SigmaB contributes to Listeria monocytogenes invasion by controlling expression of inlA and inlB

Inhibitory effects of Leuconostoc mesenteroides 1RM3 isolated from narezushi, a fermented fish with rice, on Listeria monocytogenes infection to Caco-2 cells and A/J mice

Listeria monocytogenes causes listeriosis in humans mainly through consumption of ready-to-eat foods. Immunocompromised persons, the elderly, and pregnant women and their fetuses or newborns are at highest risk for the infection. To isolate probiotic lactic acid bacteria (LAB) with inhibitory effects against L. monocytogenes, we screened for acid and bile resistant LABs from narezushi, a traditional salted and long-fermented fish with cooked rice. Then, inhibitory effects of the selected LABs on L. monocytogenes invasion and infection of human enterocyte Caco-2 cells and Listeria-susceptible A/J mice were determined. From a total of 231 LAB isolates, we selected five acid and bile resistant isolates (four were Lactobacillus plantarum and one was Leuconostoc mesenteroides). Among the five isolates, Ln. mesenteroides (Lnm-1RM3) showed the highest inhibition against L. monocytogenes invasion into Caco-2 cells. In the case of L. monocytogenes orally infected A/J mice, recovery of the pathogen from the spleen was suppressed by drinking water containing 9 log CFU/ml of Lnm-1RM3 cells. The inhibitory effects were also shown by heat-killed Lnm-1RM3 cells. These results suggest that live and also heat-killed Lnm-1RM3 cell intake might prevent L. monocytogenes entero-gastric invasion and infection.

The metalloprotease of Listeria monocytogenes is regulated by pH

Listeria monocytogenes is an intracytosolic bacterial pathogen. Among the factors contributing to escape from vacuoles are a phosphatidylcholine phospholipase C (PC-PLC) and a metalloprotease (Mpl). Both enzymes are translocated across the bacterial membrane as inactive proproteins, whose propeptides serve in part to maintain them in association with the bacterium. We have shown that PC-PLC maturation is regulated by Mpl and pH and that Mpl maturation occurs by autocatalysis. In this study, we tested the hypothesis that Mpl activity is pH regulated. To synchronize the effect of pH on bacteria, the cytosolic pH of infected cells was manipulated immediately after radiolabeling de novo-synthesized bacterial proteins. Immunoprecipitation of secreted Mpl from host cell lysates revealed the presence of the propeptide and catalytic domain in samples treated at pH 6.5 but not at pH 7.3. The zymogen was present in small amounts under all conditions. Since proteases often remain associated with their respective propeptide following autocatalysis, we aimed at determining whether pH regulates autocatalysis or secretion of the processed enzyme. For this purpose, we used an Mpl construct that contains a Flag tag at the N terminus of its catalytic domain and antibodies that can distinguish N-terminal and non-N-terminal Flag. By fluorescence microscopy, we observed the Mpl zymogen associated with the bacterium at physiological pH but not following acidification. Mature Mpl was not detected in association with the bacterium at either pH. Using purified proteins, we determined that processing of the PC-PLC propeptide by mature Mpl is also pH sensitive. These results indicate that pH regulates the activity of Mpl on itself and on PC-PLC.

Regulated shift from helical to polar localization of Listeria monocytogenes cell wall-anchored proteins

Many virulence factors of Gram-positive bacterial pathogens are covalently anchored to the peptidoglycan (PG) by sortase enzymes. However, for rod-shaped bacteria little is known about the spatiotemporal organization of these surface proteins in the cell wall. Here we report the three-dimensional (3D) localization of the PG-bound virulence factors InlA, InlH, InlJ, and SvpA in the envelope of Listeria monocytogenes under different growth conditions. We found that all PG-anchored proteins are positioned along the lateral cell wall in nonoverlapping helices. However, these surface proteins can also become localized at the pole and asymmetrically distributed when specific regulatory pathways are activated. InlA and InlJ are enriched at poles when expressed at high levels in exponential-phase bacteria. InlA and InlH, which are σ(B)dependent, specifically relocalize to the septal cell wall and subsequently to the new pole in cells entering stationary phase. The accumulation of InlA and InlH in the septal region also occurs when oxidative stress impairs bacterial growth. In contrast, the iron-dependent protein SvpA is present at the old pole and is excluded from the septum and new pole of bacteria grown under low-iron conditions. We conclude that L. monocytogenes rapidly reorganizes the spatial localization of its PG proteins in response to changes in environmental conditions such as nutrient deprivation or other stresses. This dynamic control would distribute virulence factors at specific sites during the infectious process.

Role of flhA and motA in growth of Listeria monocytogenes at low temperatures

While temperature-dependent induction of flagella is a well-characterized phenomenon in Listeria monocytogenes, the essentiality of increased flagellum production during growth at low temperatures remains unclear. To study this relationship, we compared the relative expression levels of two motility genes, flhA and motA, at 3°C, 25°C and 37°C in L. monocytogenes strain EGD-e by using qRT-PCR, and compared the growth curves, motility, and flagellation between the wild-type and flhA and motA deletion mutants. The relative expression levels of flhA and motA at 3°C were significantly higher than at 37°C (p<0.01). At 3°C, the level of flhA transcripts was also significantly higher than at 25°C (p<0.01). Growth curve analysis showed that at 3°C both the growth rates and maximum optical densities of ΔflhA and ΔmotA strains at 600 nm were significantly lower than those of the wild-type (p<0.001), while no significant differences were observed between the wild-type and the mutants at 37°C, and 25°C. Mutant strains ΔflhA and ΔmotA were nonmotile at all three temperatures. At 25°C, the number of flagellated cells of ΔmotA was notably reduced compared with the wild-type, whereas ΔflhA appeared nonflagellated at all temperatures. The results suggest that flhA and motA play a role in the cold tolerance of L. monocytogenes strain EGD-e, and that motile flagella may be needed for optimal cold stress response of L. monocytogenes.

A contingency locus in prfA in a Listeria monocytogenes subgroup allows reactivation of the PrfA virulence regulator during infection in mice

A nonhemolytic Listeria monocytogenes strain isolated from a fish processing plant was avirulent in a plaque-forming assay and in a subcutaneous mouse virulence assay. However, it showed 60% lethality (9/15 mice) when 10⁹ CFU were intraperitoneally injected into mice. Hemolytic L. monocytogenes bacteria were recovered from liver and spleen of the deceased mice, and the pulsed-field gel electrophoresis patterns were indistinguishable for the nonhemolytic and the hemolytic isolates. Sequencing of prfA from the nonhemolytic strain revealed a duplication of 7 bp in the helix-turn-helix region, resulting in a truncated PrfA protein. We propose that the direct repeat of 7 bp causes a reversible inactivation of prfA and that slipped-strand mispairing regulates the phase variation in hemolytic activity and virulence. Nonhemolytic L. monocytogenes strains with identical duplications in prfA were isolated from several sources in France, as well as in Norway, suggesting that the reversible inactivation described in this study is not an isolated event.

Contribution of Listeria monocytogenes RecA to acid and bile survival and invasion of human intestinal Caco-2 cells

The food-borne pathogen Listeria monocytogenes is able to colonize the human gastro-intestinal tract and subsequently cross the intestinal barrier. Thus, for L. monocytogenes to become virulent, it must survive the low pH of the stomach, high bile concentrations in the small intestine, and invade the epithelial cells. In this study, we show that RecA, which is an important factor in DNA repair and the activator of the SOS response, contributes to the resistance against acid and bile and to the ability of L. monocytogenes to adhere and invade human intestine epithelial cells. Activation of recA was shown with a promoter reporter after exposure to low pH and high bile concentrations and during adhesion and invasion of Caco-2 intestinal epithelial cells. Furthermore, an in-frame recA deletion mutant showed reduced survival after exposure to low pH and high bile concentrations. This mutant also showed a deficiency in adhesion and invasion of Caco-2 cells. These results suggest that RecA may contribute to the colonization of the human gastro-intestinal tract and crossing of the intestinal barrier.

Blue and red light modulates SigB-dependent gene transcription, swimming motility and invasiveness in Listeria monocytogenes

BACKGROUND

In a number of gram-positive bacteria, including Listeria, the general stress response is regulated by the alternative sigma factor B (SigB). Common stressors which lead to the activation of SigB and the SigB-dependent regulon are high osmolarity, acid and several more. Recently is has been shown that also blue and red light activates SigB in Bacillus subtilis.

METHODOLOGY/PRINCIPAL FINDINGS

By qRT-PCR we analyzed the transcriptional response of the pathogen L. monocytogenes to blue and red light in wild type bacteria and in isogenic deletion mutants for the putative blue-light receptor Lmo0799 and the stress sigma factor SigB. It was found that both blue (455 nm) and red (625 nm) light induced the transcription of sigB and SigB-dependent genes, this induction was completely abolished in the SigB mutant. The blue-light effect was largely dependent on Lmo0799, proving that this protein is a genuine blue-light receptor. The deletion of lmo0799 enhanced the red-light effect, the underlying mechanism as well as that of SigB activation by red light remains unknown. Blue light led to an increased transcription of the internalin A/B genes and of bacterial invasiveness for Caco-2 enterocytes. Exposure to blue light also strongly inhibited swimming motility of the bacteria in a Lmo0799- and SigB-dependent manner, red light had no effect there.

CONCLUSIONS/SIGNIFICANCE

Our data established that visible, in particular blue light is an important environmental signal with an impact on gene expression and physiology of the non-phototrophic bacterium L. monocytogenes. In natural environments these effects will result in sometimes random but potentially also cyclic fluctuations of gene activity, depending on the light conditions prevailing in the respective habitat.

Listeria monocytogenes efficiently invades Caco-2 cells after low-temperature storage in broth and on deli meat

The objective of this study was to investigate how various growth conditions influence the virulence of Listeria monocytogenes monitored by its ability to invade the epithelial cell lines Caco-2 and INT-407. The growth conditions examined were modified atmosphere-packaged deli meat and brain heart infusion broth (BHI) with and without salt. Five strains of L. monocytogenes were selected to investigate their invasiveness and all strains invaded Caco-2 cells at higher levels than INT-407 cells. Further, the clinical strains (3443 and 3734) were more invasive (p < 0.05) than the strains isolated from meat and food-processing environments (3008, 3126, and 4140) after grown in BHI at 30 degrees C. This attenuation could not be ascribed to a defective Internalin A as all strains encoded an intact inlA gene. To determine the influence of food products on virulence, the ability of L. monocytogenes to invade Caco-2 cells was compared after growth on a fermented sausage and on cured cooked ham to that of bacteria grown in BHI broth supplemented with salt. Samples were stored under chilling conditions for up to 4 weeks. The results showed no difference (p > 0.05) in invasiveness after 7 days at 10 degrees C in BHI broth or on sausage, whereas a slight increase (p < 0.05) was observed after incubation on ham for 2 and 4 weeks compared to that in BHI broth. Most importantly, our results show that L. monocytogenes efficiently invade Caco-2 cells even after 4 weeks of storage at chilled temperature. This is highly relevant for safety assessment of this organism in food as these conditions reflect storage of ready-to-eat food products in domestic refrigerators.

Constitutive activation of PrfA tilts the balance of Listeria monocytogenes fitness towards life within the host versus environmental survival

PrfA is a key regulator of Listeria monocytogenes pathogenesis and induces the expression of multiple virulence factors within the infected host. PrfA is post-translationally regulated such that the protein becomes activated upon bacterial entry into the cell cytosol. The signal that triggers PrfA activation remains unknown, however mutations have been identified (prfA* mutations) that lock the protein into a high activity state. In this report we examine the consequences of constitutive PrfA activation on L. monocytogenes fitness both in vitro and in vivo. Whereas prfA* mutants were hyper-virulent during animal infection, the mutants were compromised for fitness in broth culture and under conditions of stress. Broth culture prfA*-associated fitness defects were alleviated when glycerol was provided as the principal carbon source; under these conditions prfA* mutants exhibited a competitive advantage over wild type strains. Glycerol and other three carbon sugars have been reported to serve as primary carbon sources for L. monocytogenes during cytosolic growth, thus prfA* mutants are metabolically-primed for replication within eukaryotic cells. These results indicate the critical need for environment-appropriate regulation of PrfA activity to enable L. monocytogenes to optimize bacterial fitness inside and outside of host cells.

Effect of osmotic stress and culture density on invasiveness of Listeria monocytogenes strains

The effect of osmotic stress on its capacity to invade the human enterocytic cell line HT-29 was studied in the early log through the stationary phase in 10 L. monocytogenes strains representing three genetically independent lineages. The results demonstrate that the transition of the bacteria from the log to the stationary phase results in a stepwise reduction of invasiveness. This effect was heterogeneous in the studied L. monocytogenes population, as the range of invasiveness reduction between the log and stationary phases varied from 10- to 380-fold depending on the strain. Ten-minute exposure to 0.3 M NaCl was sufficient to generate invasiveness alteration. No significant change in invasiveness induction caused by osmotic stress was found between the different points of the log phase (OD₆₀₀ 0.4-1.2), being significantly different in the early log phase (OD₆₀₀ 0.2-0.3) and in the stationary phase after 18 h of culture. The level of internalins and opuCA transcripts in response to osmotic stress did not correlate with invasiveness alteration in most L. monocytogenes strains. Prolongation of stress exposure to 1 h and an increase in NaCl concentration from 0.3 to 1.8 M had no significant effect on a further increase in invasiveness. Short exposure times and low NaCl concentrations were sufficient for the generation of maximal invasiveness response of L. monocytogenes. It appears that although stationary-phase bacteria exhibit lower invasiveness than log-phase bacteria, they have a greater capacity to enhance their pathogenicity in response to stress.

Transcriptomic and phenotypic analyses identify coregulated, overlapping regulons among PrfA, CtsR, HrcA, and the alternative sigma factors sigmaB, sigmaC, sigmaH, and sigmaL in Listeria monocytogenes

A set of seven Listeria monocytogenes 10403S mutant strains, each bearing an in-frame null mutation in a gene encoding a key regulatory protein, was used to characterize transcriptional networks in L. monocytogenes; the seven regulatory proteins addressed include all four L. monocytogenes alternative sigma factors (σ(B), σ(C), σ(H), and σ(L)), the virulence gene regulator PrfA, and the heat shock-related negative regulators CtsR and HrcA. Whole-genome microarray analyses, used to identify regulons for each of these 7 transcriptional regulators, showed considerable overlap among regulons. Among 188 genes controlled by more than one regulator, 176 were coregulated by σ(B), including 92 genes regulated by both σ(B) and σ(H) (with 18 of these genes coregulated by σ(B), σ(H), and at least one additional regulator) and 31 genes regulated by both σ(B) and σ(L) (with 10 of these genes coregulated by σ(B), σ(L), and at least one additional regulator). Comparative phenotypic characterization measuring acid resistance, heat resistance, intracellular growth in J774 cells, invasion into Caco-2 epithelial cells, and virulence in the guinea pig model indicated contributions of (i) σ(B) to acid resistance, (ii) CtsR to heat resistance, and (iii) PrfA, σ(B), and CtsR to virulence-associated characteristics. Loss of the remaining transcriptional regulators (i.e., sigH, sigL, or sigC) resulted in limited phenotypic consequences associated with stress survival and virulence. Identification of overlaps among the regulons provides strong evidence supporting the existence of complex regulatory networks that appear to provide the cell with regulatory redundancies, along with the ability to fine-tune gene expression in response to rapidly changing environmental conditions.

Transcriptome analysis of alkali shock and alkali adaptation in Listeria monocytogenes 10403S

Alkali stress is an important means of inactivating undesirable pathogens in a wide range of situations. Unfortunately, Listeria monocytogenes can launch an alkaline tolerance response, significantly increasing persistence of the pathogen in such environments. This study compared transcriptome patterns of alkali and non-alkali-stressed L. monocytogenes 10403S cells, to elucidate the mechanisms by which Listeria adapts and/or grows during short- or long-term alkali stress. Transcription profiles associated with alkali shock (AS) were obtained by DNA microarray analysis of midexponential cells suspended in pH 9 media for 15, 30, or 60 min. Transcription profiles associated with alkali adaptation (AA) were obtained similarly from cells grown to midexponential phase at pH 9. Comparison of AS and AA transcription profiles with control cell profiles identified a high number of differentially regulated open-reading frames in all tested conditions. Rapid (15 min) changes in expression included upregulation of genes encoding for multiple metabolic pathways (including those associated with Na+/H+ antiporters), ATP-binding cassette transporters of functional compatible solutes, motility, and virulence-associated genes as well as the σ(B) controlled stress resistance network. Slower (30 min and more) responses to AS and adaptation during growth in alkaline conditions (AA) involved a different pattern of changes in mRNA concentrations, and genes involved in proton export.

Listeria monocytogenes PrsA2 is required for virulence factor secretion and bacterial viability within the host cell cytosol

In the course of establishing its replication niche within the cytosol of infected host cells, the facultative intracellular bacterial pathogen Listeria monocytogenes must efficiently regulate the secretion and activity of multiple virulence factors. L. monocytogenes encodes two predicted posttranslocation secretion chaperones, PrsA1 and PrsA2, and evidence suggests that PrsA2 has been specifically adapted for bacterial pathogenesis. PrsA-like chaperones have been identified in a number of Gram-positive bacteria, where they are reported to function at the bacterial membrane-cell wall interface to assist in the folding of proteins translocated across the membrane; in some cases, these proteins have been found to be essential for bacterial viability. In this study, the contributions of PrsA2 and PrsA1 to L. monocytogenes growth and protein secretion were investigated in vitro and in vivo. Neither PrsA2 nor PrsA1 was found to be essential for L. monocytogenes growth in broth culture; however, optimal bacterial viability was found to be dependent upon PrsA2 for L. monocytogenes located within the cytosol of host cells. Proteomic analyses of prsA2 mutant strains in the presence of a mutationally activated allele of the virulence regulator PrfA revealed a critical requirement for PrsA2 activity under conditions of PrfA activation, an event which normally takes place within the host cell cytosol. Despite a high degree of amino acid similarity, no detectable degree of functional overlap was observed between PrsA2 and PrsA1. Our results indicate a critical requirement for PrsA2 under conditions relevant to host cell infection.

Growth temperature-dependent contributions of response regulators, σB, PrfA, and motility factors to Listeria monocytogenes invasion of Caco-2 cells

Foodborne pathogens encounter rapidly changing environmental conditions during transmission, including exposure to temperatures below 37°C. The goal of this study was to develop a better understanding of the effects of growth temperatures and temperature shifts on regulation of invasion phenotypes and invasion-associated genes in Listeria monocytogenes. We specifically characterized the effects of L. monocytogenes growth at different temperatures (30°C vs. 37°C) on (i) the contributions to Caco-2 invasion of different regulators (including σ(B), PrfA, and 14 response regulators [RRs]) and invasion proteins (i.e., InlA and FlaA), and on (ii) gadA, plcA, inlA, and flaA transcript levels and their regulation. Overall, Caco-2 invasion efficiency was higher for L. monocytogenes grown at 30°C than for bacteria grown at 37°C (p = 0.0051 for the effect of temperature on invasion efficiency; analysis of variance); the increased invasion efficiency of the parent strain 10403S (serotype 1/2a) observed after growth at 30°C persisted for 2.5 h exposure to 37°C. For L. monocytogenes grown at 30°C, the motility RRs DegU and CheY and σ(B), but not PrfA, significantly contributed to Caco-2 invasion efficiency. For L. monocytogenes grown at 37°C, none of the 14 RRs tested significantly contributed to Caco-2 invasion, whereas σ(B) and PrfA contributed synergistically to invasion efficiency. At both growth temperatures there was significant synergism between the contributions to invasion of FlaA and InlA; this synergism was more pronounced after growth at 30°C than at 37°C. Our data show that growth temperature affects invasion efficiency and regulation of virulence-associated genes in L. monocytogenes. These data support increasing evidence that a number of environmental conditions can modulate virulence-associated phenotypes of foodborne bacterial pathogens, including L. monocytogenes.

The chitinolytic activity of Listeria monocytogenes EGD is regulated by carbohydrates but also by the virulence regulator PrfA

Chitin, an insoluble polymer of N-acetyl-D-glucosamine (GlcNAc), is one of the most abundant carbohydrate polymers in marine and terrestrial environments. Chitin hydrolysis by Listeria monocytogenes depends on two chitinase-encoding genes, chiA and chiB, and the aim of this study was to investigate their regulation. Chitin induces the expression of both chitinases in late exponential growth phase, and chiA but not chiB is furthermore induced by the monomer GlcNAc. Furthermore, their expression is subjected to catabolite control. Chitinases expressed by bacterial pathogens have proven to be important not only for nutrient acquisition and environmental survival but also for infecting animals and humans. Interestingly, the central L. monocytogenes virulence gene regulator, PrfA, is required for the chitinolytic phenotype, as chitinase activity was significantly reduced in prfA mutant cells compared to its level in wild-type cells. In agreement with this, Northern blot analysis showed that the amounts of chiA and chiB transcripts upon induction by chitin were significantly lower in the prfA mutant than in the wild type. The chitinolytic activity and chiA and chiB expression were reduced in the absence of the sigB gene, indicating that σ(B) is also important for the production of chitinases. The chiA, chiB, and chiA chiB mutants were not impaired for in vitro adhesion and invasion in epithelial cell lines, but the chiA chiB double mutant showed less survival ability in a chitin-enriched medium. The regulation of chitinolytic activity in L. monocytogenes is complex, and taken together, the results indicate that the biological role of this activity may not be limited to the external environment.

Listeria monocytogenes {sigma}B has a small core regulon and a conserved role in virulence but makes differential contributions to stress tolerance across a diverse collection of strains

Listeria monocytogenes strains are classified in at least three distinct phylogenetic lineages. There are correlations between lineage classification and source of bacterial isolation; e.g., human clinical and food isolates usually are classified in either lineage I or II. However, human clinical isolates are overrepresented in lineage I, while food isolates are overrepresented in lineage II. sigma(B), a transcriptional regulator previously demonstrated to contribute to environmental stress responses and virulence in L. monocytogenes lineage II strains, was hypothesized to provide differential abilities for L. monocytogenes survival in various niches (e.g., food and human clinical niches). To determine if the contributions of sigma(B) to stress response and virulence differ across diverse L. monocytogenes strains, DeltasigB mutations were created in strains belonging to lineages I, II, IIIA, and IIIB. Paired parent and DeltasigB mutant strains were tested for survival under acid and oxidative stress conditions, Caco-2 cell invasion efficiency, and virulence using the guinea pig listeriosis infection model. Parent and DeltasigB mutant strain transcriptomes were compared using whole-genome expression microarrays. sigma(B) contributed to virulence in each strain. However, while sigma(B) contributed significantly to survival under acid and oxidative stress conditions and Caco-2 cell invasion in lineage I, II, and IIIB strains, the contributions of sigma(B) were not significant for these phenotypes in the lineage IIIA strain. A core set of 63 genes was positively regulated by sigma(B) in all four strains; different total numbers of genes were positively regulated by sigma(B) in the strains. Our results suggest that sigma(B) universally contributes to L. monocytogenes virulence but specific sigma(B)-regulated stress response phenotypes vary among strains.

Relative transcription of Listeria monocytogenes virulence genes in liver pâtés with varying NaCl content

Quantitative real time polymerase chain reaction (qRT PCR) was used to compare the relative transcription of prfA, inlA, sigB and clpC for three Listeria monocytogenes strains after incubation in i) a standard liver pâté versus brain heart infusion (BHI) broth and ii) the standard liver pâté versus three liver pâtés with reduced NaCl content of which one also has been supplied with organic acids (Ca-acetate and Ca-lactate). The three strains (EGD-e: reference strain; O57: more NaCl sensitive; 6896: more NaCl tolerant) were selected out of twelve strains based on their growth in BHI broth adjusted to 6%, 8%, 10% (w/v) NaCl. The three strains were spiked into the liver pâtés (10(9) cfu/g) and the BHI (10(9) cfu/ml) and incubated for 48 h at 7 degrees C; all incubation conditions supported growth of the strains. Extraction of intact listerial RNA from the liver pâtés was complicated by the complexity of the liver pâté matrix. However, a method has been optimized and described, and the quality of RNA extracted from liver pâtés was equal to the quality of RNA extracted from BHI. The amplification efficiencies of the six genes used for the transcription analyses (the four target genes and two reference genes, gap and rpoB) were within the acceptable range from 90% to 110% for all three strains in both liver pâté and BHI. Comparison of the three strains after incubation in the standard liver pâté and BHI showed that the relative transcription of prfA for O57 and the relative transcription of inlA and sigB for both O57 and 6896 were significantly higher when the strains were grown in BHI compared to the standard liver pâté. Reducing the NaCl content of the standard liver pâté did not change relative transcription levels of prfA, inlA, sigB or clpC (except for prfA in O57 and sigB in 6896). However, the presence of Ca-acetate and Ca-lactate induced relative transcription of the stress response gene, clpC, for all three strains. This study demonstrates that relative microbial gene transcription can be measured in complex food matrices and points to the need for designing experimental set-ups in real food matrices to replace the laboratory model systems. With respect to L. monocytogenes, it seems that the NaCl content of liver pâté can be lowered within the investigated range without significant changes in relative virulence gene transcription while more caution should be taken when adding organic acids such as acetate and lactate.

Contributions of six lineage-specific internalin-like genes to invasion efficiency of Listeria monocytogenes

Listeria monocytogenes strains are divided into at least three lineages, which seem to differ in virulence. Internalins are surface-attached or secreted proteins that encode leucine-rich repeats, and L. monocytogenes encodes species-specific as well as lineage-specific internalin and internalin-like genes. Internalins A and B have previously been shown to be critical for L. monocytogenes host cell invasion. Transcription of selected internalins is regulated by the virulence gene regulator PrfA and/or the stress-responsive alternative sigma factor sigma(B). We hypothesized that lineage-specific internalin-like genes may contribute to differential virulence and niche adaptation of the L. monocytogenes lineages. Initial quantitative real time, reverse transcriptase PCR (RT-PCR) showed that the six selected lineage-specific internalin-like genes were transcribed in cells grown at 16 degrees and 37 degrees C. Lineage-specific internalin-like gene, lineage II (lsiIIX) showed significantly higher transcript levels in log-phase cells grown at 37 degrees C as compared to 16 degrees C. The gene lsiIA was preceded by a putative sigma(B)-dependent promoter and showed sigma(B)-dependent transcription. None of the null mutants in lineage-specific internalin-like genes differed from their respective parent strain in ability to invade either human intestinal epithelial or hepatocyte-like cell lines. All three mutants in lineage I-specific internalin-like genes exhibited the same growth condition-dependent invasion phenotype as their parent strain ( approximately 1.5 log higher invasion efficiency when grown at 30 degrees C without aeration versus 37 degrees C with aeration). Despite structural similarities to internalins with known roles in host cell attachment and invasion, none of the six lineage-specific internalin-like genes characterized here appear to contribute to invasion. Combined with the observation that some nonpathogenic Listeria species also carry internalin genes, our findings suggest a broad role of Listeria internalins, not limited to attachment and invasion of human cells. Due to the wide host range of L. monocytogenes and the fact that transcription of internalin-like genes can differ considerably depending on growth condition, elucidating the function of different internalins and internalin-like genes will remain a challenge.

Life in the gut: microbial responses to stress in the gastrointestinal tract

The complex physical and chemical conditions encountered in the gut present a range of physiological challenges to both the commensal microbiota and to pathogenic microorganisms attempting to colonise the gut. The innate immune system of the host, the host's diet and the microbial population present in the gut all contribute to the chemical complexity of the environment. The huge population of microorganisms in the gut also has a significant impact on the physicochemical properties of the gut environment. By focussing on some of the key physical and chemical stresses encountered by microorganisms in the gut, some of the molecular responses are described. Some promising new experimental approaches are outlined for studying the behaviour of microorganisms and their communities within the gut environment.

Deep RNA sequencing of L. monocytogenes reveals overlapping and extensive stationary phase and sigma B-dependent transcriptomes, including multiple highly transcribed noncoding RNAs

Background

Identification of specific genes and gene expression patterns important for bacterial survival, transmission and pathogenesis is critically needed to enable development of more effective pathogen control strategies. The stationary phase stress response transcriptome, including many σ^B-dependent genes, was defined for the human bacterial pathogen Listeria monocytogenes using RNA sequencing (RNA-Seq) with the Illumina Genome Analyzer. Specifically, bacterial transcriptomes were compared between stationary phase cells of L. monocytogenes 10403S and an otherwise isogenic ΔsigB mutant, which does not express the alternative σ factor σ^B, a major regulator of genes contributing to stress response, including stresses encountered upon entry into stationary phase.

Results

Overall, 83% of all L. monocytogenes genes were transcribed in stationary phase cells; 42% of currently annotated L. monocytogenes genes showed medium to high transcript levels under these conditions. A total of 96 genes had significantly higher transcript levels in 10403S than in ΔsigB, indicating σ^B-dependent transcription of these genes. RNA-Seq analyses indicate that a total of 67 noncoding RNA molecules (ncRNAs) are transcribed in stationary phase L. monocytogenes, including 7 previously unrecognized putative ncRNAs. Application of a dynamically trained Hidden Markov Model, in combination with RNA-Seq data, identified 65 putative σ^B promoters upstream of 82 of the 96 σ^B-dependent genes and upstream of the one σ^B-dependent ncRNA. The RNA-Seq data also enabled annotation of putative operons as well as visualization of 5'- and 3'-UTR regions.

Conclusions

The results from these studies provide powerful evidence that RNA-Seq data combined with appropriate bioinformatics tools allow quantitative characterization of prokaryotic transcriptomes, thus providing exciting new strategies for exploring transcriptional regulatory networks in bacteria.

See minireivew http://jbiol.com/content/8/12/107.

Gene transcription and virulence potential of Listeria monocytogenes strains after exposure to acidic and NaCl stress

Gene transcription and virulence potential of two strains of Listeria monocytogenes, EGD-e and 4140, were compared by quantitative real-time polymerase chain reaction and in a Caco-2 in vitro model after exposure to acidic (pH 5.5) and NaCl (4.5% w/v) stress. Strain-dependent differences in gene transcription were observed both after exposure to shock (six genes) and after long-term adaptation to stress (18 genes). In the shock experiments, a transient induction of clpC and clpE was seen for both strains, while transient induction of sigB, inlA, and inlB was observed for strain 4140 only; actA was only induced in EGD-e after NaCl shock. The long-term stress experiments were included to imitate the stress conditions encountered by L. monocytogenes when present in food products. Long-term adaptation of EGD-e to acidic stress induced transcription of iap and repressed flaA, while genes related to stress response and invasion (clpC, clpP, inlA, inlB, prfA, and sigB) were induced in 4140. Long-term adaptation of EGD-e to NaCl stress increased transcription of genes important for the intracellular life cycle (actA, hly, iap, inlA, inlB, plcA, plcB, and prfA), while few changes were observed for 4140. Experiments with Caco-2 confirmed that long-term adaptation of EGD-e and 4140 to acidic and NaCl stress is capable of increasing the virulence potential: an improved adhesion to Caco-2 was observed for both EGD-e and 4140 after acidic and NaCl stress, and increased invasion was seen for both strains after long-term NaCl stress. The fact that several virulence genes were up-regulated and that adhesion and invasion properties were increased demonstrate that certain environmental conditions in food products might influence the virulence potential of L. monocytogenes.

Some Listeria monocytogenes outbreak strains demonstrate significantly reduced invasion, inlA transcript levels, and swarming motility in vitro

Listeria monocytogenes can cause a severe invasive food-borne disease known as listeriosis, and large outbreaks of this disease occur occasionally. Based on molecular-subtype data, epidemic clone (EC) strains have been defined, including ECI and ECIa, which have caused listeriosis outbreaks on different continents. While a number of molecular-subtyping studies of outbreak strains have been reported, few comprehensive data sets of virulence-associated characteristics of these strains are available. We assembled a set of human clinical isolates from 15 outbreaks that occurred worldwide between 1975 and 2002. Initial characterization of these strains showed significant variation in the ability to invade human Caco-2 intestinal epithelial cells and HepG2 hepatic cells; four strains showed consistently reduced invasion in both cell lines. DNA sequencing of inlA, which encodes a protein required for efficient Caco-2 and HepG2 invasion, showed that none of the invasion-attenuated strains contained known virulence-attenuating mutations in inlA. Phylogenetic analyses of inlA sequences revealed a well-supported clade containing a fully invasive ECI strain and three invasion-attenuated ECI strains, along with a fully invasive ECIa strain and an invasion-attenuated ECIa strain. Of the four invasion-attenuated strains, one strain showed both reduced inlA transcript levels and impaired swarming, one strain showed reduced inlA transcript levels, and two strains showed reduced swarming. Overall, our data show that (i) L. monocytogenes strains from outbreaks vary significantly in invasion efficiency and (ii) different mechanisms may contribute to reduced invasion efficiency. Association between EC strains and listeriosis outbreaks may involve characteristics other than virulence phenotypes, including survival and growth in food-associated environments.

Listeria monocytogenes sigmaB modulates PrfA-mediated virulence factor expression

Listeria monocytogenes sigma(B) and positive regulatory factor A (PrfA) are pleiotropic transcriptional regulators that coregulate a subset of virulence genes. A positive regulatory role for sigma(B) in prfA transcription has been well established; therefore, observations of increased virulence gene expression and hemolytic activity in a DeltasigB strain initially appeared paradoxical. To test the hypothesis that L. monocytogenes sigma(B) contributes to a regulatory network critical for appropriate repression as well as induction of virulence gene expression, genome-wide transcript profiling and follow-up quantitative reverse transcriptase PCR (qRT-PCR), reporter fusion, and phenotypic experiments were conducted using L. monocytogenes prfA*, prfA* DeltasigB, DeltaprfA, and DeltaprfA DeltasigB strains. Genome-wide transcript profiling and qRT-PCR showed that in the presence of active PrfA (PrfA*), sigma(B) is responsible for reduced expression of the PrfA regulon. sigma(B)-dependent modulation of PrfA regulon expression reduced the cytotoxic effects of a PrfA* strain in HepG2 cells, highlighting the functional importance of regulatory interactions between PrfA and sigma(B). The emerging model of the role of sigma(B) in regulating overall PrfA activity includes a switch from transcriptional activation at the P2(prfA) promoter (e.g., in extracellular bacteria when PrfA activity is low) to posttranscriptional downregulation of PrfA regulon expression (e.g., in intracellular bacteria when PrfA activity is high).

Listeria as an enteroinvasive gastrointestinal pathogen

The bacterium Listeria monocytogenes is the causative agent of listeriosis, a highly fatal opportunistic foodborne infection. Listeria spp. are isolated from a diversity of environmental sources, including soil, water, effluents, a large variety of foods, and the feces of humans and animals. Recent outbreaks demonstrated that L. monocytogenes can cause gastroenteritis in otherwise healthy individuals and more severe invasive disease in immunocompromised patients. Common symptoms include fever, watery diarrhea, nausea, headache, and pains in joints and muscles. The intestinal tract is the major portal of entry for L. monocytogenes, whereby strains penetrate the mucosal tissue either directly, via invasion of enterocytes, or indirectly, via active penetration of the Peyer's patches. Studies have revealed the strategy taken by the bacteria to overcome changes in oxygen tension, osmolarity, acidity, and the sterilizing effects of bile or antimicrobial peptides to adapt to conditions in the gut. In addition, L. monocytogenes has evolved species-specific strategies for intestinal entry by exploiting the interaction between the internalin protein and its receptor E-cadherin, or inducing diarrhea and an inflammatory response via the activity of its hemolytic toxin, listeriolysin. The ability of these bacteria to survive in bile-rich environments, and to induce depletion of sentinel cells such as Paneth cells that monitor the luminal burden of commensal bacteria, suggest strategies that have evolved to promote intestinal survival. Preexisting gastrointestinal disease may be a risk factor for infection of the gastrointestinal tract with L. monocytogenes. Currently, there is enough evidence to warrant consideration of L. monocytogenes as a possible etiology in outbreaks of febrile gastroenteritis, and for further studies to examine the genetic structure of Listeria strains that have a propensity to cause gastrointestinal versus systemic infections.

SigmaB- and PrfA-dependent transcription of genes previously classified as putative constituents of the Listeria monocytogenes PrfA regulon

Mounting evidence suggests that sigma(B) and PrfA coregulate transcription of multiple genes in Listeria monocytogenes, therefore, the relative contributions of sigma(B) and PrfA to transcript levels of genes identified previously as differentially regulated by PrfA were measured. Group I genes are recognized virulence genes that are positively regulated by PrfA; group II genes were reported previously as negatively regulated by PrfA; and multiple group III genes were proposed to be coregulated by sigma(B) and PrfA. Transcript levels for selected genes were measured by quantitative reverse transcriptase polymerase chain reaction (RT-PCR) in L. monocytogenes 10403S as well as in otherwise isogenic DeltasigB, DeltaprfA, and DeltasigBDeltaprfA strains grown under conditions demonstrated to induce either PrfA activity (0.2% activated charcoal) or both PrfA and sigma(B) activity (stationary phase). Although the Group I gene plcA was positively regulated by PrfA, transcript levels for the group II genes lmo0278 and lmo0178 were not affected by the prfA deletion. While the sigB deletion significantly affected transcript levels for the selected group III genes (i.e., lmo0596, lmo0654, bsh, and opuCA), with lower transcript levels in the DeltasigB strains under all conditions tested, transcript levels for these genes were not significantly affected by the prfA deletion. Our results suggest that the regulatory interactions between PrfA and sigma(B) contribute to PrfA's predominant role as a direct regulator of virulence genes critical for invasion and intracellular survival in L. monocytogenes 10403S, while sigma(B) regulates a wider range of virulence and stress response genes.

Differential regulation of Listeria monocytogenes internalin and internalin-like genes by sigmaB and PrfA as revealed by subgenomic microarray analyses

The Listeria monocytogenes genome contains more than 20 genes that encode cell surface-associated internalins. To determine the contributions of the alternative sigma factor sigma(B) and the virulence gene regulator PrfA to internalin gene expression, a subgenomic microarray was designed to contain two probes for each of 24 internalin-like genes identified in the L. monocytogenes 10403S genome. Competitive microarray hybridization was performed on RNA extracted from (i) the 10403S parent strain and an isogenic Delta sigB strain; (ii) 10403S and an isogenic Delta prfA strain; (iii) a (G155S) 10403S derivative that expresses the constitutively active PrfA (PrfA*) and the Delta prfA strain; and (iv) 10403S and an isogenic Delta sigB Delta prfA strain. Sigma(B)- and PrfA-dependent transcription of selected genes was further confirmed by quantitative reverse-transcriptase polymerase chain reaction. For the 24 internalin-like genes examined, (i) both sigma(B) and PrfA contributed to transcription of inlA and inlB, (ii) only sigma(B) contributed to transcription of inlC2, inlD, lmo0331, and lmo0610; (iii) only PrfA contributed to transcription of inlC and lmo2445; and (iv) neither sigma(B) nor PrfA contributed to transcription of the remaining 16 internalin-like genes under the conditions tested.

Identification of components of the sigma B regulon in Listeria monocytogenes that contribute to acid and salt tolerance

Sigma B (sigma(B)) is an alternative sigma factor that controls the transcriptional response to stress in Listeria monocytogenes and is also known to play a role in the virulence of this human pathogen. In the present study we investigated the impact of a sigB deletion on the proteome of L. monocytogenes grown in a chemically defined medium both in the presence and in the absence of osmotic stress (0.5 M NaCl). Two new phenotypes associated with the sigB deletion were identified using this medium. (i) Unexpectedly, the strain with the DeltasigB deletion was found to grow faster than the parent strain in the growth medium, but only when 0.5 M NaCl was present. This phenomenon was independent of the carbon source provided in the medium. (ii) The DeltasigB mutant was found to have unusual Gram staining properties compared to the parent, suggesting that sigma(B) contributes to the maintenance of an intact cell wall. A proteomic analysis was performed by two-dimensional gel electrophoresis, using cells growing in the exponential and stationary phases. Overall, 11 proteins were found to be differentially expressed in the wild type and the DeltasigB mutant; 10 of these proteins were expressed at lower levels in the mutant, and 1 was overexpressed in the mutant. All 11 proteins were identified by tandem mass spectrometry, and putative functions were assigned based on homology to proteins from other bacteria. Five proteins had putative functions related to carbon utilization (Lmo0539, Lmo0783, Lmo0913, Lmo1830, and Lmo2696), while three proteins were similar to proteins whose functions are unknown but that are known to be stress inducible (Lmo0796, Lmo2391, and Lmo2748). To gain further insight into the role of sigma(B) in L. monocytogenes, we deleted the genes encoding four of the proteins, lmo0796, lmo0913, lmo2391, and lmo2748. Phenotypic characterization of the mutants revealed that Lmo2748 plays a role in osmotolerance, while Lmo0796, Lmo0913, and Lmo2391 were all implicated in acid stress tolerance to various degrees. Invasion assays performed with Caco-2 cells indicated that none of the four genes was required for mammalian cell invasion. Microscopic analysis suggested that loss of Lmo2748 might contribute to the cell wall defect observed in the DeltasigB mutant. Overall, this study highlighted two new phenotypes associated with the loss of sigma(B). It also demonstrated clear roles for sigma(B) in both osmotic and low-pH stress tolerance and identified specific components of the sigma(B) regulon that contribute to the responses observed.

inlA premature stop codons are common among Listeria monocytogenes isolates from foods and yield virulence-attenuated strains that confer protection against fully virulent strains

Previous studies showed that a considerable proportion of Listeria monocytogenes isolates obtained from foods carry a premature stop codon (PMSC) mutation in inlA that leads to production of a truncated and secreted InlA. To further elucidate the role these mutations play in virulence of L. monocytogenes, we created isogenic mutants, including (i) natural isolates where an inlA PMSC was reverted to a wild-type inlA allele (without a PMSC) and (ii) natural isolates where a PMSC mutation was introduced into a wild-type inlA allele; isogenic mutant sets were constructed to represent two distinct inlA PMSC mutations. Phenotypical and transcriptional analysis data showed that inlA PMSC mutations do not have a polar effect on the downstream inlB. Isogenic and natural strains carrying an inlA PMSC showed significantly reduced invasion efficiencies in Caco-2 and HepG2 cell lines as well as reduced virulence in oral guinea pig infections. Guinea pigs were also orally infected with a natural strain carrying the most common inlA PMSC mutation (vaccinated group), followed by challenge with a fully virulent L. monocytogenes strain 15 days postvaccination to probe potentially immunizing effects of exposure to L. monocytogenes with inlA PMSC mutations. Vaccinated guinea pigs showed reduced bacterial loads in internal organs and improved weight gain postchallenge, indicating reduced severity of infections in guinea pigs exposed to natural strains with inlA PMSC mutations. Our data support that (i) inlA PMSC mutations are causally associated with attenuated virulence in mammalian hosts and (ii) naturally occurring virulence-attenuated L. monocytogenes strains commonly found in food confer protective immunity.

Modulation of stress and virulence in Listeria monocytogenes

Listeria monocytogenes can respond rapidly to changing environmental conditions, as illustrated by its ability to transition from a saprophyte to an orally transmitted facultative intracellular pathogen. Differential associations between various alternative sigma factors and a core RNA polymerase provide a transcriptional mechanism for regulating bacterial gene expression that is crucial for survival in rapidly changing conditions. Alternative sigma factors are key components of complex L. monocytogenes regulatory networks that include multiple transcriptional regulators of stress-response and virulence genes, regulation of genes encoding other regulators, and regulation of small RNAs. In this article, the contributions of various sigma factors to L. monocytogenes stress response and virulence are described.

Temporal transcriptomic analysis of the Listeria monocytogenes EGD-e sigmaB regulon

Background

The opportunistic food-borne gram-positive pathogen Listeria monocytogenes can exist as a free-living microorganism in the environment and grow in the cytoplasm of vertebrate and invertebrate cells following infection. The general stress response, controlled by the alternative sigma factor, σ^B, has an important role for bacterial survival both in the environment and during infection. We used quantitative real-time PCR analysis and immuno-blot analysis to examine σ^B expression during growth of L. monocytogenes EGD-e. Whole genome-based transcriptional profiling was used to identify σ^B-dependent genes at different growth phases.

Results

We detected 105 σ^B-positively regulated genes and 111 genes which appeared to be under negative control of σ^B and validated 36 σ^B-positively regulated genes in vivo using a reporter gene fusion system.

Conclusion

Genes comprising the σ^B regulon encode solute transporters, novel cell-wall proteins, universal stress proteins, transcriptional regulators and include those involved in osmoregulation, carbon metabolism, ribosome- and envelope-function, as well as virulence and niche-specific survival genes such as those involved in bile resistance and exclusion. Ten of the σ^B-positively regulated genes of L. monocytogenes are absent in L. innocua. A total of 75 σ^B-positively regulated listerial genes had homologs in B. subtilis, but only 33 have been previously described as being σ^B-regulated in B. subtilis even though both species share a highly conserved σ^B-dependent consensus sequence. A low overlap of genes may reflects adaptation of these bacteria to their respective environmental conditions.

Comparative analysis of the sigma B-dependent stress responses in Listeria monocytogenes and Listeria innocua strains exposed to selected stress conditions

The alternative sigma factor sigma(B) contributes to transcription of stress response and virulence genes in diverse gram-positive bacterial species. The composition and functions of the Listeria monocytogenes and Listeria innocua sigma(B) regulons were hypothesized to differ due to virulence differences between these closely related species. Transcript levels in stationary-phase cells and in cells exposed to salt stress were characterized by microarray analyses for both species. In L. monocytogenes, 168 genes were positively regulated by sigma(B); 145 of these genes were preceded by a putative sigma(B) consensus promoter. In L. innocua, 64 genes were positively regulated by sigma(B). sigma(B) contributed to acid stress survival in log-phase cells for both species but to survival in stationary-phase cells only for L. monocytogenes. In summary, (i) the L. monocytogenes sigma(B) regulon includes >140 genes that are both directly and positively regulated by sigma(B), including genes encoding proteins with importance in stress response, virulence, transcriptional regulation, carbohydrate metabolism, and transport; (ii) a number of L. monocytogenes genes encoding flagellar proteins show higher transcript levels in the Delta sigB mutant, and both L. monocytogenes and L. innocua Delta sigB null mutants have increased motility compared to the respective isogenic parent strains, suggesting that sigma(B) affects motility and chemotaxis; and (iii) although L. monocytogenes and L. innocua differ in sigma(B)-dependent acid stress resistance and have species-specific sigma(B)-dependent genes, the L. monocytogenes and L. innocua sigma(B) regulons show considerable conservation, with a common set of at least 49 genes that are sigma(B) dependent in both species.

Resistance of Listeria monocytogenes F2365 cells to synthetic gastric fluid is greater following growth on ready-to-eat deli turkey meat than in brain heart infusion broth

Ready-to-eat (RTE) deli meats have been categorized as high-risk foods for contraction of foodborne listeriosis. Several recent listeriosis outbreaks have been associated with the consumption of RTE deli turkey meat. In this study, we examined whether the growth of Listeria monocytogenes F2365 on commercially prepared RTE deli turkey meat causes listerial cells to become more resistant to inactivation by synthetic gastric fluid (SGF). Listerial cells grown on turkey meat to late logarithmic-early stationary phase were significantly more resistant to SGF at pH 7.0, 5.0, or 3.5 than listerial cells grown in brain heart infusion (BHI) broth. The pH was lower in the fluid in packages of turkey meat than in BHI broth (6.5 versus 7.5). However, listerial cells grown in BHI broth adjusted to a lower pH (6.0) did not exhibit enhanced resistance to SGF. The lesser resistance to SGF of listerial cells grown in BHI broth may be due, in part, to the presence of glucose (0.2%). This study indicates the environment presented by the growth of L. monocytogenes on deli turkey meat affects its ability to survive conditions it encounters in the gastrointestinal tract.

Phenotypic and transcriptomic analyses demonstrate interactions between the transcriptional regulators CtsR and Sigma B in Listeria monocytogenes

Listeria monocytogenes sigma(B) positively regulates the transcription of class II stress response genes; CtsR negatively regulates class III stress response genes. To identify interactions between these two stress response systems, we constructed L. monocytogenes DeltactsR and DeltactsR DeltasigB strains, as well as a DeltactsR strain expressing ctsR in trans under the control of an IPTG (isopropyl-beta-d-thiogalactopyranoside)-inducible promoter. These strains, along with a parent and a DeltasigB strain, were assayed for motility, heat resistance, and invasion of human intestinal epithelial cells, as well as by whole-genome transcriptomic and quantitative real-time PCR analyses. Both DeltactsR and DeltactsR DeltasigB strains had significantly higher thermotolerances than the parent strain; however, full heat sensitivity was restored to the DeltactsR strain when ctsR was expressed in trans. Although log-phase DeltactsR was not reduced in its ability to infect human intestinal cells, the DeltactsR DeltasigB strain showed significantly lower invasion efficiency than either the parent strain or the DeltasigB strain, indicating that interactions between CtsR and sigma(B) contribute to invasiveness. Statistical analyses also confirmed interactions between the ctsR and the sigB null mutations in both heat resistance and invasion phenotypes. Microarray transcriptomic analyses and promoter searches identified (i) 42 CtsR-repressed genes, (ii) 22 genes with lower transcript levels in the DeltactsR strain, and (iii) at least 40 genes coregulated by both CtsR and sigma(B), including genes encoding proteins with confirmed or plausible roles in virulence and stress response. Our data demonstrate that interactions between CtsR and sigma(B) play an important role in L. monocytogenes stress resistance and virulence.

Toward an improved laboratory definition of Listeria monocytogenes virulence

Listeria monocytogenes is an opportunistic foodborne pathogen that encompasses a diversity of strains with varied virulence. The ability to rapidly determine the pathogenic potential of L. monocytogenes strains is integral to the control and prevention campaign against listeriosis. Early methods for assessing L. monocytogenes virulence include in vivo bioassays and in vitro cell assays. While in vivo bioassays provide a measurement of all virulence determinants of L. monocytogenes, they are not applied routinely due to their reliance on experimental animals whose costs have become increasingly prohibitive. As a low cost alternative, in vitro cell assays are useful for estimating the virulence of L. monocytogenes strains. However, these assays are often slow, and at times variable. Prior attempts to ascertain L. monocytogenes virulence by targeting virulence-associated proteins and genes have been largely unsuccessful, since many of the assay targets are present in both virulent and avirulent strains. Recent identification of novel virulence-specific genes (particularly internalin gene inlJ) has opened a new avenue for rapid, sensitive, and precise differentiation of virulent L. monocytogenes strains from avirulent strains. The application of DNA sequencing technique also offers an additional tool for assessing L. monocytogenes virulence potential. By providing an update on the laboratory methods that have been reported for the determination of L. monocytogenes pathogenicity, this review discusses future research needs that may help achieve an improved laboratory definition of L. monocytogenes virulence.

Internalins: a complex family of leucine-rich repeat-containing proteins in Listeria monocytogenes

The Listeria monocytogenes genome includes a large family of proteins harbouring leucine-rich repeats known as internalins (Inl). The generation of novel mutants and comparative analysis of Inl variability among Listeria and other bacterial genomes suggest that beyond the extensively-studied invasins, InlA and InlB, additional internalins also play important functions in the infectious process.

Oxygen restriction increases the infective potential of Listeria monocytogenes in vitro in Caco-2 cells and in vivo in guinea pigs

Background

Listeria monocytogenes has been implicated in several food borne outbreaks as well as sporadic cases of disease. Increased understanding of the biology of this organism is important in the prevention of food borne listeriosis.

The infectivity of Listeria monocytogenes ScottA, cultivated with and without oxygen restriction, was compared in vitro and in vivo. Fluorescent protein labels were applied to allow certain identification of Listeria cells from untagged bacteria in in vivo samples, and to distinguish between cells grown under different conditions in mixed infection experiments.

Results

Infection of Caco-2 cells revealed that Listeria cultivated under oxygen-restricted conditions were approximately 100 fold more invasive than similar cultures grown without oxygen restriction. This was observed for exponentially growing bacteria, as well as for stationary-phase cultures.

Oral dosage of guinea pigs with Listeria resulted in a significantly higher prevalence (p < 0.05) of these bacteria in jejunum, liver and spleen four and seven days after challenge, when the bacterial cultures had been grown under oxygen-restricted conditions prior to dosage. Additionally, a 10–100 fold higher concentration of Listeria in fecal samples was observed after dosage with oxygen-restricted bacteria. These differences were seen after challenge with single Listeria cultures, as well as with a mixture of two cultures grown with and without oxygen restriction.

Conclusion

Our results show for the first time that the environmental conditions to which L. monocytogenes is exposed prior to ingestion are decisive for its in vivo infective potential in the gastrointestinal tract after passage of the gastric barrier. This is highly relevant for safety assessment of this organism in food.

The PrfA virulence regulon

The PrfA protein, a member of the Crp/Cap-Fnr family of bacterial transcription factors, controls the expression of key virulence determinants of the facultative intracellular pathogen Listeria monocytogenes. Each of the steps of the listerial intracellular infection cycle-host cell invasion, phagosomal escape, cytosolic replication, and direct cell-to-cell spread-is mediated by products of the PrfA regulon. Only 10 of the 2853 genes of the L. monocytogenes EGDe genome have been confirmed as bona fide (directly regulated) members of this regulon, a number surprisingly small given the apparent complexity of listerial intracellular parasitism. PrfA activates transcription by binding as a dimer to a palindromic promoter element of canonical sequence tTAACanntGTtAa, with seven invariant nucleotides (in capitals) and a two-mismatch tolerance. PrfA integrates a number of environmental and bacteria-derived signals to ensure the correct spatio-temporal and niche-adapted expression of the regulon, with maximum induction in the host cell cytosol and repression in the environmental habitat. Regulation operates through changes in PrfA activity-presumably by cofactor-mediated allosteric shift-and concentration, and involves transcriptional, translational and post-translational control mechanisms. There is evidence that PrfA exerts a more global influence on L. monocytogenes physiology via indirect mechanisms.

Temperature-dependent expression of Listeria monocytogenes internalin and internalin-like genes suggests functional diversity of these proteins among the listeriae

The Listeria monocytogenes genome contains genes encoding several internalins and internalin-like proteins. As L. monocytogenes is present in many environments and can infect numerous, diverse host species, the environmental temperature was hypothesized to be a signal that might affect internalin gene transcription. A subgenomic microarray was used to investigate temperature-dependent transcription of 24 members of the internalin gene family in L. monocytogenes 10403S. The levels of internalin gene transcripts for cells grown at 37 degrees C were compared to the levels of transcripts for cells grown at 16, 30, and 42 degrees C using competitive microarray hybridization, and the results were confirmed by performing quantitative reverse transcriptase PCR for 14 internalin genes. Based on these studies, the internalin genes can be grouped into the following five temperature-dependent categories: (i) four sigma(B)-dependent internalin genes (inlC2, inlD, lmo0331, and lmo0610) with the highest levels of transcripts at 16 degrees C and generally the lowest levels of transcripts at 37 degrees C; (ii) three partially PrfA-dependent internalin genes (inlA, inlB, and inlC) with the lowest levels of transcripts at 16 degrees C and the highest levels of transcripts at 37 and 42 degrees C; (iii) four genes (inlG, inlJ, lmo0514, and lmo1290) with the lowest levels of transcripts at 16 degrees C and the highest levels of transcripts at 30 and/or 37 degrees C; (iv) one gene (lmo0327) with the highest levels of transcripts at 16 degrees C and low levels of transcripts at higher temperatures; and (v) 12 internalin genes with no differences in the levels of transcripts at the temperatures used in this study. The temperature-dependent transcription patterns suggest that the relative importance of different internalins varies by environment, which may provide insight into the specific functions of these proteins.

The alternative sigma factor sigma B and the virulence gene regulator PrfA both regulate transcription of Listeria monocytogenes internalins

Some Listeria monocytogenes internalins are recognized as contributing to invasion of mammalian tissue culture cells. While PrfA is well established as a positive regulator of L. monocytogenes virulence gene expression, the stress-responsive sigma(B) has been recognized only recently as contributing to expression of virulence genes, including some that encode internalins. To measure the relative contributions of PrfA and sigma(B) to internalin gene transcription, we used reverse transcription-PCR to quantify transcript levels for eight internalin genes (inlA, inlB, inlC, inlC2, inlD, inlE, inlF, and inlG) in L. monocytogenes 10403S and in isogenic Delta prfA, Delta sigB, and Delta sigB Delta prfA strains. Strains were grown under defined conditions to produce (i) active PrfA, (ii) active sigma(B) and active PrfA, (iii) inactive PrfA, and (iv) active sigma(B) and inactive PrfA. Under the conditions tested, sigma(B) and PrfA contributed differentially to the expression of the various internalins such that (i) both sigma(B) and PrfA contributed to inlA and inlB transcription, (ii) only PrfA contributed to inlC transcription, (iii) only sigma(B) contributed to inlC2 and inlD transcription, and (iv) neither sigma(B) nor PrfA contributed to inlF and inlG transcription. inlE transcript levels were undetectable. The important role for sigma(B) in regulating expression of L. monocytogenes internalins suggests that exposure of this organism to environmental stress conditions, such as those encountered in the gastrointestinal tract, may activate internalin transcription. Interplay between sigma(B) and PrfA also appears to be critical for regulating transcription of some virulence genes, including inlA, inlB, and prfA.

Evaluation of a tissue culture-based approach for differentiating between virulent and avirulent Vibrio parahaemolyticus strains based on cytotoxicity

The ability of only a subset of Vibrio parahaemolyticus strains to cause human infection underscores the need for an analytical method that can effectively differentiate between pathogenic strains and those that do not cause disease. We tested the feasibility of a tissue culture-based assay to determine whether clinical isolates could be differentiated from nonclinical isolates based on relative isolate cytopathogenicity. To screen for cytotoxic capability, we measured relative extracellular lactate dehydrogenase as an indicator of host cell damage in five different mammalian cell lines in the presence of V. parahaemolyticus. Isolates originating from clinical sources exhibited 15.5 to 59.3% relative cytotoxicity, whereas those originating from food sources exhibited 4.4 to 54.9% relative cytotoxicity. In the presence of -1.2 x 10(6) cells, cytotoxicity was 1.6- to 3.5-fold higher (P < 0.05) for clinical isolates than for nonclinical isolates in L2, Henle 407, and Caco-2 cell lines. V. parahaemolyticus serotype O3:K6 clinical isolates had 1.6- to 2.1-fold higher cytotoxicity than did the non-O3:K6 clinical isolates, with significantly higher cytotoxicity in HeLa, J774A.1, and Henle 407 cells than in L2 and Caco-2 cells. Because V. parahaemolyticus often is found in oysters, the effect of the presence of an oyster matrix on assay efficacy was also tested with L2 cells. The cytotoxicity elicited by a highly cytotoxic V. parahaemolyticus isolate was not affected by the presence of oyster tissue, suggesting that an oyster matrix will not interfere with assay sensitivity. In the present format, this assay can detect the presence of > 10(5) cells of a virulent V. parahaemolyticus strain in an oyster matrix.

Listeria monocytogenes F2365 carries several authentic mutations potentially leading to truncated gene products, including inlB, and demonstrates atypical phenotypic characteristics

Searches of the genome annotation of Listeria monocytogenes F2365, an isolate from the 1985 listeriosis epidemic in California, showed that this strain carries 20 authentic mutations resulting in premature stop codons, including a nonsense mutation in inlB. Here we showed that L. monocytogenes F2365 demonstrates atypical virulence-associated characteristics, including significantly (P < 0.05) reduced invasion efficiency in Caco-2 cells as compared with a closely related lineage I serotype 4b strain as well as significantly (P < 0.05) greater variation in invasiveness when grown under different conditions compared with standard laboratory control and other lineage I serotype 4b strains. In addition, L. monocytogenes F2365 demonstrated distinct growth characteristics, including a significantly (P < 0.05) reduced exponential growth rate when compared with laboratory control and other lineage I serotype 4b outbreak-associated strains as well as a significantly (P < 0.05) longer lag phase duration time compared with another lineage I serotype 4b strain. Our results support that L. monocytogenes F2365 is characterized by genotypic and phenotypic properties that are atypical of other L. monocytogenes strains.

Identification of new noncoding RNAs in Listeria monocytogenes and prediction of mRNA targets

To identify noncoding RNAs (ncRNAs) in the pathogenic bacterium Listeria monocytogenes, we analyzed the intergenic regions (IGRs) of strain EGD-e by in silico-based approaches. Among the twelve ncRNAs found, nine are novel and specific to the Listeria genus, and two of these ncRNAs are expressed in a growth-dependent manner. Three of the ncRNAs are transcribed in opposite direction to overlapping open reading frames (ORFs), suggesting that they act as antisense on the corresponding mRNAs. The other ncRNA genes appear as single transcription units. One of them displays five repeats of 29 nucleotides. Five of these new ncRNAs are absent from the non-pathogenic species L. innocua, raising the possibility that they might be involved in virulence. To predict mRNA targets of the ncRNAs, we developed a computational method based on thermodynamic pairing energies and known ncRNA–mRNA hybrids. Three ncRNAs, including one of the putative antisense ncRNAs, were predicted to have more than one mRNA targets. Several of them were shown to bind efficiently to the ncRNAs suggesting that our in silico approach could be used as a general tool to search for mRNA targets of ncRNAs.

SigmaB activation under environmental and energy stress conditions in Listeria monocytogenes

To measure sigmaB activation in Listeria monocytogenes under environmental or energy stress conditions, quantitative reverse transcriptase PCR (TaqMan) was used to determine the levels of transcripts for the sigmaB -dependent opuCA and clpC genes in strains having null mutations in genes encoding regulator of sigma B proteins (rsbT and rsbV) and sigma B (sigB) and in the L. monocytogenes wild-type 10403S strain under different stress conditions. The DeltasigB, DeltarsbT, and DeltarsbV strains previously exhibited increased hemolytic activities compared to the hemolytic activity of the wild-type strain; therefore, transcript levels for hly were also determined. RsbT, RsbV, and sigmaB were all required for opuCA expression during growth under carbon-limiting conditions or following exposure to pH 4.5, salt, ethanol, or the protonophore carbonyl cyanide m-chlorophenylhydrazone (CCCP). Expression of clpC was RsbT, RsbV, and sigmaB dependent in the presence of CCCP but not under the other conditions. hly expression was not RsbT, RsbV, or sigmaB dependent in the presence of either CCCP or salt. opuCA transcript levels did not increase in the presence of rapidly lethal stresses (i.e., pH 2.5 or 13 mM cumene hydroperoxide) despite the enhanced survival of the wild type compared with the survival of the mutant strains under these conditions. These findings highlight the importance of complementing phenotypic characterizations with gene expression studies to identify direct and indirect effects of null mutations in regulatory genes, such as sigB. Overall, our data show that while sigmaB activation occurs through a single pathway under both environmental and energy stress conditions, regulation of expression of some stress response and virulence genes in the sigmaB regulon (e.g., clpC) appears to require networks involving multiple transcriptional regulators.