 342 (Kp342), enhance plant growth and nutrition. Others, such as Salmonella enterica serovar Typhimurium (S. typhimurium), are human pathogens that contaminate raw produce. Several lines of evidence are presented here to support the hypothesis that plant defense response pathways regulate colonization by endophytic bacteria. An ethylene-insensitive mutant of Medicago truncatula is hypercolonized by Kp342 compared to the parent genotype. Addition of ethylene, a signal molecule for induced systemic resistance in plants, decreased endophytic colonization in Medicago spp. This ethylene-mediated inhibition of endophytic colonization was reversed by addition of the ethylene action inhibitor, 1-methylcyclopropene. Colonization of Medicago spp. by S. typhimurium also was affected by exogenous ethylene. Mutants lacking flagella or a component of the type III secretion system of Salmonella pathogenicity island 1 (TTSS-SPI1) colonize the interior of Medicago spp. in higher numbers than the wild type. Arabidopsis defense response-related genotypes indicated that only salicylic acid (SA)-independent defense responses contribute to restricting colonization by Kp342. In contrast, colonization by S. typhimurium is affected by both SA-dependent and -independent responses. S. typhimurium mutants further delineated these responses, suggesting that both flagella and TTSS-SPI1 effectors can be recognized. Flagella act primarily through SA-independent responses (compromising SA accumulation still affected colonization in the absence of flagella). Removal of a TTSS-SPI1 effector resulted in hypercolonization regardless of whether the genotype was affected in either SA-dependent or SA-independent responses. Consistent with these results, S. typhimurium activates the promoter of PR1, a SA-dependent pathogenesis-related gene, while S. typhimurium mutants lacking the TTSS-SPI1 failed to activate this promoter. These observations suggest approaches to reduce contamination of raw produce by human enteric pathogens and to increase the number of growth-promoting bacteria in plants.

Mechanisms of resistance in Salmonella enterica adapted to erythromycin, benzalkonium chloride and triclosan

The potential for adaptive resistance of S. enterica serovar Enteritidis, Typhimurium and Virchow to increasing sub-lethal concentrations of erythromycin, benzalkonium chloride and triclosan was investigated to identify mechanisms underlying resistance. Permeability changes of the outer membrane, including LPS, cell surface charge, hydrophobicity and the presence of an active efflux in the adapted strain compared with the parent were studied. Examination of the outer membrane and LPS did not reveal any significant changes, although most of the pre-adapted strains were notably less hydrophobic than resistant strains. More than one type of active efflux was identified in all strains investigated, on the basis of restored sensitivity in the presence of the inhibitors reserpine and carbonyl cyanide 3-chlorophenylhydrazone (CCCP). Cell surface hydrophobicity and the presence of active efflux could contribute to the resistance of S. enterica to the antibacterial agents studied here.

Isolation of mini-Tn5Km2 insertion mutants of Salmonella enterica serovar Oranienburg sensitive to NaCl-induced osmotic stress

We previously reported that the viability of Salmonella Oranienburg strains under NaCl stress was variable and depended on the strain's origin; food strains were resistant and patient strains sensitive to NaCl. Therefore, we mutagenized a food strain with a mini-Tn5Km2 transposon. Of 2,400 mutants screened, 15 NaCl-sensitive mutants were isolated, and 7 genes associated with NaCl-sensitivity were identified. The intact genes complemented their own food-strain mutants, but not patient-strain mutants, suggesting that the difference in NaCl-sensitivity between food and patient S. Oranienburg strains might not arise from a single gene mutation, but from change in multiple osmoregulatory mechanisms in Salmonella.

A pH-sensitive function and phenotype: evidence that EutH facilitates diffusion of uncharged ethanolamine in Salmonella enterica

The eutH gene is part of an operon that allows Salmonella enterica to use ethanolamine as a sole source of nitrogen, carbon, and energy. Although the sequence of EutH suggests a role in transport, eutH mutants use ethanolamine normally under standard conditions (pH 7.0). These mutants fail to use ethanolamine at a low pH. Evidence is presented that protonated ethanolamine (Eth0) does not enter cells, while uncharged ethanolamine (Eth0) diffuses freely across the membrane. The external concentration of Eth0 varies with the pH (pK=9.5). At pH 7.0, the standard ethanolamine concentration (41 mM) provides enough Eth0 for an influx rate that can support growth with or without EutH. When a lowered pH and/or ethanolamine concentration reduced the Eth0 concentration below 25 microM, EutH was needed to facilitate diffusion. EutH+ cells grew normally at Eth0 concentrations above 3 microM, close to the Km (9 microM) of the first degradative enzyme, ethanolamine ammonia lyase. It is suggested that EutH facilitates diffusion of Eth0. As predicted for a transporter, EutH contributed to the toxicity of ethanolamine seen under some conditions; furthermore, fusion of EutH to fluorescent Yfp protein provided evidence that EutH is a membrane protein.

Cell-to-cell signalling in Escherichia coli and Salmonella enterica

Cell-to-cell signalling in prokaryotes that leads to co-ordinated behaviour has been termed quorum sensing. This type of signalling can have profound impacts on microbial community structure and host-microbe interactions. The Gram-negative quorum-sensing systems were first discovered and extensively characterized in the marine Vibrios. Some components of the Vibrio systems are present in the classical genetic model organisms Escherichia coli and Salmonella enterica. Both organisms encode a signal receptor of the LuxR family, SdiA, but not a corresponding signal-generating enzyme. Instead, SdiA of Salmonella detects and responds to signals generated only by other microbial species. Conversely, E. coli and Salmonella encode the signal-generating component of a second system (a LuxS homologue that generates AI-2), but the sensory apparatus for AI-2 differs substantially from the Vibrio system. The only genes currently known to be regulated by AI-2 in Salmonella encode an active uptake and modification system for AI-2. Therefore, it is not yet clear whether Salmonella uses AI-2 as a signal molecule or whether AI-2 has some other function. In E. coli, the functions of both SdiA and AI-2 are unclear due to pleiotropy. Genetic strategies to identify novel signalling systems have been performed with E. coli and Providencia stuartii. Several putative signalling systems have been identified, one that uses indole as a signal and another that releases what appears to be a peptide. The latter system has homologues in E. coli and Salmonella, as well as other bacteria, plants and animals. In fact, the protease components from Providencia and Drosophila are functionally interchangeable.

The omptin family of enterobacterial surface proteases/adhesins: from housekeeping in Escherichia coli to systemic spread of Yersinia pestis

The omptins are a family of enterobacterial surface proteases/adhesins that share high sequence identity and a conserved beta-barrel fold in the outer membrane. The omptins are multifunctional, and the individual omptins exhibit differing virulence-associated functions. The Pla plasminogen activator of Yersinia pestis contributes by several mechanisms to bacterial invasiveness and the systemic, uncontrolled proteolysis in plague. Pla proteolytically activates the human proenzyme plasminogen and inactivates the antiprotease alpha2-antiplasmin, and its binding to laminin localizes the uncontrolled plasmin activity onto basement membranes. These properties enhance bacterial migration through tissue barriers. Pla also degrades circulating complement proteins and functions in bacterial invasion into human epithelial cells. PgtE of Salmonella enterica and OmpT of Escherichia coli have been shown to degrade cationic antimicrobial peptides from epithelial cells or macrophages. PgtE and SopA of Shigella flexneri appear important in the intracellular phases of salmonellosis and shigellosis, whereas functions of OmpT have mainly been associated with protein degradation in E. coli cells. The differing virulence roles and functions have been attributed to minor sequence variations at the surface-exposed regions important for substrate recognition, to the dependence of omptin functions on lipopolysaccharide, and to the different regulation of omptin expression.

The Salmonella SPI1 effector SopB stimulates nitric oxide production long after invasion

The ability of Salmonella enterica to invade and replicate within host cells depends on two type III secretion systems (TTSSs) encoded on pathogenicity islands 1 and 2 (SPI1 and SPI2). The current paradigm holds that these systems translocate two classes of effectors that operate sequentially and independently. In essence, the SPI1 TTSS mediates early events (i.e. invasion) whereas the SPI2 TTSS mediates post-invasion processes (i.e. replication, vacuole maturation). Contrary to this model, we have found in infected macrophages that a SPI1 effector, SopB/SigD, increased inducible nitric oxide synthase levels and nitric oxide production, host cell process previously known only to be a target of the SPI2 TTSS. Furthermore, SopB protein and message persist many hours after invasion. Our findings reveal an unanticipated potential for dialogue between the SPI1 and SPI2 TTSS and the host cell response.

Attachment of Salmonella Poona to cantaloupe rind and stem scar tissues as affected by temperature of fruit and inoculum

A negative temperature differential between fruits or vegetables and the water in which they are immersed theoretically enhances infiltration of water and any microorganisms it might contain into tissues. The effect of temperature differentials between cantaloupes and wash water, each at 4 and 30 degrees C, on changes in cantaloupe weight and populations of Salmonella enterica Poona recovered from rinds and stem scar tissues of Eastern and Western (shipper) types of cantaloupes was assessed. The percent weight increase in Western cantaloupes was significantly greater (P < or = 0.05) than that in Eastern cantaloupes for all cantaloupe and inoculum temperature combinations. Salmonella Poona attachment to or infiltration of Eastern but not Western cantaloupe rind is enhanced when the fruit is at 4 degrees C, compared with 30 degrees C, regardless of the temperature of the immersion suspension. The number of Salmonella Poona cells recovered from rind tissue of Western cantaloupes at 30 degrees C immersed in inoculum at 30 degrees C was significantly less (P < or = 0.05) than that recovered from rind tissues of cantaloupes at 4 or 30 degrees C that were immersed in inoculum at 4 degrees C. Salmonella Poona in immersion water can adhere to or infiltrate surface tissues of cantaloupes. The populations of Salmonella Poona recovered from stem scar tissues of Eastern and Western types of cantaloupes were not significantly (P > 0.05) affected by cantaloupe and inoculum temperature combinations. Populations of cells adhering to or infiltrating various cantaloupe tissues is not dictated entirely by temperature differentials between fruits and immersion suspensions: rather, it also apparently is influenced by structures unique to surface tissues.

Comparison of 40 Salmonella enterica serovars injured by thermal, high-pressure and irradiation stress

AIM

To investigate and compare the inherent resistance of 40 Salmonella serovars to heat, irradiation and high-pressure stress.

METHODS AND RESULTS

D10 values for each of the three stresses were calculated for four serovars, chosen as representatives from a catalogue of 40. Based on these results, conditions for each stress were defined, which produced, on average, a three-log reduction in viability. Heat stress (57 degrees C for 13 min), high-pressure stress (350 MPa for 10 min at 20 degrees C) and irradiation stress (1.5 kGy at 20 degrees C) were applied to all 40 serovars in the collection. Injury and loss of viability for all serovars were determined.

CONCLUSIONS

Cluster analysis identified five groupings of isolates in terms of resistance to the applied stresses. The independent response of each isolate to all three stresses suggests that there is no relationship between resistances.

SIGNIFICANCE AND IMPACT OF THE STUDY

Each serovar is inherently different. For modelling of real-life food preservation processing the most resistant isolates for that process should be chosen. The results also emphasize the importance of including multiple stress resistant strains when food preservation systems apply multiple stresses.

Changes in Salmonella enterica serovar Oranienburg viability caused by NaCl-induced osmotic stress is related to DNA relaxation by the H-NS protein during host infection

The NaCl sensitivity of Salmonella enterica serovar Oranienburg strains depends on their origin. We found previously that food- and patient-origin isolates in an outbreak were, respectively, NaCl-resistant and NaCl-sensitive, and the NaCl-resistant strain of food-origin isolates became NaCl-sensitive after passage of the strain through mice [FEMS Microbiol. Lett. 212 (2002) 87]. Here, we report that this phenotypic difference is mainly dependent on topological changes regulated by H-NS, a bacterial histone-like nucleoid protein that binds non-specifically to DNA. That is, this phenotypic difference was caused by changes in DNA topology during infection of the host. Based on these findings, we propose this mechanism has a key role in promoting the survival of Salmonella under osmotic stress.

Induction of antimicrobial pathways during early-phase immune response to Salmonella spp. in murine macrophages: gamma interferon (IFN-gamma) and upregulation of IFN-gamma receptor alpha expression are required for NADPH phagocytic oxidase gp91-stimulated oxidative burst and control of virulent Salmonella spp

The effect of gamma interferon (IFN-gamma) on elevation of reactive oxygen species and the viability of virulent wild-type and avirulent mutants of Salmonella enterica serovar Typhimurium and S. enterica serovar Infantis was studied in a murine macrophage cell line (J774.2 cells). S. enterica serovar Typhimurium 14028 phoP and a rough lipopolysaccharide mutant of S. enterica serovar Infantis 1326/28 (phi(r)) (avirulent mutants) induced NADPH phagocytic oxidase gp91 (gp91(phox)) activity and a significant (P < 0.05) elevation of reactive oxygen species within 12 h without coculture with IFN-gamma. This coincided with reduced survival of S. enterica serovar Typhimurium14028 phoP or stasis of S. enterica serovar Infantis phi(r). Fluorometric studies indicated that expression of IFN-gamma on infected J774.2 cells was not significantly (P > 0.05) elevated. However, studies with the virulent S. enterica serovar Typhimurium strains showed that a comparable level of control of bacterial numbers could only be achieved by coculture with IFN-gamma. This coincided with significant upregulation of IFN-gamma receptor alpha expression on the surface of J774.2 cells and was completely abolished by N-acetyl-L-cysteine captopril (an inhibitor of reactive oxygen species). Delay in reactive oxygen species induction due to a requirement for IFN-gamma and upregulation of IFN-gamma receptor alpha in macrophages infected with virulent salmonellae may result in greater dissemination of virulent salmonellae in host tissue.

Functions and effectors of the Salmonella pathogenicity island 2 type III secretion system

Salmonella enterica uses two functionally distinct type III secretion systems encoded on the pathogenicity islands SPI-1 and SPI-2 to transfer effector proteins into host cells. A major function of the SPI-1 secretion system is to enable bacterial invasion of epithelial cells and the principal role of SPI-2 is to facilitate the replication of intracellular bacteria within membrane-bound Salmonella-containing vacuoles (SCVs). Studies of mutant bacteria defective for SPI-2-dependent secretion have revealed a variety of functions that can be attributed to this secretion system. These include an inhibition of various aspects of endocytic trafficking, an avoidance of NADPH oxidase-dependent killing, the induction of a delayed apoptosis-like host cell death, the control of SCV membrane dynamics, the assembly of a meshwork of F-actin around the SCV, an accumulation of cholesterol around the SCV and interference with the localization of inducible nitric oxide synthase to the SCV. Several effector proteins that are translocated across the vacuolar membrane in a SPI-2-dependent manner have now been identified. These are encoded both within and outside SPI-2. The characteristics of these effectors, and their relationship to the physiological functions listed above, are the subject of this review. The emerging picture is of a multifunctional system, whose activities are explained in part by effectors that control interactions between the SCV and intracellular membrane compartments.

Adrenomedullin expression by gastric epithelial cells in response to infection

Many surface epithelial cells express adrenomedullin, a multifunctional peptide found in a wide number of body and cell systems. Recently, we and others have proposed that adrenomedullin has an important novel role in host defense. This peptide has many properties in common with other cationic antimicrobial peptides, including the human beta-defensins. Upon exposure of human gastric epithelial cells to viable cells of invasive or noninvasive strains of Helicobacter pylori, Escherichia coli, Salmonella enterica, or Streptococcus bovis, a significant increase in adrenomedullin secretion from these cells was demonstrated. Adrenomedullin gene expression was also increased in response to these microorganisms. Similar observations were noted when these cells were incubated with proinflammatory cytokines such as interleukin 1 alpha (IL-1 alpha), IL-6, tumor necrosis factor alpha and lipopolysaccharide. In cultured cells and an animal infection model, increased adrenomedullin peptide and gene expression was demonstrated when exposed to E. coli or Mycobacterium paratuberculosis, respectively. The data suggest there is a strong association between epithelial infection, inflammation, and adrenomedullin expression, which may have clinical relevance. The regulation of adrenomedullin expression may have therapeutic applications, such as improving or enhancing mucosal immunity.

Comparison of early ileal invasion by Salmonella enterica serovars Choleraesuis and Typhimurium

The mechanisms of Salmonella serovar-host specificity are not well defined. Pig ileal loops were used to compare phenotypic differences in early cellular invasion between non-host-adapted Salmonella serovar Typhimurium (SsT) and host-adapted Salmonella serovar Choleraesuis (SsC). By 10 minutes postinoculation, both serovars invaded a small number of M cells, enterocytes, and goblet cells. Multiple SsC organisms (up to 6 per cell) simultaneously invaded M cells, whereas SsT often invaded as one to two organisms per M cell. Internalization of both serovars resulted in vacuoles containing a single bacterium. The follicle-associated epithelium (FAE) of SsC-inoculated loops responded with more filopodia and lamellipodia although exhibiting less cell swelling than SsT. Additionally, SsT showed an enhanced affinity for sites of cell extrusion compared with SsC at 60 minutes. These results suggest: 1) both SsC and SsT exhibit non-cell-specific invasion as early as 10 minutes postinoculation, 2) Salmonella serovars exhibit differences in early invasion of FAE and M cells, and 3) cells undergoing extrusion may provide a site for preferential adherence by SsT and SsC.

Acid shock accumulation of sigma S in Salmonella enterica involves increased translation, not regulated degradation

Enteric pathogens such as Salmonella enterica and Escherichia coli face the daunting task of surviving passage through the extremely acid pH of the stomach in order to establish an infection in the host intestinal tract. These organisms have evolved elaborate stress response systems that aid in survival. The alternative sigma factor sigma(S) is a key regulator of many stress responses in S. enterica and is regulated at the levels of transcription, translation, and protein stability. Of these control mechanisms, proteolysis has been considered paramount in determining sigma(S) levels in the cell. Until the current report, acid shock was thought to increase sigma(S) levels by directly regulating degradation. However, mutant strains unable to degrade sigma(S) still exhibited acid shock induction of sigma(S). We demonstrate here that RPOS translation is a major focus of acid stress control and is responsible for the observed increase in sigma(S) levels. A series of deletions of the 566-nucleotide untranslated region of the RPOS mRNA were constructed to examine the importance of this regulatory region in acid shock induction of RPOS. Progressive deletions starting from the 5' end of the RPOS message produced alternating loss and recovery of acid shock control. The results suggest that competing stem-loop structures work in concert to control the acid shock induction of RPOS. Further, the half-life of sigma(S) was unchanged in response to acid shock and over-expression of the MviA recognition protein resulted in constitutive sigma(S) degradation under acid stress conditions. The data indicate that in log phase, nonstressed cells increasing sigma(S) production is sufficient to increase protein half-life. In toto, these results suggest that acid shock stabilization of sigma(S) is the result of increased synthesis via translational control and does not involve changes in the activity of the MviA (RssB/SprE) ClpXP degradation complex. Therefore, constitutive degradation may enable the cell to reset the level of sigma(S) once acid stress is alleviated.

In vitro adhesion of an avian pathogenic Escherichia coli O78 strain to surfaces of the chicken intestinal tract and to ileal mucus

The role of fimbria in adherence of an avian pathogenic Escherichia coli (APEC) O78 strain 789 to chicken intestine was studied. Bacterial adhesion to tissue sections representing the regions within the chicken intestinal tract was determined by using immunohistochemical methods. E. coli 789 grown to express the type 1 fimbria adhered efficiently to the crop epithelium, to the lamina propria of intestinal villi, and to the apical surfaces of both the mature as well as the crypt-located enterocytes in intestinal villi, whereas no adhesion to mucus-producing goblet cells was detected. The adhesion was inhibited by mannoside and the role of type 1 fimbriae in the observed adhesion was confirmed with a recombinant strain expressing type 1 fimbriae genes cloned from E. coli and Salmonella enterica. E. coli 789 strain grown to favor AC/I fimbriae expression as well as the recombinant E. coli strain expressing the fac genes adhered to goblet cells but only poorly to the other epithelial sites. E. coli strain 789 as well as S. enterica serovar Typhimurium IR715 and S. enterica serovar Enteriditis TN2 strains were able to multiply in ileal mucus medium. The type 1 fimbria expressing bacteria adhered to the ileal mucus, whereas the AC/I fimbriated strains showed poor adherence to the mucus. The adhesion of E. coli 789 onto the crop epithelium and the follicle associated epithelium of the chicken ileum was efficiently inhibited by an adhesive strain ST1 of Lactobacillus crispatus isolated from chicken, whereas poor inhibition of E. coli adherence was observed with the weakly adhesive L. crispatus strain 134mi. The type 1 fimbriae may be important in colonization of the chicken intestine by APEC and Salmonella.

Low-Shear modeled microgravity alters the Salmonella enterica serovar typhimurium stress response in an RpoS-independent manner

We have previously demonstrated that low-shear modeled microgravity (low-shear MMG) serves to enhance the virulence of a bacterial pathogen, Salmonella enterica serovar Typhimurium. The Salmonella response to low-shear MMG involves a signaling pathway that we have termed the low-shear MMG stimulon, though the identities of the low-shear MMG stimulon genes and regulatory factors are not known. RpoS is the primary sigma factor required for the expression of genes that are induced upon exposure to different environmental-stress signals and is essential for virulence in mice. Since low-shear MMG induces a Salmonella acid stress response and enhances Salmonella virulence, we reasoned that RpoS would be a likely regulator of the Salmonella low-shear MMG response. Our results demonstrate that low-shear MMG provides cross-resistance to several environmental stresses in both wild-type and isogenic rpoS mutant strains. Growth under low-shear MMG decreased the generation time of both strains in minimal medium and increased the ability of both strains to survive in J774 macrophages. Using DNA microarray analysis, we found no evidence of induction of the RpoS regulon by low-shear MMG but did find that other genes were altered in expression under these conditions in both the wild-type and rpoS mutant strains. Our results indicate that, under the conditions of these studies, RpoS is not required for transmission of the signal that induces the low-shear MMG stimulon. Moreover, our studies also indicate that low-shear MMG can be added to a short list of growth conditions that can serve to preadapt an rpoS mutant for resistance to multiple environmental stresses.

Differences in attachment of Salmonella enterica serovars and Escherichia coli O157:H7 to alfalfa sprouts

Numerous Salmonella enterica and Escherichia coli O157:H7 outbreaks have been associated with contaminated sprouts. We examined how S. enterica serovars, E. coli serotypes, and nonpathogenic bacteria isolated from alfalfa sprouts grow on and adhere to alfalfa sprouts. Growth on and adherence to sprouts were not significantly different among different serovars of S. enterica, but all S. enterica serovars grew on and adhered to alfalfa sprouts significantly better than E. coli O157:H7. E. coli O157:H7 was essentially rinsed from alfalfa sprouts with repeated washing steps, while 1 to 2 log CFU of S. enterica remained attached per sprout. S. enterica Newport adhered to 3-day-old sprouts as well as Pantoea agglomerans and 10-fold more than Pseudomonas putida and Rahnella aquatilis, whereas the growth rates of all four strains throughout seed sprouting were similar. S. enterica Newport and plant-associated bacteria adhered 10- to 1,000-fold more than E. coli O157:H7; however, three of four other E. coli serotypes, isolated from cabbage roots exposed to sewage water following a spill, adhered to sprouts better than E. coli O157:H7 and as well as the Pseudomonas and Rahnella strains. Therefore, attachment to alfalfa sprouts among E. coli serotypes is variable, and nonpathogenic strains of E. coli to be used as surrogates for the study of pathogenic E. coli may be difficult to identify and should be selected carefully, with knowledge of the biology being examined.