Reactivities of genus-specific monoclonal antibody EM-6E11 against Listeria species and serotypes of Listeria monocytogenes grown in nonselective and selective enrichment broth media

Expression of Surface Protein LapB by a Wide Spectrum of Listeria monocytogenes Serotypes as Demonstrated with Anti-LapB Monoclonal Antibodies

Protein antigens expressed on the surface of all strains of Listeria monocytogenes and absent from nonpathogenic Listeria spp. are presumably useful targets for pathogen identification, detection, and isolation using specific antibodies (Abs). To seek such surface proteins expressed in various strains of L. monocytogenes for diagnostic applications, we focused on a set of surface proteins known to be involved or putatively involved in L. monocytogenes virulence and identified Listeria adhesion protein B (LapB) as a candidate based on the bioinformatics analysis of whole-genome sequences showing that the gene coding for LapB was present in L. monocytogenes strains and absent from strains of other Listeria spp. Immunofluorescence microscopy (IFM), performed with rabbit polyclonal antibodies against the recombinant LapB protein (rLapB) of L. monocytogenes serotype 4b strain L10521, confirmed expression of LapB on the surface. A panel of 48 mouse monoclonal antibodies (MAbs) to rLaB was generated, and 7 of them bound strongly to the surface of L. monocytogenes cells as demonstrated using IFM. Further characterization of these 7 anti-LapB MAbs, using an enzyme-linked immunosorbent assay (ELISA), revealed that 6 anti-LapB MAbs (M3484, M3495, M3500, M3509, M3517, and M3519) reacted strongly with 46 (86.8%) of 53 strains representing 10 of the 12 serotypes tested (1/2a, 1/2b, 1/2c, 3a, 3b, 3c, 4ab, 4b, 4d, and 4e). These results indicate that LapB, together with companion anti-LapB MAbs, can be targeted as a biomarker for the detection and isolation of various L. monocytogenes strains from contaminated foods.

IMPORTANCE

Strains of L. monocytogenes are traditionally grouped into serotypes. Identification of a surface protein expressed in all or the majority of at least 12 serotypes would aid in the development of surface-binding monoclonal antibodies (MAbs) for detection and isolation of L. monocytogenes from foods. Bioinformatics analysis revealed that the gene coding for Listeria adhesion protein B (LapB), a surface protein involved in L. monocytogenes virulence, was present in L. monocytogenes strains and absent from other Listeria spp. Polyclonal antibodies against recombinant LapB (rLapB) detected the exposed epitopes on the surface of L. monocytogenes Production and extensive assessment of 48 MAbs to rLapB showed that 6 anti-LapB MAbs (M3484, M3495, M3500, M3509, M3517, and M3519) detected the expression of LapB in a wide range of L. monocytogenes isolates representing 10 of 12 serotypes tested, suggesting that LapB, together with specific MAbs, can be targeted as a biomarker for pathogen detection and isolation.

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.

Characterization of new hybridoma clones producing monoclonal antibodies reactive against both live and heat-killed Listeria monocytogenes

The objective of this study is to develop high affinity monoclonal antibody (MAb) probes recognizing all major serotypes of Listeria monocytogenes cells. From 500 candidate hybridoma clones, 2 new monoclonal antibody-producing hybridomas were selected and evaluated. MAbs 22D10 and 24F6 reacted strongly with live cells of most serotypes of L. monocytogenes except 4c and 4e and with some L. innocua strains; MAb 22D10 reacted strongly with both live and heat-killed cells (100 degrees C for 20 min) of Listeria. Both MAbs 22D10 and 24F6 did not show any cross-reactions with the other non-Listeria G(+) bacteria tested in ELISA. The mixture of EM-7G1 and 22D10 or 24F6 reacted with all 13 major serotypes of live L. monocytogenes except serotype 4c, while none of these 3 MAbs when tested alone did so. MAb 22D10 mixed with 7G1 reacted with all heat-killed L. monocytogenes serotypes except 4c and 4e. In Western blots, MAbs 22D10 and 24F6 reacted with 1 major protein band of 66 kDa in extracts from L. monocytogenes, but with 2 major protein bands of 66 kDa and 76 kDa in extracts from L. innocua. These results suggest that MAbs 22D10 and 24F6 have high affinity for 11 of 13 serotypes of L. monocytogenes, both live and heat-killed cells. MAbs 22D10 and 24F6--in combination with species-specific MAb EM-7G1--should be useful candidates for use in an ELISA sandwich assays for detecting L. monocytogenes in RTE meat and poultry products.

Differential expression of InlB and ActA in Listeria monocytogenes in selective and nonselective enrichment broths

AIM

To investigate the effect of selective and nonselective media on the expression of ActA and InlB proteins in Listeria monocytogenes.

METHODS AND RESULTS

Polyclonal antibodies to InlB and ActA were used in western blotting to determine the effect of selective (BLEB, UVM, and FB) or nonselective (BHI and LB) enrichment broths or hotdog exudates. Of the 13 L. monocytogenes serotypes tested, 11 and 12 serotypes showed a strong InlB expression in brain heart infusion (BHI) and Luria-Bertani (LB), respectively, while only seven and one serotypes showed a strong ActA expression in these two respective broths, and others showed a weaker or no expression. On the contrary, in selective broths, expression of InlB was either very weak or undetectable. However, ActA expression was stronger in 12 serotypes when grown in buffered Listeria enrichment broth (BLEB), 11 in University of Vermont medium (UVM), and 10 in Fraser broth (FB). When tested in hotdog exudates, InlB and ActA were detected in serotypes grown at 37 degrees C but not at 4 degrees C. Transmission electron microscopy, enzyme-linked immunosorbent assay, and mRNA analysis further supported these observations.

CONCLUSION

Overall, selective enrichment broths promote ActA while nonselective broths promote InlB expression.

SIGNIFICANCE AND IMPACT OF THE STUDY

As commonly recommended enrichment broths show differential InlB and ActA expression, proper media must be selected to avoid false results during antibody-based detection of L. monocytogenes.

Development of Listeria monocytogenes-specific immunomagnetic beads using a single-chain antibody fragment

A method for coupling single-chain antibody fragments (scFvs) to immunomagnetic beads (IMBs) was developed and evaluated using scFvs specific for Listeria monocytogenes. A plasmid vector, pBAD380, was constructed that allowed the expression of histidine-tagged biotinylated scFvs in Escherichia coli. The gene encoding a scFv specific for L. monocytogenes was cloned into pBAD380 and the 6-histidine-tagged biotinylated anti-L. monocytogenes scFvs were coupled to streptavidin-coated IMBs. The ability of the anti-L. monocytogenes scFv-IMBs to capture L. monocytogenes and other Listeria species was evaluated in comparison to commercially available anti-Listeria IMBs. The anti-L. monocytogenes scFv-IMBs displayed higher efficiencies of capture (1.38-19.04%) for most strains of L. monocytogenes than were observed for the anti-Listeria IMBs (0.05-3.35%); also, the anti-L. monocytogenes scFv-IMBs exhibited improved specificity for L. monocytogenes as determined by cell capture efficiency in pure and mixed cultures.

Selective enrichment media affect the antibody-based detection of stress-exposed Listeria monocytogenes due to differential expression of antibody-reactive antigens identified by protein sequencing

Selective enrichment broths are frequently used to recover stressed Listeria cells to detectable levels, but the ability of antibodies to detect these cells from various commonly used enrichment media is unknown. In this study, a polyclonal (PAb) and monoclonal (MAb) antibody were used to examine the variation in antigen expression on healthy or stress-recovered Listeria monocytogenes cells grown in brain heart infusion broth, buffered Listeria enrichment broth (BLEB), Listeria repair broth (LRB), University of Vermont medium (UVM), and Fraser broth (FB) for immunodetection. Indirect enzyme-linked immunosorbent assay (ELISA) data showed that L. monocytogenes subjected to stresses (acid, cold, heat, and salt) and then grown in BLEB gave the highest reaction with the anti-Listeria PAb while those grown in LRB gave the highest reaction with the MAb C11E9. Cells grown in UVM and FB gave poor ELISA values with both antibodies. Western blotting with PAb revealed differential expression of surface proteins of 62, 58, 50, 43, and 30 kDa on L. monocytogenes cells, with most proteins displaying elevated expression in BLEB and LRB but reduced or no expression in UVM or FB. Similar differential expressions were noticed for C11E9. PAb-reactive proteins were identified as putative LPXTG-motif cell-wall anchor-domain protein (62 kDa; lmo0610), flavocytochrome C fumarate reductase chain A homolog protein (58 kDa; lmo0355), enolase (50 kDa; lmo2455), glyceraldehyde 3-phosphate dehydrogenase (43 kDa; lmo2459), and hypothetical phospho-sugar binding protein (30 kDa; lmo0041), respectively, and the MAb-reactive 66-kDa protein was confirmed to be N-acetylmuramidase (lmo2691). In conclusion, BLEB and LRB favorably supported increased expression of antigens and proved to be superior to UVM and FB for immunodetection of stressed L. monocytogenes cells.

Single-chain Fv antibody with specificity for Listeria monocytogenes

Single chain antibodies (scFv) exhibiting specific binding to Listeria monocytogenes strains were isolated from a pool of random scFvs expressed on the surface of filamentous bacteriophages. Positive selection (panning) using L. monocytogenes was used to enrich for phage clones with the desired binding affinity, and negative selection using L. innocua and L. ivanovii was used to remove phages expressing cross-reactive antibody fragments. A single phage clone, P4:A8, was selected using two independent panning schemes. A rapid assay was devised to determine phage antibody binding specificity and was used to develop a selectivity profile for individual phage clones. The P4:A8 clone was screened against a panel of bacteria consisting of eight strains of L. monocytogenes, one each of the other six species of Listeria and nine other relevant bacterial species. A collection of individual clones from the penultimate panning was also screened against a subset of the panel of bacteria. The selectivity profiles indicate that multiple clones, including P4:A8, exhibit binding to one or more strains of L. monocytogenes without cross-reactivity toward any other species in the panel. This is the first report of a species-specific antibody for viable cells of L. monocytogenes (i.e., the ability to bind to L. monocytogenes without cross-reactivity toward any other species of Listeria).

Single-cell microbiology: tools, technologies, and applications

The field of microbiology has traditionally been concerned with and focused on studies at the population level. Information on how cells respond to their environment, interact with each other, or undergo complex processes such as cellular differentiation or gene expression has been obtained mostly by inference from population-level data. Individual microorganisms, even those in supposedly "clonal" populations, may differ widely from each other in terms of their genetic composition, physiology, biochemistry, or behavior. This genetic and phenotypic heterogeneity has important practical consequences for a number of human interests, including antibiotic or biocide resistance, the productivity and stability of industrial fermentations, the efficacy of food preservatives, and the potential of pathogens to cause disease. New appreciation of the importance of cellular heterogeneity, coupled with recent advances in technology, has driven the development of new tools and techniques for the study of individual microbial cells. Because observations made at the single-cell level are not subject to the "averaging" effects characteristic of bulk-phase, population-level methods, they offer the unique capacity to observe discrete microbiological phenomena unavailable using traditional approaches. As a result, scientists have been able to characterize microorganisms, their activities, and their interactions at unprecedented levels of detail.

Expression of cellular antigens of Listeria monocytogenes that react with monoclonal antibodies C11E9 and EM-7G1 under acid-, salt- or temperature-induced stress environments

AIMS

To study the expression of cellular antigens of Listeria monocytogenes that react with monoclonal antibodies (MAbs) C11E9 and EM-7G1 under acid-, salt- or temperature-induced stress environments.

METHODS AND RESULTS

The reaction patterns of antibodies to L. monocytogenes held in stressful environments for a short duration (3 h) or grown for extended periods (16-72 h) were investigated. During both short or prolonged exposure to stress environments of high temperature (45 degrees C) and NaCl (>1.5%, w/v), reactions of whole cells of L. monocytogenes to antibodies were severely affected as determined by ELISA and by the reduced expression of the antibody-reactive 66 kDa antigen in the Western blot assay. Conversely, cold (4-15 degrees C) or acid (pH 2-3) stress environments had very little effect on antigen expression or antibody reaction. Additionally, heat-killed cells showed reduced reactions to these antibodies when compared with unheated cells. Artificially created stress environments in hotdog slurry also affected the antigen expression in L. monocytogenes. Immunoelectron microscopy revealed that the antibody-reactive antigens were uniformly present on the surface of the cells. Morphological characteristics following growth in stressed environments revealed that heat stress at 45 degrees C caused L. monocytogenes cells to be elongated and to form clumps; whereas, osmotic stress (5.5% NaCl, w/v) caused filamentous appearance with multiple septa along the length of the cell.

CONCLUSIONS

These results indicated that MAb C11E9 or EM-7G1 could detect L. monocytogenes from cold or acid-stress environments; however, they may show weaker reactions with heat or osmotically stressed cells or cells grown at 4 degrees C.

SIGNIFICANCE AND IMPACT OF THE STUDY

Bacteria in food are routinely subjected to various stresses, induced by cold, heat, salt or acid during processing and storage. Whether stresses would modify the expression of cellular antigens of L. monocytogenes is of a great concern for immunodetections in food products.

Comparison of selective and non-selective enrichment media in the detection of Listeria monocytogenes from meat containing Listeria innocua

AIMS

This study investigated whether the higher incidence of recovery from meat of Listeria innocua compared with L. monocytogenes could be due to the laboratory media used, leading to an artificially lower detection of the pathogenic species, L. monocytogenes.

METHODS AND RESULTS

Minced beef was inoculated with L. monocytogenes, L. innocua, or a mixture of these species, and stored at 0 or 10 degrees C under vacuum or aerobic conditions for up to 28 days. Listeria were recovered from the minced beef using selective (University of Vermont Medium, UVM) and non-selective (Buffered Peptone Water, BPW) enrichment broths after 0, 14, and 28 days of storage. In general, there were no significant differences (P < 0.05) between the numbers of L. monocytogenes recovered from minced beef samples after 24 h enrichment in BPW and the numbers recovered using UVM. In addition, the presence of L. innocua in meat samples containing L. monocytogenes did not significantly (P < 0.05) affect the numbers of L. monocytogenes recovered using either enrichment broth. In most cases there were no significant differences (P < 0.05) between the numbers of L. innocua recovered from minced beef samples after 24 h enrichment in BPW compared with numbers recovered using UVM.

CONCLUSION

Listeria innocua was found to have no significant competitive advantage over L. monocytogenes in selective or non-selective enrichment media.

SIGNIFICANCE AND IMPACT OF THE STUDY

The results suggest that, in some instances, the use of a selective enrichment broth offers no advantage over a non-selective enrichment broth for the recovery of Listeria species from minced beef.