Identification of IspC, an 86-kilodalton protein target of humoral immune response to infection with Listeria monocytogenes serotype 4b, as a novel surface autolysin

Design of a novel affinity probe using the cell wall-binding domain of a Listeria monocytogenes autolysin for pathogen detection

IMPORTANCE

Human listeriosis is caused by consuming foods contaminated with the bacterial pathogen Listeria monocytogenes, leading to the development of a severe and life-threatening foodborne illness. Detection of L. monocytogenes present in food and food processing environments is crucial for preventing Listeria infection. The L. monocytogenes peptidoglycan hydrolase IspC anchors non-covalently to the bacterial surface through its C-terminal cell wall-binding domain (CWBD), CWBDIspC. This study explored the surface binding property of CWBDIspC to design, construct, characterize, and assess an affinity molecular probe for detecting L. monocytogenes. CWBDIspC recognized a cell wall ligand lipoteichoic acid that remains evenly displayed and mostly unoccupied on the bacterial surface for interaction with the exogenously added CWBDIspC. CWBDIspC, when fused to the enhanced green fluorescent protein reporter or covalently conjugated onto magnetic beads, exhibited the functionality as an antibody alternative for rapid detection and efficient separation of the pathogen.

LygA retention on the surface of Listeria monocytogenes via its interaction with wall teichoic acid modulates bacterial homeostasis and virulence

Wall teichoic acid (WTA) is the abundant cell wall-associated glycopolymer in Gram-positive bacteria, playing crucial roles in surface proteins retention, bacterial homeostasis, and virulence. Hypervirulent serovar (SV) 4h Listeria monocytogenes is a newly designated serotype with only galactosylated (Gal) type II WTA. Although the surface association of some proteins relies on the WTA glycosylation, the nature and function of the noncovalent interactions between cell wall-associated proteins and WTA are less known. In this study, we found Gal-WTA plays a key role in modulating the novel glycine-tryptophan (GW) domain-containing autolysin protein LygA through direct interactions. An SV 4h strain deficient in WTA galactosylation (XYSNΔgalT) showed a dramatic reduction of LygA on the cell surface, significantly decreasing the autolytic activity, impairing the bacterial colonization in colon and brain. Notably, we demonstrated LygA binds to Gal-WTA with high affinity through the GW domain and that the extent of binding increases with the number of GW domains. Moreover, we confirmed the direct Gal-dependent binding of the GW protein Auto from the type I WTA strain, which has no interaction with l-rhamnosylated WTA, indicating that the complexity of both WTA and GW proteins can affect the coordination patterns. Altogether, our findings suggest that both the glycosylation patterns of WTA and a fixed numbers of GW domains are closely associated with the retention of LygA on the cell surface, which facilitates L. monocytogenes infection by promoting bacteria colonization in intestine and brain.

Potential Roles and Functions of Listerial Virulence Factors during Brain Entry

Although it rarely induces disease in humans, Listeria monocytogenes (Lm) is important due to the frequency of serious pathological conditions—such as sepsis and meningitis—it causes in those few people that do get infected. Virulence factors (VF) of Lm—especially those involved in the passage through multiple cellular barriers of the body, including internalin (Inl) family members and listeriolysin O (LLO)—have been investigated both in vitro and in vivo, but the majority of work was focused on the mechanisms utilized during penetration of the gut and fetoplacental barriers. The role of listerial VF during entry into other organs remain as only partially solved puzzles. Here, we review the current knowledge on the entry of Lm into one of its more significant destinations, the brain, with a specific focus on the role of various VF in cellular adhesion and invasion.

Identification of Surface Protein Biomarkers of Listeria monocytogenes via Bioinformatics and Antibody-Based Protein Detection Tools

The Gram-positive bacterium Listeria monocytogenes causes a significant percentage of the fatalities among foodborne illnesses in humans. Surface proteins specifically expressed in a wide range of L. monocytogenes serotypes under selective enrichment culture conditions could serve as potential biomarkers for detection and isolation of this pathogen via antibody-based methods. Our study aimed to identify such biomarkers. Interrogation of the L. monocytogenes serotype 4b strain F2365 genome identified 130 putative or known surface proteins. The homologues of four surface proteins, LMOf2365_0578, LMOf2365_0581, LMOf2365_0639, and LMOf2365_2117, were assessed as biomarkers due to the presence of conserved regions among strains of L. monocytogenes which are variable among other Listeria species. Rabbit polyclonal antibodies against the four recombinant proteins revealed the expression of only LMOf2365_0639 on the surface of serotype 4b strain LI0521 cells despite PCR detection of mRNA transcripts for all four proteins in the organism. Three of 35 monoclonal antibodies (MAbs) to LMOf2365_0639, MAbs M3643, M3644, and M3651, specifically recognized 42 (91.3%) of 46 L. monocytogenes lineage I and II isolates grown in nonselective brain heart infusion medium. While M3644 and M3651 reacted with 14 to 15 (82.4 to 88.2%) of 17 L. monocytogenes lineage I and II isolates, M3643 reacted with 22 (91.7%) of 24 lineage I, II, and III isolates grown in selective enrichment media (UVM1, modified Fraser, Palcam, and UVM2 media). The three MAbs exhibited only weak reactivities (the optical densities at 414 nm were close to the cutoff value) to some other Listeria species grown in selective enrichment media. Collectively, the data indicate the potential of LMOf2365_0639 as a surface biomarker of L. monocytogenes, with the aid of specific MAbs, for pathogen detection, identification, and isolation in clinical, environmental, and food samples.

IMPORTANCE

L. monocytogenes is traditionally divided into at least 12 serotypes. Currently, there are no monoclonal antibodies (MAbs) available that are capable of binding to the surface of L. monocytogenes strains representing all 12 serotypes. Such antibodies would be useful and are needed for the development of methods to detect and isolate L. monocytogenes from food samples. In our study, we aimed to identify surface proteins that possess regions of well-conserved amino acid sequences among various serotypes and then to employ them as antigen targets (biomarkers) for the development of MAbs. Through bioinformatics and protein expression analysis, we identified one of the four putative surface protein candidates, LMOf2365_0639, encoded by the genome of the L. monocytogenes serotype 4b strain F2365, as a useful surface biomarker. Extensive assessment of 35 MAbs raised against LMOf2365_0639 in our study revealed three MAbs (M3643, M3644, and M3651) that recognized a wide range of L. monocytogenes isolates.

Draft Genome Sequence of Listeria monocytogenes Strain LI0521 (syn. HPB7171), Isolated in 1983 during an Outbreak in Massachusetts Caused by Contaminated Cheese

Listeria monocytogenes, a pathogenic food-borne bacterium, is the causative agent of both sporadic and outbreak cases of human listeriosis. Here, we present the genome sequence of L. monocytogenes reference strain LI0521, isolated during an outbreak involving contaminated cheese, which has been used as the model during several proteomic studies.

How Listeria monocytogenes organizes its surface for virulence

Listeria monocytogenes is a Gram-positive pathogen responsible for the manifestation of human listeriosis, an opportunistic foodborne disease with an associated high mortality rate. The key to the pathogenesis of listeriosis is the capacity of this bacterium to trigger its internalization by non-phagocytic cells and to survive and even replicate within phagocytes. The arsenal of virulence proteins deployed by L. monocytogenes to successfully promote the invasion and infection of host cells has been progressively unveiled over the past decades. A large majority of them is located at the cell envelope, which provides an interface for the establishment of close interactions between these bacterial factors and their host targets. Along the multistep pathways carrying these virulence proteins from the inner side of the cytoplasmic membrane to their cell envelope destination, a multiplicity of auxiliary proteins must act on the immature polypeptides to ensure that they not only maturate into fully functional effectors but also are placed or guided to their correct position in the bacterial surface. As the major scaffold for surface proteins, the cell wall and its metabolism are critical elements in listerial virulence. Conversely, the crucial physical support and protection provided by this structure make it an ideal target for the host immune system. Therefore, mechanisms involving fine modifications of cell envelope components are activated by L. monocytogenes to render it less recognizable by the innate immunity sensors or more resistant to the activity of antimicrobial effectors. This review provides a state-of-the-art compilation of the mechanisms used by L. monocytogenes to organize its surface for virulence, with special focus on those proteins that work “behind the frontline”, either supporting virulence effectors or ensuring the survival of the bacterium within its host.

Monoclonal antibodies recognizing the surface autolysin IspC of Listeria monocytogenes serotype 4b: epitope localization, kinetic characterization, and cross-reaction studies

Listeria monocytogenes serotype 4b is responsible for a high percentage of fatal cases of food-borne infection. In a previous study, we created 15 monoclonal antibodies (MAbs) against a ∼77 kDa antigen that is associated with the cell surface of live L. monocytogenes serotype 4b cells. Here we report an extensive characterization of these MAbs to further their development as diagnostic reagents. The ∼77 kDa target antigen was identified by mass spectrometry and N-terminal sequencing to be IspC, a novel surface associated autolysin. Epitope localization experiments revealed that each of the 15 MAbs recognized the C-terminal cell-wall binding domain of IspC. The presence of IspC was shown to be highly conserved within L. monocytogenes serotype 4b, as evidenced by a strong reaction between anti-IspC MAbs and all 4b isolates. To determine the range of cross-reactivity with other L. monocytogenes serotypes ELISA was used to test each MAb against multiple isolates from each of the L. monocytogenes serotypes. Of the 15 MAbs, five: M2774, M2775, M2780, M2790 and M2797, showed specificity for L. monocytogenes serotype 4b and only cross reacted with serotype 4ab isolates. The kinetics of the interaction between each of the MAbs and IspC was measured using surface plasmon resonance. The MAbs M2773, M2792, M2775, M2797 and M2781 each had very low dissociation constants (4.5 × 10⁻⁹ to 1.2 × 10⁻⁸ M). While several of these antibodies have properties which could be useful in diagnostic tests, the combined high fidelity and affinity of M2775 for the IspC protein and serotype 4b isolates, makes it a particularly promising candidate for use in the development of a specific L. monocytogenes serotype 4b diagnostic test.

Unveiling the expression characteristics of IspC, a cell wall-associated peptidoglycan hydrolase in Listeria monocytogenes, during growth under stress conditions

Listeria monocytogenes serotype 4b is a food-borne pathogen of public health concern, since it accounts for approximately 40% of human listeriosis cases. We have recently identified IspC, a surface-localized peptidoglycan hydrolase, as the antigen recognized by a number of monoclonal antibodies (MAbs) produced against a serotype 4b strain for diagnostic applications. To determine whether IspC, which is well conserved among various serotype 4b strains, is a useful diagnostic marker in antibody-based methods, we assessed the expression of IspC in L. monocytogenes cultured under normal and stress conditions. A functional promoter directing the transcription of the ispC gene was identified upstream of the ispC open reading frame by constructing a promoterless lacZ gene fusion with the putative ispC promoter region and by 5' rapid amplification of cDNA ends analysis. Using both the lacZ reporter gene system and immunofluorescent staining with an IspC-specific MAb, we provide evidence that IspC is expressed on the cell surface in all growth conditions tested (temperature, osmotic stress, pH, ethanol, oxidative stress, anaerobic conditions, carbon source, and type of growth media) that allow for cellular division, although the level of ispC gene expression varies. These results demonstrated the usefulness of IspC as an excellent diagnostic marker for the serotype 4b strains and imply that IspC, in conjunction with specific MAbs, can be targeted for detection and isolation of L. monocytogenes serotype 4b strains directly from food, environmental, and clinical samples with minimal or no need for culture enrichment.

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.

Bacterial peptidoglycan degrading enzymes and their impact on host muropeptide detection

Peptidoglycan (PGN) is a major component of the bacterial cell envelope in both Gram-positive and Gram-negative bacteria. These muropeptides can be produced or modified by the activity of bacterial glycolytic and peptidolytic enzymes referred to as PGN hydrolases and autolysins. Some of these bacterial enzymes are crucial for bacterial pathogenicity and have been shown to modulate muropeptide release and/or host innate immune responses. The ability of muropeptides to modulate host responses is due to the fact that eukaryotes do not produce PGN and have instead evolved numerous strategies to detect intact PGN and PGN fragments (muropeptides). Here we review the structure of PGN and introduce the various bacterial enzymes known to degrade or modify bacterial PGN. Host factors involved in PGN and muropeptide detection are also briefly discussed, as are examples of how specific bacterial pathogens use PGN degradation and modification to subvert host innate immunity.

A pair of ligation-independent Escherichia coli expression vectors for rapid addition of a polyhistidine affinity tag to the N- or C-termini of recombinant proteins

6x His tag is one of the most widely used affinity fusion tags that facilitates detection and purification of recombinant proteins. However, the location of this tag within a particular type of protein may influence the expression, solubility, and bioactivity of the protein, and the optimal location needs to be determined experimentally. To provide a tool for rapid generation of 6x His tags at the N- or C-terminus of any recombinant protein, we have constructed a pair of Escherichia coli expression vectors-pLIC-NHis and pLIC-CHis-based on the pET30a vector, for ligation-independent cloning (LIC). Construction of this new pair of LIC vectors was accomplished by replacement of the multiple cloning site of pET30a with two specifically designed LIC cloning sites. A target gene derived by PCR with a pair of predesigned primers can be inserted into the LIC site of pLIC-NHis for expression of recombinant proteins fused with the N-terminal sequence MHHHHHHG or into that of pLIC-CHis for expression of recombinant proteins with the C-terminal sequence THHHHHH. Successful expression of two normal mammalian prion proteins and five bacterial proteins in E. coli using this pair of LIC vectors reveals that these vectors are valuable tools for the production of recombinant His-tagged proteins in E. coli.

A novel cell wall-anchored peptidoglycan hydrolase (autolysin), IspC, essential for Listeria monocytogenes virulence: genetic and proteomic analysis

We have recently concluded that a Listeria monocytogenes 86 kDa immunogenic surface protein, IspC, is a cell wall-anchored peptidoglycan hydrolase (autolysin), capable of degrading the cell wall peptidoglycan of the bacterium itself. To determine if this enzyme has any biological functions and/or plays a role in virulence, we in-frame-deleted the ispC gene from the L. monocytogenes chromosome. This DeltaispC mutant exhibited complete abrogation of expression of IspC and displayed no defects in in vitro growth, colony and microscopic morphologies, or biochemical characteristics. Lack of IspC led to attenuated virulence in mice, evidenced by a significant reduction in bacterial counts in livers and brains and no mortality compared with the wild-type. Furthermore, the data from assays using various eukaryotic cells for adhesion, invasion, actin tail formation, plaque formation and intracellular growth indicated that the mutant was severely attenuated in virulence in a cell culture model in a cell type-dependent manner. The findings that (i) the mutant was impaired for adhesion to certain eukaryotic cells, and (ii) both purified IspC and its C-terminal cell wall-binding domain were capable of binding sheep choroid plexus (SCP) epithelial cells and Vero cells, supported the role of IspC as an adhesin in virulence. The DeltaispC mutant exhibited a marked defect in adhesion to and invasion of SCP cells but not human brain microvascular endothelial cells (HBMEC), suggesting that IspC is necessary for crossing the blood-cerebrospinal fluid barrier. Proteomic and immunological analysis showed a reduced surface expression of some known or putative virulence factors (e.g. ActA, InlC2 and a flagellin homologue, FlaA) due to IspC deficiency. Altogether, this study demonstrates that IspC, expressed as a minor autolysin in vitro, is not important for cell division or separation but is essential for full virulence of L. monocytogenes in vivo.