SigmaB-dependent and sigmaB-independent mechanisms contribute to transcription of Listeria monocytogenes cold stress genes during cold shock and cold growth

Antimicrobial Activity of Sertraline on Listeria monocytogenes

We explored the antimicrobial activity of sertraline on Listeria monocytogenes and further investigated the effects of sertraline on biofilm formation and the virulence gene expression of L. monocytogenes. The minimum inhibitory concentration and minimum bactericidal concentration for sertraline against L. monocytogenes were in the range of 16-32 μg/mL and 64 μg/mL, respectively. Sertraline-dependent damage of the cell membrane and a decrease in intracellular ATP and pH_in in L. monocytogenes were observed. In addition, sertraline reduced the biofilm formation efficiency of the L. monocytogenes strains. Importantly, low concentrations (0.1 μg/mL and 1 μg/mL) of sertraline significantly down-regulated the expression levels of various L. monocytogens virulence genes (prfA, actA, degU, flaA, sigB, ltrC and sufS). These results collectively suggest a role of sertraline for the control of L. monocytogenes in the food industry.

Dual RNA sequencing of group B Streptococcus-infected human monocytes reveals new insights into host-pathogen interactions and bacterial evasion of phagocytosis

Streptococcus agalactiae, also known as Group B Streptococcus (GBS) is a frequent cause of infections, including bacteraemia and other acute diseases in adults and immunocompromised individuals. We developed a novel system to study GBS within human monocytes to define the co-transcriptome of intracellular GBS (iGBS) and host cells simultaneously using dual RNA-sequencing (RNA-seq) to better define how this pathogen responds to host cells. Using human U937 monocytes and genome-sequenced GBS reference strain 874,391 in antibiotic protection assays we validated a system for dual-RNA seq based on measures of GBS and monocyte viability to ensure that the bacterial and host cell co-transcriptome reflected mainly intracellular (iGBS) rather than extracellular GBS. Elucidation of the co-transcriptome revealed 1119 dysregulated transcripts in iGBS with most genes, including several that encode virulence factors (e.g., scpB, hvgA, ribD, pil2b) exhibiting activation by upregulated expression. Infection with iGBS resulted in significant remodelling of the monocyte transcriptome, with 7587 transcripts differentially expressed including 7040 up-regulated and 547 down-regulated. qPCR confirmed that the most strongly activated genes included sht, encoding Streptococcal Histidine Triad Protein. An isogenic GBS mutant strain deficient in sht revealed a significant effect of this gene on phagocytosis of GBS and survival of the bacteria during systemic infection in mice. Identification of a novel contribution of sht to GBS virulence shows the co-transcriptome responses elucidated in GBS-infected monocytes help to shape the host-pathogen interaction and establish a role for sht in the response of the bacteria to phagocytic uptake. This study provides comprehension of concurrent transcriptional responses that occur in GBS and human monocytes that shape the host-pathogen interaction.

Listeria monocytogenes - How This Pathogen Survives in Food-Production Environments?

The foodborne pathogen Listeria monocytogenes is the causative agent of human listeriosis, a severe disease, especially dangerous for the elderly, pregnant women, and newborns. Although this infection is comparatively rare, it is often associated with a significant mortality rate of 20-30% worldwide. Therefore, this microorganism has an important impact on food safety. L. monocytogenes can adapt, survive and even grow over a wide range of food production environmental stress conditions such as temperatures, low and high pH, high salt concentration, ultraviolet lights, presence of biocides and heavy metals. Furthermore, this bacterium is also able to form biofilm structures on a variety of surfaces in food production environments which makes it difficult to remove and allows it to persist for a long time. This increases the risk of contamination of food production facilities and finally foods. The present review focuses on the key issues related to the molecular mechanisms of the pathogen survival and adaptation to adverse environmental conditions. Knowledge and understanding of the L. monocytogenes adaptation approaches to environmental stress factors will have a significant influence on the development of new, efficient, and cost-effective methods of the pathogen control in the food industry, which is critical to ensure food production safety.

Pathogenicity and virulence of Listeria monocytogenes: A trip from environmental to medical microbiology

Listeria monocytogenes is a saprophytic gram-positive bacterium, and an opportunistic foodborne pathogen that can produce listeriosis in humans and animals. It has evolved an exceptional ability to adapt to stress conditions encountered in different environments, resulting in a ubiquitous distribution. Because some food preservation methods and disinfection protocols in food-processing environments cannot efficiently prevent contaminations, L. monocytogenes constitutes a threat to human health and a challenge to food safety. In the host, Listeria colonizes the gastrointestinal tract, crosses the intestinal barrier, and disseminates through the blood to target organs. In immunocompromised individuals, the elderly, and pregnant women, the pathogen can cross the blood-brain and placental barriers, leading to neurolisteriosis and materno-fetal listeriosis. Molecular and cell biology studies of infection have proven L. monocytogenes to be a versatile pathogen that deploys unique strategies to invade different cell types, survive and move inside the eukaryotic host cell, and spread from cell to cell. Here, we present the multifaceted Listeria life cycle from a comprehensive perspective. We discuss genetic features of pathogenic Listeria species, analyze factors involved in food contamination, and review bacterial strategies to tolerate stresses encountered both during food processing and along the host's gastrointestinal tract. Then we dissect host-pathogen interactions underlying listerial pathogenesis in mammals from a cell biology and systemic point of view. Finally, we summarize the epidemiology, pathophysiology, and clinical features of listeriosis in humans and animals. This work aims to gather information from different fields crucial for a comprehensive understanding of the pathogenesis of L. monocytogenes.

Development of a database and standardized approach for rpoB sequence-based subtyping and identification of aerobic spore-forming Bacillales

Aerobic spore-forming Bacillales are a highly diverse and ubiquitous group that includes organisms that cause foodborne illnesses and food spoilage. Classical microbiological and biochemical identification of members of the order Bacillales represents a challenge due to the diversity of organisms in this group as well as the fact that the phenotypic-based taxonomic assignment of some named species in this group is not consistent with their phylogenomic characteristics. DNA-sequencing-based tools, on the other hand, can be fast and cost-effective, and can provide for a more reliable identification and characterization of Bacillales isolates. In comparison to 16S rDNA, rpoB was shown to better discriminate between Bacillales isolates and to allow for improved taxonomic assignment to the species level. However, the lack of a publicly accessible rpoB database, as well as the lack of standardized protocols for rpoB-based typing and strain identification, is a major challenge. Here, we report (i) the curation of a DNA sequence database for rpoB-based subtype classification of Bacillales isolates; (ii) the development of standardized protocols for generating rpoB sequence data, and a scheme for rpoB-based initial taxonomic identification of Bacillales isolates at the species level; and (iii) the integration of the database in a publicly accessible online platform that allows for the analysis of rpoB sequence data from uncharacterized Bacillales isolates. Specifically, we curated a database of DNA sequences for a 632-nt internal variable region within the rpoB gene from representative Bacillales reference type strains and a large number of isolates that we have previously isolated and characterized through multiple projects. As of May 21, 2021, the rpoB database contained more than 8350 rpoB sequences representing 1902 distinct rpoB allelic types that can be classified into 160 different genera. The database also includes 1129 rpoB sequences for representative Bacillales reference type strains as available on May 21, 2021 in the NCBI database. The rpoB database is integrated into the online Food Microbe Tracker platform (www.foodmicrobetracker.com) and can be queried using the integrated BLAST tool to initially subtype and taxonomically identify aerobic and facultative anaerobic spore-formers. While whole-genome sequencing is increasingly used in bacterial taxonomy, the rpoB sequence-based identification scheme described here provides a valuable tool as it allows for rapid and cost-effective initial isolate characterization, which can help to identify and characterize foodborne pathogens and food spoilage bacteria. In addition, the database and primers described here can also be adopted for metagenomics approaches that include rpoB as a target, improving discriminatory power and identification over what can be achieved using 16S rDNA as a target.

Proteomic Analysis of Listeria monocytogenes FBUNT During Biofilm Formation at 10°C in Response to Lactocin AL705

Listeria monocytogenes is one of the major food-related pathogens and is able to survive and multiply under different stress conditions. Its persistence in industrial premises and foods is partially due to its ability to form biofilm. Thus, as a natural strategy to overcome L. monocytogenes biofilm formation, the treatment with lactocin AL705 using a sublethal dose (20AU/ml) was explored. The effect of the presence of the bacteriocin on the biofilm formation at 10°C of L. monocytogenes FBUNT was evaluated for its proteome and compared to the proteomes of planktonic and sessile cells grown at 10°C in the absence of lactocin. Compared to planktonic cells, adaptation of sessile cells during cold stress involved protein abundance shifts associated with ribosomes function and biogenesis, cell membrane functionality, carbohydrate and amino acid metabolism, and transport. When sessile cells were treated with lactocin AL705, proteins’ up-regulation were mostly related to carbohydrate metabolism and nutrient transport in an attempt to compensate for impaired energy generation caused by bacteriocin interacting with the cytoplasmic membrane. Notably, transport systems such as β-glucosidase IIABC (lmo0027), cellobiose (lmo2763), and trehalose (lmo1255) specific PTS proteins were highly overexpressed. In addition, mannose (lmo0098), a specific PTS protein indicating the adaptive response of sessile cells to the bacteriocin, was downregulated as this PTS system acts as a class IIa bacteriocin receptor. A sublethal dose of lactocin AL705 was able to reduce the biofilm formation in L. monocytogenes FBUNT and this bacteriocin induced adaptation mechanisms in treated sessile cells. These results constitute valuable data related to specific proteins targeting the control of L. monocytogenes biofilm upon bacteriocin treatment.

Impact of osmotic stress on the phosphorylation and subcellular location of Listeria monocytogenes stressosome proteins

Listeria monocytogenes responds to environmental stress using a supra-macromolecular complex, the stressosome, to activate the stress sigma factor SigB. The stressosome structure, inferred from in vitro-assembled complexes, consists of the core proteins RsbR (here renamed RsbR1) and RsbS and, the kinase RsbT. The active complex is proposed to be tethered to the membrane and to support RsbR1/RsbS phosphorylation by RsbT and the subsequent release of RsbT following signal perception. Here, we show in actively-growing cells that L. monocytogenes RsbR1 and RsbS localize mostly in the cytosol in a fully phosphorylated state regardless of osmotic stress. RsbT however distributes between cytosolic and membrane-associated pools. The kinase activity of RsbT on RsbR1/RsbS and its requirement for maximal SigB activation in response to osmotic stress were demonstrated in vivo. Cytosolic RsbR1 interacts with RsbT, while this interaction diminishes at the membrane when RsbR1 paralogues (RsbR2, RsbR3 and RsbL) are present. Altogether, the data support a model in which phosphorylated RsbR1/RsbS may sustain basal SigB activity in unstressed cells, probably assuring a rapid increase in such activity in response to stress. Our findings also suggest that in vivo the active RsbR1-RsbS-RsbT complex forms only transiently and that membrane-associated RsbR1 paralogues could modulate its assembly.

Transcriptomic and Phenotypic Analyses of the Sigma B-Dependent Characteristics and the Synergism between Sigma B and Sigma L in Listeria monocytogenes EGD-e

Numerous gene expression and stress adaptation responses in L. monocytogenes are regulated through alternative sigma factors σ^B and σ^L. Stress response phenotypes and transcriptomes were compared between L. monocytogenes EGD-e and its ΔsigB and ΔsigBL mutants. Targeted growth phenotypic analysis revealed that the ΔsigB and ΔsigBL mutants are impaired during growth under cold and organic-acid stress conditions. Phenotypic microarrays revealed increased sensitivity in both mutants to various antimicrobial compounds. Genes de-regulated in these two mutants were identified by genome-wide transcriptome analysis during exponential growth in BHI. The ΔsigB and ΔsigBL strains repressed 198 and 254 genes, respectively, compared to the parent EGD-e strain at 3 °C, whereas 86 and 139 genes, respectively, were repressed in these mutants during growth at 37 °C. Genes repressed in these mutants are involved in various cellular functions including transcription regulation, energy metabolism and nutrient transport functions, and viral-associated processes. Exposure to cold stress induced a significant increase in σ^B and σ^L co-dependent genes of L. monocytogenes EGD-e since most (62%) of the down-regulated genes uncovered at 3 °C were detected in the ΔsigBL double-deletion mutant but not in ΔsigB or ΔsigL single-deletion mutants. Overall, the current study provides an expanded insight into σ^B and σ^L phenotypic roles and functional interactions in L. monocytogenes. Besides previously known σ^B- and σ^L-dependent genes, the transcriptomes defined in ΔsigB and ΔsigBL mutants reveal several new genes that are positively regulated by σ^B alone, as well as those co-regulated through σ^B- and σ^L-dependent mechanisms during L. monocytogenes growth under optimal and cold-stress temperature conditions.

Transcriptome Sequencing of Listeria monocytogenes Reveals Major Gene Expression Changes in Response to Lactic Acid Stress Exposure but a Less Pronounced Response to Oxidative Stress

Listeria monocytogenes is a well-characterized pathogen that represents a major threat to food safety. In this study, we examine the chromosomal and plasmid transcriptomes of two different L. monocytogenes strains, 6179 [belonging to sequence type (ST) 121] and R479a (ST8), in response to 30 min exposure to oxidative (0.01% hydrogen peroxide) and acid (1% lactic acid, pH 3.4) stress. The exposure to oxidative stress resulted in 102 and 9 differentially expressed (DE) genes in the chromosomal transcriptomes of 6179 and R479a, respectively. In contrast, 2280 and 2151 DE genes were observed in the respective chromosomal transcriptomes of 6179 and R479a in response to lactic acid stress. During lactic acid stress, we observed upregulation of numerous genes known to be involved in the L. monocytogenes stress response, including multiple members of the σ^B regulon, many of which have not been functionally characterized. Among these genes, homologs of lmo2230 were highly upregulated in both strains. Most notably, the σ^B-dependent non-coding RNA Rli47 was by far the most highly expressed gene in both 6179 and R479a, accounting for an average of 28 and 38% of all mapped reads in the respective chromosomal transcriptomes. In response to oxidative stress, one DE gene was identified in the 6179 plasmid transcriptome, and no DE genes were observed in the transcriptome of the R479a plasmid. However, lactic acid exposure resulted in upregulation of the stress response gene clpL, among others, on the 6179 plasmid. In R479a, a number of uncharacterized plasmid genes were upregulated, indicating a potential role in stress response. Furthermore, an average of 65% of all mapped transcriptome reads for the R479a plasmid following acid stress were mapped to an intergenic region bearing similarity to riboswitches involved in transition metal resistance. The results of this study support the conclusion that members of the σ^B regulon, particularly lmo2230 and the non-coding RNA Rli47, play an integral role in the response of L. monocytogenes to acid stress. Furthermore, we report the first global transcriptome sequencing analysis of L. monocytogenes plasmid gene expression and identify a putative, plasmid-encoded riboswitch with potential involvement in response to acid exposure.

Listeria monocytogenes Biofilm Adaptation to Different Temperatures Seen Through Shotgun Proteomics

Listeria monocytogenes is a foodborne pathogen that can cause invasive severe human illness (listeriosis) in susceptible patients. Most human listeriosis cases appear to be caused by consumption of refrigerated ready-to-eat foods. Although initial contamination levels in foods are usually low, the ability of these bacteria to survive and multiply at low temperatures allows it to reach levels high enough to cause disease. This study explores the set of proteins that might have an association with L. monocytogenes adaptation to different temperatures. Cultures were grown in biofilm, the most widespread mode of growth in natural and industrial realms. Protein extractions were performed from three different growth temperatures (10, 25, and 37°C) and two growth phases (early stage and mature biofilm). L. monocytogenes subproteomes were targeted using three extraction methods: trypsin-enzymatic shaving, biotin-labeling and cell fractionation. The different subproteomes obtained were separated and analyzed by shotgun proteomics using high-performance liquid chromatography combined with tandem mass spectrometry (LC-OrbiTrap LTQVelos, ThermoFisher Scientific). A total of 141 (biotinylation), 98 (shaving) and 910 (fractionation) proteins were identified. Throughout the 920 unique proteins identified, many are connected to basic cell functions, but some are linked with thermoregulation. We observed some noteworthy protein abundance shifts associated with the major adaptation to cold mechanisms present in L. monocytogenes, namely: the role of ribosomes and the stressosome with a higher abundance of the general stress protein Ctc (Rl25) and the general stress transcription factor sigma B (σB), changes in cell fluidity and motility seen by higher levels of foldase protein PrsA2 and flagellin (FlaA), the uptake of osmolytes with a higher abundance of glycine betaine (GbuB) and carnitine transporters (OpucA), and the relevance of the overexpression of chaperone proteins such as cold shock proteins (CspLA and Dps). As for 37°C, we observed a significantly higher percentage of proteins associated with transcriptional or translational activity present in higher abundance upon comparison with the colder settings. These contrasts of protein expression throughout several conditions will enrich databases and help to model the regulatory circuitry that drives adaptation of L. monocytogenes to environments.

Systematic review of the Listeria monocytogenes σB regulon supports a role in stress response, virulence and metabolism

Aim: Among the alternative sigma factors of Listeria monocytogenes, σB controls the largest regulon. The aim of this study was to perform a comprehensive review of σB-regulated genes, and the functions they confer. Materials & methods: A systematic search of PubMed and Web of Knowledge was carried out to identify members of the σB regulon based on experimental evidence of σB-dependent transcription and presence of a consensus σB-dependent promoter. Results: The literature review identified σB-dependent transcription units encompassing 304 genes encoding different functions including stress response and virulence. Conclusion: Our review supports the well-known roles of σB in virulence and stress response and provides new insight into novel roles for σB in metabolism and overall resilience of L. monocytogenes.

MALDI mass spectrometry imaging and in situ microproteomics of Listeria monocytogenes biofilms

MALDI-TOF Mass spectrometry Imaging (MSI) is a surface-sampling technology that can determine spatial information and relative abundance of analytes directly from biological samples. Human listeriosis cases are due to the ingestion of contaminated foods with the pathogenic bacteria Listeria monocytogenes. The reduction of water availability in food workshops by decreasing the air relative humidity (RH) is one strategy to improve the control of bacterial contamination. This study aims to develop and implement an MSI approach on L. monocytogenes biofilms and proof of concept using a dehumidified stress condition. MSI allowed examining the distribution of low molecular weight proteins within the biofilms subjected to a dehumidification environment, mimicking the one present in a food workshop (10 °C, 75% RH). Furthermore, a LC-MS/MS approach was made to link the dots between MSI and protein identification. Five identified proteins were assigned to registered MSI m/z, including two cold-shock proteins and a ligase involved in cell wall biogenesis. These data demonstrate how imaging can be used to dissect the proteome of an intact bacterial biofilm giving new insights into protein expression relating to a dehumidification stress adaptation. Data are available via ProteomeXchange with identifier PXD010444.

BIOLOGICAL SIGNIFICANCE

The ready-to-eat food processing industry has the daily challenge of controlling the contamination of surfaces and machines with spoilage and pathogenic microorganisms. In some cases, it is a lost cause due to these microorganisms' capacity to withstand the cleaning treatments, like desiccation procedures. Such a case is the ubiquitous Gram-positive Bacterium Listeria monocytogenes. Its surface proteins have particular importance for the interaction with its environment, being important factors contributing to adaptation to stress conditions. There are few reproducibly techniques to obtain the surface proteins of Gram-positive cells. Here, we developed a workflow that enables the use of MALDI imaging on Gram-positive bacterium biofilms to study the impact of dehumidification on sessile cells. It will be of the most interest to test this workflow with different environmental conditions and potentially apply it to other biofilm-forming bacteria.

Strand specific RNA-sequencing and membrane lipid profiling reveals growth phase-dependent cold stress response mechanisms in Listeria monocytogenes

The human pathogen Listeria monocytogenes continues to pose a challenge in the food industry, where it is known to contaminate ready-to-eat foods and grow during refrigerated storage. Increased knowledge of the cold-stress response of this pathogen will enhance the ability to control it in the food-supply-chain. This study utilized strand-specific RNA sequencing and whole cell fatty acid (FA) profiling to characterize the bacterium's cold stress response. RNA and FAs were extracted from a cold-tolerant strain at five time points between early lag phase and late stationary-phase, both at 4°C and 20°C. Overall, more genes (1.3×) were suppressed than induced at 4°C. Late stationary-phase cells exhibited the greatest number (n = 1,431) and magnitude (>1,000-fold) of differentially expressed genes (>2-fold, p<0.05) in response to cold. A core set of 22 genes was upregulated at all growth phases, including nine genes required for branched-chain fatty acid (BCFA) synthesis, the osmolyte transporter genes opuCBCD, and the internalin A and D genes. Genes suppressed at 4°C were largely associated with cobalamin (B12) biosynthesis or the production/export of cell wall components. Antisense transcription accounted for up to 1.6% of total mapped reads with higher levels (2.5×) observed at 4°C than 20°C. The greatest number of upregulated antisense transcripts at 4°C occurred in early lag phase, however, at both temperatures, antisense expression levels were highest in late stationary-phase cells. Cold-induced FA membrane changes included a 15% increase in the proportion of BCFAs and a 15% transient increase in unsaturated FAs between lag and exponential phase. These increases probably reduced the membrane phase transition temperature until optimal levels of BCFAs could be produced. Collectively, this research provides new information regarding cold-induced membrane composition changes in L. monocytogenes, the growth-phase dependency of its cold-stress regulon, and the active roles of antisense transcripts in regulating its cold stress response.

The Role of Stress and Stress Adaptations in Determining the Fate of the Bacterial Pathogen Listeria monocytogenes in the Food Chain

The foodborne pathogen Listeria monocytogenes is a highly adaptable organism that can persist in a wide range of environmental and food-related niches. The consumption of contaminated ready-to-eat foods can cause infections, termed listeriosis, in vulnerable humans, particularly those with weakened immune systems. Although these infections are comparatively rare they are associated with high mortality rates and therefore this pathogen has a significant impact on food safety. L. monocytogenes can adapt to and survive a wide range of stress conditions including low pH, low water activity, and low temperature, which makes it problematic for food producers who rely on these stresses for preservation. Stress tolerance in L. monocytogenes can be explained partially by the presence of the general stress response (GSR), a transcriptional response under the control of the alternative sigma factor sigma B (σB) that reconfigures gene transcription to provide homeostatic and protective functions to cope with the stress. Within the host σB also plays a key role in surviving the harsh conditions found in the gastrointestinal tract. As the infection progresses beyond the GI tract L. monocytogenes uses an intracellular infectious cycle to propagate, spread and remain protected from the host's humoral immunity. Many of the virulence genes that facilitate this infectious cycle are under the control of a master transcriptional regulator called PrfA. In this review we consider the environmental reservoirs that enable L. monocytogenes to gain access to the food chain and discuss the stresses that the pathogen must overcome to survive and grow in these environments. The overlap that exists between stress tolerance and virulence is described. We review the principal measures that are used to control the pathogen and point to exciting new approaches that might provide improved means of control in the future.

Differences in the Expression of Cold Stress-Related Genes and in the Swarming Motility Among Persistent and Sporadic Strains of Listeria monocytogenes

The persistence of certain Listeria monocytogenes strains in food-related environments suggests niche adaptation of these strains and therefore constitutes a major risk to consumer health and results in economic losses for the food producer. In this study, a set of 23 L. monocytogenes isolates, including a group of persistent and a group of sporadic strains, was evaluated regarding their swarming motility at 11°C. In each group, significant (p<0.05) differences in motility were observed. The transcript levels of nine cold stress-related genes were analyzed by real-time quantitative PCR in two representatives of persistent (CBISA3077) and sporadic (CBISA3049) strains isolated from the dairy environment, and significant (p<0.05) differences between the two strains were observed. The persistent strain showed significantly higher transcript levels of dtpT and sigB genes, and significantly lower levels of flaA, oppA, lmo1722, and lmo0866 genes. In the persistent strain, the upregulation of sigB, involved in the tolerance to low temperature and to osmotic stress, could account for the persistence of this strain in its original dairy environment. In a similar way, the downregulation of two helicase-encoding genes lmo1722 and lmo0866, in this strain, may be an evolutionary trait that could facilitate cold stress adaptation. Even though this analysis should be extended to more sporadic and more persistent strains, the results presented here strongly suggest gene expression networks differently adjusted, in the two strains, to the low-temperature environment from where they were collected. Moreover, our findings suggest that bacterial motility per se should not be considered a key feature for the persistence of L. monocytogenes in the food environment.

σ(B) affects biofilm formation under the dual stress conditions imposed by adding salt and low temperature in Listeria monocytogenes

The food-borne pathogenic bacteria Listeria monocytogenes can form biofilms on various surfaces including food-processing equipment. Biofilms offer survival benefits to the organisms entrapped against environmental insults. Moreover, the σ(B) transcription factor of L. monocytogenes plays an important role in its survival under various stress conditions. In this study, we evaluated whether σ(B) contributes to biofilm formation when L. monocytogenes is grown under various temperatures and media. When the wild-type strain was grown under static biofilm culture below ambient temperature (15°C) for 72 h, the difference in viable cell number (in both planktonic and biofilm cells) between the wild-type and ΔsigB mutant increased by adding NaCl to BHI broth (9% salt BHI > 6% salt BHI > BHI, w/v), and the specific activity of β-galactosidase was highly induced in the wild-type strain grown in 6% salt containing BHI broth. Furthermore, we measured surface-adhered biofilm forming ability using the crystal violet staining method. The wild-type strain formed a four times larger biofilm than that of the ΔsigB mutant in 6% salt-BHI medium at 15°C over a 72 h incubation and also showed the highest level of β-galactosidase specific activity. However, both the wild-type and ΔsigB mutant L. monocytogenes were defective for forming a biofilm in 9% salt-BHI medium at 15°C. Our results suggest that σ(B) plays an enhanced role in surface-adhered biofilm formation when L. monocytogenes encounters dual stress conditions, such as 6% NaCl and low temperature.

Transcriptional and phenotypic responses of Listeria monocytogenes to chlorine dioxide

Significant food-borne disease outbreaks have occurred from consumption of ready-to-eat foods, including produce, contaminated with Listeria monocytogenes. Challenging food matrices (e.g., cantaloupe, sprouts) with limited processing steps postharvest to reduce pathogen loads have underscored a need for new mitigation strategies. Chlorine dioxide (ClO2) is increasingly being used in produce and other food systems to reduce food-borne pathogen levels. The goal of this study was to characterize the transcriptional response and survival of L. monocytogenes 10403S exposed to ClO2. The transcriptional profile of log-phase cells exposed to 300 mg/liter ClO2 for 15 min was defined by whole-genome microarray. A total of 340 genes were significantly differentially expressed. Among the differentially expressed genes, 223 were upregulated (fold change ≥ 1.5; adjusted P value < 0.05) in role categories responsible for protein fate, cellular processes, and energy metabolism. There were 113 and 16 genes differentially expressed belonging to regulatory networks of σ(B) and CtsR, respectively. We assessed L. monocytogenes 10403S survival after exposure to 100, 300, and 500 mg/liter aqueous ClO2 in brain heart infusion (BHI) broth; there was a significant difference between cells exposed to 500 mg/liter ClO2 and those exposed to all other conditions over time (P value < 0.05). Isogenic ΔsigB and ΔctsR mutants exposed to 300 mg/liter ClO2 were more sensitive to ClO2 than the wild type under the same conditions. These results provide an initial insight into the mechanisms that L. monocytogenes employs to survive sublethal ClO2 and further our understanding of the inactivation mechanisms of this increasingly used sanitizer.

Effects of growth phase and temperature on σB activity within a Listeria monocytogenes population: evidence for RsbV-independent activation of σB at refrigeration temperatures

The alternative sigma factor σ^B of Listeria monocytogenes is responsible for regulating the transcription of many of the genes necessary for adaptation to both food-related stresses and to conditions found within the gastrointestinal tract of the host. The present study sought to investigate the influence of growth phase and temperature on the activation of σ^B within populations of L. monocytogenes EGD-e wild-type, ΔsigB, and ΔrsbV throughout growth at both 4°C and 37°C, using a reporter fusion that couples expression of EGFP to the strongly σ^B-dependent promoter of lmo2230. A similar σ^B activation pattern within the population was observed in wt-egfp at both temperatures, with the highest induction of σ^B occurring in the early exponential phase of growth when the fluorescent population rapidly increased, eventually reaching the maximum in early stationary phase. Interestingly, induction of σ^B activity was heterogeneous, with only a proportion of the cells in the wt-egfp population being fluorescent above the background autofluorescence level. Moreover, significant RsbV-independent activation of σ^B was observed during growth at 4°C. This result suggests that an alternative route to σ^B activation exists in the absence of RsbV, a finding that is not explained by the current model for σ^B regulation.

Development and optimization of an EGFP-based reporter for measuring the general stress response in Listeria monocytogenes

A characteristic of the food-borne pathogen Listeria monocytogenes is its tolerance to the harsh conditions found both in minimally processed foods and the human gastrointestinal tract. This trait is partly under the control of the alternative sigma factor sigma B (σ(B)). To study the mechanisms that trigger the activation of σ(B) , and hence the development of stress tolerance, we have developed a fluorescent reporter fusion that allows the real-time activity of σ(B) to be monitored. The reporter, designated Plmo2230::egfp, fuses the strong σ(B)-dependent promoter from the lmo2230 gene (which encodes a putative arsenate reductase) to a gene encoding enhanced green fluorescence protein (EGFP). The reporter was integrated into the genomes of the wild-type strain L. monocytogenes EGD-e as well as two mutant derivatives lacking either sigB or rsbV. The resulting strains were used to study σ(B) activation in response to growth phase and hyperosmotic stress. The wild-type was strongly fluorescent in stationary phase or in cultures with added NaCl and this fluorescence was abolished in both the sigB and rsbV backgrounds, consistent with the σ(B)-dependency of the lmo2230 promoter. During sudden osmotic upshock (addition of 0.5 M NaCl during growth) a real-time increase in fluorescence was observed microscopically, reaching maximal activation after 30 min. Flow cytometry was used to study the activation of σ(B) at a population level by hyperosmotic stress during exponential growth. A strong and proportional increase in fluorescence was observed as the salt concentration increased from 0 to 0.9 M NaCl. Interestingly, there was considerable heterogeneity within the population and a significant proportion of cells failed to induce a high level of fluorescence, suggesting that σ(B) activation occurs stochastically in response to hyperosmotic stress. Thus the Plmo2230::egfp is a powerful tool that will allow the stress response to be better studied in this important human pathogen.

The Listeria monocytogenes σB regulon and its virulence-associated functions are inhibited by a small molecule

The stress-responsive alternative sigma factor σ^B is conserved across diverse Gram-positive bacterial genera. In Listeria monocytogenes, σ^B regulates transcription of >150 genes, including genes contributing to virulence and to bacterial survival under host-associated stress conditions, such as those encountered in the human gastrointestinal lumen. An inhibitor of L. monocytogenes σ^B activity was identified by screening ~57,000 natural and synthesized small molecules using a high-throughput cell-based assay. The compound fluoro-phenyl-styrene-sulfonamide (FPSS) (IC₅₀ = 3.5 µM) downregulated the majority of genes previously identified as members of the σ^B regulon in L. monocytogenes 10403S, thus generating a transcriptional profile comparable to that of a 10403S ΔsigB strain. Specifically, of the 208 genes downregulated by FPSS, 75% had been identified previously as positively regulated by σ^B. Downregulated genes included key virulence and stress response genes, such as inlA, inlB, bsh, hfq, opuC, and bilE. From a functional perspective, FPSS also inhibited L. monocytogenes invasion of human intestinal epithelial cells and bile salt hydrolase activity. The ability of FPSS to inhibit σ^B activity in both L. monocytogenes and Bacillus subtilis indicates its utility as a specific inhibitor of σ^B across multiple Gram-positive genera.

The σ^B transcription factor regulates expression of genes responsible for bacterial survival under changing environmental conditions and for virulence; therefore, this alternative sigma factor is important for transmission of L. monocytogenes and other Gram-positive bacteria. Regulation of σ^B activity is complex and tightly controlled, reflecting the key role of this factor in bacterial metabolism. We present multiple lines of evidence indicating that fluoro-phenyl-styrene-sulfonamide (FPSS) specifically inhibits activity of σ^B across Gram-positive bacterial genera, i.e., in both Listeria monocytogenes and Bacillus subtilis. Therefore, FPSS is an important new tool that will enable novel approaches for exploring complex regulatory networks in L. monocytogenes and other Gram-positive pathogens and for investigating small-molecule applications for controlling pathogen transmission.

Rapid, transient, and proportional activation of σ(B) in response to osmotic stress in Listeria monocytogenes

The osmotic activation of sigma B (σ(B)) in Listeria monocytogenes was studied by monitoring expression of four known σ(B)-dependent genes, opuCA, lmo2230, lmo2085, and sigB. Activation was found to be rapid, transient, and proportional to the magnitude of the osmotic stress applied, features that underpin the adaptability of this pathogen.

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.

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.

Proteomics for the elucidation of cold adaptation mechanisms in Listeria monocytogenes

Listeria monocytogenes, one of the major food-related pathogens, is the aetiological agent of listeriosis, a potentially life-threatening illness. It is able to survive in hostile environments and stress conditions such as those encountered in food-processing technologies (high salt concentration, wide range of pH and temperature, low water availability) and it also thrives at temperatures ranging from -0.4 to 45 °C. In this study, expression proteomics was applied to gain insight into key cellular events that allow L. monocytogenes to survive and multiply even at refrigeration temperatures. Interestingly, we observed that the adaptation processes mainly affect biochemical pathways related to protein synthesis and folding, nutrient uptake and oxidative stress. Furthermore, proteins implicated in metabolic pathways for energy production, such as glycolysis and Pta-AckA pathway, were present to a higher level in the cells grown at 4 °C. This suggests that, on the whole, cells exhibit an enhanced demand for energy to sustain cold growth. Proteomics may represent a key tool in deciphering specific mechanisms underlying cold adaptation response and, more widely, cell machinery.

Physical and antibiotic stresses require activation of the RsbU phosphatase to induce the general stress response in Listeria monocytogenes

Among pathogenic strains of Listeria monocytogenes, the σ^B transcription factor has a pivotal role in the outcome of food-borne infections. This factor is activated by diverse stresses to provide general protection against multiple challenges, including those encountered during gastrointestinal passage. It also acts with the PrfA regulator to control virulence genes needed for entry into intestinal lumen cells. Environmental and nutritional signals modulate σ^B activity via a network that operates by the partner switching mechanism, in which protein interactions are controlled by serine phosphorylation. This network is well characterized in the related bacterium Bacillus subtilis. A key difference in Listeria is the presence of only one input phosphatase, RsbU, instead of the two found in B. subtilis. Here, we aim to determine whether this sole phosphatase is required to convey physical, antibiotic and nutritional stress signals, or if additional pathways might exist. To that end, we constructed L. monocytogenes 10403S strains bearing single-copy, σ^B-dependent opuCA–lacZ reporter fusions to determine the effects of an rsbU deletion under physiological conditions. All stresses tested, including acid, antibiotic, cold, ethanol, heat, osmotic and nutritional challenge, required RsbU to activate σ^B. This was of particular significance for cold stress activation, which occurs via a phosphatase-independent mechanism in B. subtilis. We also assayed the effects of the D80N substitution in the upstream RsbT regulator that activates RsbU. The mutant had a phenotype consistent with low and uninducible phosphatase activity, but nonetheless responded to nutritional stress. We infer that RsbU activity but not its induction is required for nutritional signalling, which would enter the network downstream from RsbU.

Evaluation of cold growth and related gene transcription responses associated with Listeria monocytogenes strains of different origins

The cold growth phenotypes and transcriptional activation of cold stress adaptation genes was evaluated amongst Listeria monocytogenes strains from human listeriosis cases, food products and associated production environments. Significant cold growth phenotypic variation was observed during growth of such strains in rich (BHI) as well as chemically defined minimal (MDM) nutrient conditions. While all twenty analyzed strains grew in BHI at 4 degrees C, only eight of these strains, mostly those recovered from human listeriosis cases, were also able to grow in MDM under similar cold stress. The cold growth phenotypes observed in BHI were used to define two categories of five strains each, which either displayed enhanced and poor cold tolerance relative to the rest of the strain collection. The first group (GP1) consisted of strains characterized by short lag times, whilst the second group (GP2) comprised of strains displaying prolonged lag times before growth resumption during incubation in BHI cultures at 4 degrees C. Transcription level activation of sigB, cspA and pgpH gene expression associated with cold stress exposure in a selection of GP1 and GP2 strains was assessed. Despite similar cold dependent sigB transcript induction between these two strain groups, there were significant differences observed in cold stress dependent induction of cspA and pgpH transcripts. Cold tolerant GP1 strains displayed relatively higher transcriptional activation of cspA and pgpH after cold stress exposure compared to the cold sensitive GP2 strains. This study highlights strain variability in cold stress tolerance phenotypes, as well as in strain capacity to activate specific cold adaptation gene expression responses. In addition the study also shows that enhanced and poor cold growth phenotypes are associated with particular strain capacity to activate important cold stress gene expression responses upon transition of L. monocytogenes into low temperature environments.

Identification of Bacillus cereus genes specifically expressed during growth at low temperatures

The mechanisms involved in the ability of Bacillus cereus to multiply at low temperatures were investigated. It was assumed that many genes involved in cold acclimation would be upregulated at low temperatures. Recombinase-based in vivo expression technology (IVET) was adapted to the detection of the transient activation of B. cereus promoters during growth at 10 degrees C. Four independent screenings of a promoter library from type strain ATCC 14579 were performed, and 17 clones were isolated. They corresponded to 17 promoter regions that displayed reproducibly elevated expression at 10 degrees C relative to expression at 30 degrees C. This analysis revealed several genes that may be important for B. cereus to grow successfully under the restrictive conditions of cold habitats. Among them, a locus corresponding to open reading frames BC5402 to BC5398, harboring a lipase-encoding gene and a putative transcriptional regulator, was identified three times. While a mutation in the putative regulator-encoding gene did not cause any particular phenotype, a mutant deficient in the lipase-encoding gene showed reduced growth abilities at low temperatures compared with the parental strain. The mutant did not change its fatty acid profiles in the same way as the wild type when grown at 12 degrees C instead of 37 degrees C. This study demonstrates the feasibility of a promoter trap strategy for identifying cold-induced genes. It outlines a first picture of the different processes involved in B. cereus cold acclimation.

sigma(B) and sigma(L) contribute to Listeria monocytogenes 10403S response to the antimicrobial peptides SdpC and nisin

The ability of the foodborne pathogen Listeria monocytogenes to survive antimicrobial treatments is a public health concern; therefore, this study was designed to investigate genetic mechanisms contributing to antimicrobial response in L. monocytogenes. In previous studies, the putative bacteriocin immunity gene lmo2570 was predicted to be regulated by the stress responsive alternative sigma factor, sigma(B). As the alternative sigma factor sigma(L) controls expression of genes important for resistance to some antimicrobial peptides, we hypothesized roles for lmo2570, sigma(B), and sigma(L) in L. monocytogenes antimicrobial response. Results from phenotypic characterization of a L. monocytogenes lmo2570 null mutant suggested that this gene does not contribute to resistance to nisin or to SdpC, an antimicrobial peptide produced by some strains of Bacillus subtilis. While lmo2570 transcript levels were confirmed to be sigma(B) dependent, they were sigma(L) independent and were not affected by the presence of nisin under the conditions used in this study. In spot-on-lawn assays with the SdpC-producing B. subtilis EG351, the L. monocytogenes DeltasigB, DeltasigL, and DeltasigB/DeltasigL strains all showed increased sensitivity to SdpC, indicating that both sigma(B) and sigma(L) regulate genes contributing to SdpC resistance. Nisin survival assays showed that sigma(B) and sigma(L) both affect L. monocytogenes sensitivity to nisin in broth survival assays; that is, a sigB null mutant is more resistant than the parent strain to nisin, while a sigB null mutation in DeltasigL background leads to reduced nisin resistance. In summary, while the sigma(B)-dependent lmo2570 does not contribute to resistance of L. monocytogenes to nisin or SdpC, both sigma(B) and sigma(L) contribute to the L. monocytogenes antimicrobial response.

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.

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.

In vivo transcriptional profiling of Listeria monocytogenes and mutagenesis identify new virulence factors involved in infection

Listeria monocytogenes is a human intracellular pathogen able to colonize host tissues after ingestion of contaminated food, causing severe invasive infections. In order to gain a better understanding of the nature of host-pathogen interactions, we studied the L. monocytogenes genome expression during mouse infection. In the spleen of infected mice, approximately 20% of the Listeria genome is differentially expressed, essentially through gene activation, as compared to exponential growth in rich broth medium. Data presented here show that, during infection, Listeria is in an active multiplication phase, as revealed by the high expression of genes involved in replication, cell division and multiplication. In vivo bacterial growth requires increased expression of genes involved in adaptation of the bacterial metabolism and stress responses, in particular to oxidative stress. Listeria interaction with its host induces cell wall metabolism and surface expression of virulence factors. During infection, L. monocytogenes also activates subversion mechanisms of host defenses, including resistance to cationic peptides, peptidoglycan modifications and release of muramyl peptides. We show that the in vivo differential expression of the Listeria genome is coordinated by a complex regulatory network, with a central role for the PrfA-SigB interplay. In particular, L. monocytogenes up regulates in vivo the two major virulence regulators, PrfA and VirR, and their downstream effectors. Mutagenesis of in vivo induced genes allowed the identification of novel L. monocytogenes virulence factors, including an LPXTG surface protein, suggesting a role for S-layer glycoproteins and for cadmium efflux system in Listeria virulence.

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.

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.

Contributions of two-component regulatory systems, alternative sigma factors, and negative regulators to Listeria monocytogenes cold adaptation and cold growth

The ability of Listeria monocytogenes to grow at refrigeration temperatures is critical for transmission of this foodborne pathogen. We evaluated the contributions of different transcriptional regulators and two-component regulatory systems to L. monocytogenes cold adaptation and cold growth. L. monocytogenes parent strain 10403S and selected isogenic null mutants in genes encoding four alternative sigma factors (sigB, sigH, sigC, and sigL), two regulators of sigmaB (rsbT and rsbV), two negative regulators (ctsR and hrcA), and 15 two-component response regulators were grown in brain heart infusion broth at 4 degrees C with (i) a high-concentration starting inoculum (10(8) CFU/ml), (ii) a low-concentration starting inoculum (102 CFU/ml), and (iii) a high-concentration starting inoculum of cold-adapted cells. With a starting inoculum of 10(8) CFU/ml, null mutants in genes encoding selected alternative sigma factors (DeltasigH, DeltasigC, and DeltasigL), a negative regulator (DeltactsR), regulators of sigmaB (DeltarsbT and DeltarsbV), and selected two-component response regulators (DeltalisR, Deltalmo1172, and Deltalmo1060) had significantly reduced growth (P < 0.05) compared with the parent strain after 12 days at 4 degrees C. The growth defect for DeltasigL was limited and was not confirmed by optical density (OD600) measurement data. With a starting inoculum of 102 CFU/ml and after monitoring growth at 4 degrees C over 84 days, only the DeltactsR strain had a consistent but limited growth defect; the other mutant strains had either no growth defects or limited growth defects apparent at only one or two of the nine sampling points evaluated during the 84-day growth period (DeltasigB, DeltasigC, and Deltalmo1172). With a 10(8) CFU/ml starting inoculum of cold-adapted cells, none of the mutant strains that had a growth defect when inoculation was performed with cells pregrown at 37 degrees C had reduced growth as compared with the parent strain after 12 days at 4 degrees C, suggesting a specific defect in the ability of these mutant strains to adapt to 4 degrees C after growth at 37 degrees C. Our data indicate (i) selected sigma factors and two-component regulators may contribute to cold adaptation even though two-component regulatory systems, alternative sigma factors, and the negative regulators CtsR and HrcA appear to have limited contributions to L. monocytogenes growth at 4 degrees C in rich media, and (ii) inoculum concentration and pregrowth conditions affect the L. monocytogenes cold-growth phenotype.

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.

Microarray-based characterization of the Listeria monocytogenes cold regulon in log- and stationary-phase cells

Whole-genome microarray experiments were performed to define the Listeria monocytogenes cold growth regulon and to identify genes differentially expressed during growth at 4 and 37 degrees C. Microarray analysis using a stringent cutoff (adjusted P < 0.001; >/=2.0-fold change) revealed 105 and 170 genes that showed higher transcript levels in logarithmic- and stationary-phase cells, respectively, at 4 degrees C than in cells grown at 37 degrees C. A total of 74 and 102 genes showed lower transcript levels in logarithmic- and stationary-phase cells, respectively, grown at 4 degrees C. Genes with higher transcript levels at 4 degrees C in both stationary- and log-phase cells included genes encoding a two-component response regulator (lmo0287), a cold shock protein (cspL), and two RNA helicases (lmo0866 and lmo1722), whereas a number of genes encoding virulence factors and heat shock proteins showed lower transcript levels at 4 degrees C. Selected genes that showed higher transcript levels at 4 degrees C during both stationary and log phases were confirmed by quantitative reverse transcriptase PCR. Our data show that (i) a large number of L. monocytogenes genes are differentially expressed at 4 and 37 degrees C, with more genes showing higher transcript levels than lower transcript levels at 4 degrees C, (ii) L. monocytogenes genes with higher transcript levels at 4 degrees C include a number of genes and operons with previously reported or plausible roles in cold adaptation, and (iii) L. monocytogenes genes with lower transcript levels at 4 degrees C include a number of virulence and virulence-associated genes as well as some heat shock genes.