Characterization of Listeria monocytogenes protein Lmo0327 with murein hydrolase activity

Aquatic environment drives the emergence of cell wall-deficient dormant forms in Listeria

Stressed bacteria can enter a dormant viable but non-culturable (VBNC) state. VBNC pathogens pose an increased health risk as they are undetectable by growth-based techniques and can wake up back into a virulent state. Although widespread in bacteria, the mechanisms governing this phenotypic switch remain elusive. Here, we investigate the VBNC state transition in the human pathogen Listeria monocytogenes. We show that bacteria starved in mineral water become VBNC by converting into osmotically stable cell wall-deficient coccoid forms, a phenomenon that occurs in other Listeria species. We reveal the bacterial stress response regulator SigB and the autolysin NamA as major actors of VBNC state transition. We lastly show that VBNC Listeria revert to a walled and virulent state after passage in chicken embryos. Our study provides more detail on the VBNC state transition mechanisms, revealing wall-free bacteria naturally arising in aquatic environments as a potential survival strategy in hypoosmotic and oligotrophic conditions.

A Listeria monocytogenes ST2 clone lacking chitinase ChiB from an outbreak of non-invasive gastroenteritis

An outbreak with a remarkable Listeria monocytogenes clone causing 163 cases of non-invasive listeriosis occurred in Germany in 2015. Core genome multi locus sequence typing grouped non-invasive outbreak isolates and isolates obtained from related food samples into a single cluster, but clearly separated genetically close isolates obtained from invasive listeriosis cases. A comparative genomic approach identified a premature stop codon in the chiB gene, encoding one of the two L. monocytogenes chitinases, which clustered with disease outcome. Correction of this premature stop codon in one representative gastroenteritis outbreak isolate restored chitinase production, but effects in infection experiments were not found. While the exact role of chitinases in virulence of L. monocytogenes is still not fully understood, our results now clearly show that ChiB-derived activity is not required to establish L. monocytogenes gastroenteritis in humans. This limits a possible role of ChiB in human listeriosis to later steps of the infection.

Chitinase Expression in Listeria monocytogenes Is Influenced by lmo0327, Which Encodes an Internalin-Like Protein

The chitinolytic system of Listeria monocytogenes thus far comprises two chitinases, ChiA and ChiB, and a lytic polysaccharide monooxygenase, Lmo2467. The role of the system in the bacterium appears to be pleiotropic, as besides mediating the hydrolysis of chitin, the second most ubiquitous carbohydrate in nature, the chitinases have been deemed important for the colonization of unicellular molds, as well as mammalian hosts. To identify additional components of the chitinolytic system, we screened a transposon mutant library for mutants exhibiting impaired chitin hydrolysis. The screening yielded a mutant with a transposon insertion in a locus corresponding to lmo0327 of the EGD-e strain. lmo0327 encodes a large (1,349 amino acids [aa]) cell wall-associated protein that has been proposed to possess murein hydrolase activity. The single inactivation of lmo0327, as well as of lmo0325 that codes for a putative transcriptional regulator functionally related to lmo0327, led to an almost complete abolishment of chitinolytic activity. The effect could be traced at the transcriptional level, as both chiA and chiB transcripts were dramatically decreased in the lmo0327 mutant. In accordance with that, we could barely detect ChiA and ChiB in the culture supernatants of the mutant strain. Our results provide new information regarding the function of the lmo0325-lmo0327 locus in L. monocytogenes and link it to the expression of chitinolytic activity.

IMPORTANCE Many bacteria from terrestrial and marine environments express chitinase activities enabling them to utilize chitin as the sole source of carbon and nitrogen. Interestingly, several bacterial chitinases may also be involved in host pathogenesis. For example, in the important foodborne pathogen Listeria monocytogenes, the chitinases ChiA and ChiB and the lytic polysaccharide monooxygenase Lmo2467 are implicated in chitin assimilation but also act as virulence factors during the infection of mammalian hosts. Therefore, it is important to identify their regulators and induction cues to understand how the different roles of the chitinolytic system are controlled and mediated. Here, we provide evidence for the importance of lmo0327 and lmo0325, encoding a putative internalin/autolysin and a putative transcriptional activator, respectively, in the efficient expression of chitinase activity in L. monocytogenes and thereby provide new information regarding the function of the lmo0325-lmo0327 locus.

Transcriptomic Analysis of the Adaptation of Listeria monocytogenes to Lagoon and Soil Matrices Associated with a Piggery Environment: Comparison of Expression Profiles

Understanding how Listeria monocytogenes, the causative agent of listeriosis, adapts to the environment is crucial. Adaptation to new matrices requires regulation of gene expression. To determine how the pathogen adapts to lagoon effluent and soil, two matrices where L. monocytogenes has been isolated, we compared the transcriptomes of L. monocytogenes CIP 110868 20 min and 24 h after its transfer to effluent and soil extract. Results showed major variations in the transcriptome of L. monocytogenes in the lagoon effluent but only minor modifications in the soil. In both the lagoon effluent and in the soil, genes involved in mobility and chemotaxis and in the transport of carbohydrates were the most frequently represented in the set of genes with higher transcript levels, and genes with phage-related functions were the most represented in the set of genes with lower transcript levels. A modification of the cell envelop was only found in the lagoon environment. Finally, the differential analysis included a large proportion of regulators, regulons, and ncRNAs.

InlL from Listeria monocytogenes Is Involved in Biofilm Formation and Adhesion to Mucin

The bacterial etiological agent of listeriosis, Listeria monocytogenes, is an opportunistic intracellular foodborne pathogen. The infection cycle of L. monocytogenes is well-characterized and involves several key virulence factors, including internalins A and B. While 35 genes encoding internalins have been identified in L. monocytogenes, less than half of them have been characterized as yet. Focusing on lmo2026, it was shown this gene encodes a class I internalin, InlL, exhibiting domains potentially involved in adhesion. Following a functional genetic approach, InlL was demonstrated to be involved in initial bacterial adhesion as well as sessile development in L. monocytogenes. In addition, InlL enables binding to mucin of type 2, i.e., the main secreted mucin making up the mucus layer, rather than to surface-located mucin of type 1. InlL thus appears as a new molecular determinant contributing to the colonization ability of L. monocytogenes.

Recombinant Expression of a Putative Amidase Cloned from the Genome of Listeria monocytogenes that Lyses the Bacterium and its Monolayer in Conjunction with a Protease

Listeria monocytogenes is a Gram-positive, non-spore forming, catalase-positive rod that is a major bacterial food-borne disease agent associated with uncooked meats, including poultry, uncooked vegetables, soft cheeses, and unpasteurized milk. The bacterium may be carried by animals without signs of disease, can replicate at refrigeration temperatures, and is frequently associated with biofilms. There is a need to discover innovative pathogen intervention technologies for this bacterium. Consequently, bioinformatic analyses were used to identify genes encoding lytic protein sequences in the genomes of L. monocytogenes isolates. PCR primers were designed that amplified nucleotide sequences of a putative N-acetylmuramoyl-L-alanine amidase gene from L. monocytogenes strain 4b. The resultant amplification product was cloned into an expression vector, propagated in Escherichia coli Rosetta strains, and the recombinant protein was purified to homogeneity. Gene and protein sequencing confirmed that the predicted and chemically determined amino acid sequence of the recombinant protein designated PlyLM was a putative N-acetylmuramoyl-L-alanine amidase. The recombinant lytic protein was capable of lysing both the parental L. monocytogenes strain as well as other strains of the bacterium in spot and MIC/MIB assays, but was not active against other bacteria beyond the genus. A microtiter plate assay was utilized to assay for the ability of the recombinant lysin protein to potentially aid with digestion of a L. monocytogenes biofilm. Protease or lysozyme digestion alone did not significantly reduce the L. monocytogenes biofilm. Although the recombinant protein alone reduced the biofilm by only 20%, complete digestion of the bacterial monolayer was accomplished in conjunction with a protease.

Domain function dissection and catalytic properties of Listeria monocytogenes p60 protein with bacteriolytic activity

The major extracellular protein p60 of Listeria monocytogenes (Lm-p60) is an autolysin that can hydrolyze the peptidoglycan of bacterial cell wall and has been shown to be required for L. monocytogenes virulence. The predicted three-dimensional structure of Lm-p60 showed that Lm-p60 could be split into two independent structural domains at the amino acid residue 270. Conserved motif analysis showed that V30, D207, S395, and H444 are the key amino acid residues of the corresponding motifs. However, not only the actual functions of these two domains but also the catalytic properties of Lm-p60 are unclear. We try to express recombinant Lm-p60 and identify the functions of two domains by residue substitution (V30A, D207A, S395A, and H444A) and peptide truncation. The C-terminal domain was identified as catalytic element and N-terminal domain as substrate recognition and binding element. Either N-terminal domain truncation or C-terminal domain truncation presents corresponding biological activity. The catalytic activity of Lm-p60 with a malfunctioned substrate-binding domain was decreased, while the substrate binding was not affected by a mulfunctioned catalytic domain. With turbidimetric method, we determined the optimal conditions for the bacteriolytic activity of Lm-p60 against Micrococcus lysodeikficus. The assay for the effect of Lm-p60 on the bacteriolytic activity of lysozyme revealed that the combined use of Lm-p60 protein with lysozyme showed a strong synergistic effect on the bacteriolytic activity.

Remodeling of the Listeria monocytogenes cell wall inside eukaryotic cells

Listeria monocytogenes is an intracellular Gram-positive bacterial pathogen that produces many types of surface proteins. To get insights into its intracellular lifestyle, we used high-resolution mass spectrometry to characterize the cell wall proteome of bacteria proliferating within the eukaryotic cell. The relative amount of a few surface proteins was found notoriously different in intracellular bacteria. Internalin A (InlA), which is covalently bound to the peptidoglycan and plays a central role in bacterial entry into non-phagocytic eukaryotic cells, was present in high amounts in the cell wall of intracellular bacteria. Our study also revealed that the actin assembly-inducing protein ActA co-purified with peptidoglycan isolated from intracellular bacteria. Growth of L. monocytogenes in minimal media reproduced the predominance of InlA in the cell wall and the association of ActA with peptidoglycan. Intriguingly, bacteria grown in this condition used ActA for efficient invasion of host cells. These findings suggest that the adaptation of L. monocytogenes to the intracellular lifestyle involves changes in the relative abundance of certain surface proteins and in their mode of association to the peptidoglycan. These alterations, probably promoted by yet-unknown changes in the cell wall architecture, may instruct these proteins to perform different functions outside and inside the host cell.

N-acetylglucosamine-6-phosphate deacetylase (NagA) of Listeria monocytogenes EGD, an essential enzyme for the metabolism and recycling of amino sugars

The main aim of our study was to determine the physiological function of NagA enzyme in the Listeria monocytogenes cell. The primary structure of the murein of L. monocytogenes is very similar to that of Escherichia coli, the main differences being amidation of diaminopimelic acid and partial de-N-acetylation of glucosamine residues. NagA is needed for the deacetylation of N-acetyl-glucosamine-6 phosphate to glucosamine-6 phosphate and acetate. Analysis of the L. monocytogenes genome reveals the presence of two proteins with NagA domain, Lmo0956 and Lmo2108, which are cytoplasmic putative proteins. We introduced independent mutations into the structural genes for the two proteins. In-depth characterization of one of these mutants, MN1, deficient in protein Lmo0956 revealed strikingly altered cell morphology, strongly reduced cell wall murein content and decreased sensitivity to cell wall hydrolase, mutanolysin and peptide antibiotic, colistin. The gene products of operon 150, consisting of three genes: lmo0956, lmo0957, and lmo0958, are necessary for the cytosolic steps of the amino-sugar-recycling pathway. The cytoplasmic de-N-acetylase Lmo0956 of L. monocytogenes is required for cell wall peptidoglycan and teichoic acid biosynthesis and is also essential for bacterial cell growth, cell division, and sensitivity to cell wall hydrolases and peptide antibiotics.

Actin polymerization drives septation of Listeria monocytogenes namA hydrolase mutants, demonstrating host correction of a bacterial defect

The Gram-positive bacterial cell wall presents a structural barrier that requires modification for protein secretion and large-molecule transport as well as for bacterial growth and cell division. The Gram-positive bacterium Listeria monocytogenes adjusts cell wall architecture to promote its survival in diverse environments that include soil and the cytosol of mammalian cells. Here we provide evidence for the enzymatic flexibility of the murein hydrolase NamA and demonstrate that bacterial septation defects associated with a loss of NamA are functionally complemented by physical forces associated with actin polymerization within the host cell cytosol. L. monocytogenes ΔnamA mutants formed long bacterial chains during exponential growth in broth culture; however, normal septation could be restored if mutant cells were cocultured with wild-type L. monocytogenes bacteria or by the addition of exogenous NamA. Surprisingly, ΔnamA mutants were not significantly attenuated for virulence in mice despite the pronounced exponential growth septation defect. The physical force of L. monocytogenes-mediated actin polymerization within the cytosol was sufficient to sever ΔnamA mutant intracellular chains and thereby enable the process of bacterial cell-to-cell spread so critical for L. monocytogenes virulence. The inhibition of actin polymerization by cytochalasin D resulted in extended intracellular bacterial chains for which septation was restored following drug removal. Thus, despite the requirement for NamA for the normal septation of exponentially growing L. monocytogenes cells, the hydrolase is essentially dispensable once L. monocytogenes gains access to the host cell cytosol. This phenomenon represents a notable example of eukaryotic host cell complementation of a bacterial defect.

Comprehensive appraisal of the extracellular proteins from a monoderm bacterium: theoretical and empirical exoproteomes of Listeria monocytogenes EGD-e by secretomics

Defined as proteins actively transported via secretion systems, secreted proteins can have radically different subcellular destinations in monoderm (Gram-positive) bacteria. From degradative enzymes in saprophytes to virulence factors in pathogens, secreted proteins are the main tools used by bacteria to interact with their surroundings. The etiological agent of listeriosis, Listeria monocytogenes, is a Gram-positive facultative intracellular foodborne pathogen, whose ecological niche is the soil and as such should be primarily considered as a ubiquitous saprophyte. Recent advances on protein secretion systems in this species prompted us to investigate the exoproteome. First, an original and rational bioinformatic strategy was developed to mimic the protein exportation steps leading to the extracellular localization of secreted proteins; 79 exoproteins were predicted as secreted via Sec, 1 exoprotein via Tat, 4 bacteriocins via ABC exporters, 3 exoproteins via holins, and 3 exoproteins via the WXG100 system. This bioinformatic analysis allowed for defining a databank of the mature protein set in L. monocytogenes, which was used for generating the theoretical exoproteome and for subsequent protein identification by proteomics. 2-DE proteomic analyses were performed over a wide pI range to experimentally cover the largest protein spectrum possible. A total of 120 spots could be resolved and identified, which corresponded to 50 distinct proteins. These exoproteins were essentially virulence factors, degradative enzymes, and proteins of unknown functions, which exportation would essentially rely on the Sec pathway or nonclassical secretion. This investigation resulted in the first comprehensive appraisal of the exoproteome of L. monocytogenes EGD-e based on theoretical and experimental secretomic analyses, which further provided indications on listerial physiology in relation with its habitat and lifestyle. The novel and rational strategy described here is generic and has been purposely designed for the prediction of proteins localized extracellularly in monoderm bacteria.

Deciphering the intracellular metabolism of Listeria monocytogenes by mutant screening and modelling

Background

The human pathogen Listeria monocytogenes resides and proliferates within the cytoplasm of epithelial cells. While the virulence factors essentially contributing to this step of the infection cycle are well characterized, the set of listerial genes contributing to intracellular replication remains to be defined on a genome-wide level.

Results

A comprehensive library of L. monocytogenes strain EGD knockout mutants was constructed upon insertion-duplication mutagenesis, and 1491 mutants were tested for their phenotypes in rich medium and in a Caco-2 cell culture assay. Following sequencing of the plasmid insertion site, 141 different genes required for invasion of and replication in Caco-2 cells were identified. Ten in-frame deletion mutants were constructed that confirmed the data. The genes with known functions are mainly involved in cellular processes including transport, in the intermediary metabolism of sugars, nucleotides and lipids, and in information pathways such as regulatory functions. No function could be ascribed to 18 genes, and a counterpart of eight genes is missing in the apathogenic species L. innocua. Mice infection studies revealed the in vivo requirement of IspE (Lmo0190) involved in mevalonate synthesis, and of the novel ABC transporter Lmo0135-0137 associated with cysteine transport. Based on the data of this genome-scale screening, an extreme pathway and elementary mode analysis was applied that demonstrates the critical role of glycerol and purine metabolism, of fucose utilization, and of the synthesis of glutathione, aspartate semialdehyde, serine and branched chain amino acids during intracellular replication of L. monocytogenes.

Conclusion

The combination of a genetic screening and a modelling approach revealed that a series of transporters help L. monocytogenes to overcome a putative lack of nutrients within cells, and that a high metabolic flexibility contributes to the intracellular replication of this pathogen.

TelA contributes to the innate resistance of Listeria monocytogenes to nisin and other cell wall-acting antibiotics

Nisin is a class I bacteriocin (lantibiotic), which is employed by the food and veterinary industries and exhibits potent activity against numerous pathogens. However, this activity could be further improved through the targeting and inhibition of factors that contribute to innate nisin resistance. Here we describe a novel locus, lmo1967, which is required for optimal nisin resistance in Listeria monocytogenes. The importance of this locus, which is a homologue of the tellurite resistance gene telA, was revealed after the screening of a mariner random mutant bank of L. monocytogenes for nisin-susceptible mutants. The involvement of telA in nisin resistance was confirmed through an analysis of a nonpolar deletion mutant. In addition to being 4-fold-more susceptible to nisin, the ΔtelA strain was also 8-fold-more susceptible to gallidermin and 2-fold-more susceptible to cefuroxime, cefotaxime, bacitracin, and tellurite. This is the first occasion upon which telA has been investigated in a Gram-positive organism and also represents the first example of a link being established between a telA gene and resistance to cell envelope-acting antimicrobials.

Oleanolic acid and ursolic acid affect peptidoglycan metabolism in Listeria monocytogenes

The plant pentacyclic triterpenoids, oleanolic and ursolic acids, inhibit the growth and survival of many bacteria, particularly Gram-positive species, including pathogenic ones. The effect of these compounds on the facultative human pathogen Listeria monocytogenes was examined. Both acids affected cell morphology and enhanced autolysis of the bacterial cells. Autolysis of isolated cell walls was inhibited by oleanolic acid, but the inhibitory activity of ursolic acid was less pronounced. Both compounds inhibited peptidoglycan turnover and quantitatively affected the profile of muropeptides obtained after digestion of peptidoglycan with mutanolysin. These results suggest that peptidoglycan metabolism is a cellular target of oleanolic and ursolic acids.

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.

Listeria monocytogenes surface proteins: from genome predictions to function

The genome of the human food-borne pathogen Listeria monocytogenes is predicted to encode a high number of surface proteins. This abundance likely reflects the ability of this bacterium to survive in diverse environments, including soil, food, and the human host. This review focuses on the various mechanisms by which listerial proteins are attached at the bacterial surface and their many functions, including peptidoglycan metabolism, protein processing, adhesion to host cells, and invasion of host tissues. Extensive in silico analysis of the domains or motifs present in these mosaic proteins reveals that diverse structural features allow the surface proteome to interact with diverse bacterial or host components. This diversity offers new clues about the molecular bases of Listeria pathogenesis.

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.