Glycotyping and specific separation of Listeria monocytogenes with a novel bacteriophage protein toolkit.

The Gram-positive pathogen Listeria monocytogenes can be subdivided into at least 12 different serovars, based on differential expression of a set of somatic and flagellar antigens. Of note, strains belonging to serovars 1/2a, 1/2b and 4b cause the vast majority of foodborne listeriosis cases and outbreaks. The standard protocol for serovar determination involves an agglutination method using a set of sera containing cell surface-recognizing antibodies. However, this procedure is imperfect in both precision and practicality, due to discrepancies resulting from subjective interpretation. Further, the exact antigenic epitopes remain unclear, due to the preparation of the absorbed sera and the complex nature of polyvalent antibody binding. Here, we present a novel method for quantitative somatic antigen differentiation using a set of recombinant affinity proteins (cell-wall-binding domains and receptor-binding proteins) derived from a collection of Listeria bacteriophages. These proteins enable rapid, objective and precise identification of the different teichoic acid glycopolymer structures, which represent the O-antigens, and allow a near-complete differentiation. This glycotyping approach confirmed serovar designations of over 60 previously characterized Listeria strains. Using select phage receptor-binding proteins coupled to paramagnetic beads, we also demonstrate the ability to specifically isolate serovar 1/2 or 4b cells from a mixed culture. In addition, glycotyping led to the discovery that strains designated as SV 4e actually possess an intermediate 4b-4d teichoic acid glycosylation pattern, underpinning the high discerning power and precision of this novel technique. IMPORTANCEListeria monocytogenes is a ubiquitous opportunistic pathogen that presents a major concern to the food industry due to its propensity to cause food-borne illness. The Listeria genus contains fifteen different serovars, with most of the variance depending on the wall-associated teichoic acid glycopolymers, which confer somatic antigenicity. Strains belonging to serovars 1/2 and 4b cause the vast majority of listeriosis cases and outbreaks, meaning that regulators, as well as the food industry itself, have an interest in rapidly identifying isolates of these particular serovars in food processing environments. Current methods for phenotypic serovar differentiation are slow and lack accuracy, and the food industry could benefit from new technologies allowing serovar-specific isolation. Therefore, the novel method described here for rapid glycotype determination could present a valuable asset to detect and control this bacterium.