Antigenic, taxonomic and epidemiological studies on Listeria monocytogenes and related species
Listeria monocytogenes and the closely related species L ivanovii, L innocua, L welshimeri and L seeligeri are Gram-positive diphtheroid-like bacteria widespread in nature. L monocytogenes causes human infections infrequently, usually during the perinatal period and in immunocompromised adults. L innocua, L welshimeri and L seeligeri are non pathogenic and L ivanovii is usually associated with ovine abortions. Serological diagnosis of infection with L monocytogenes is at present unsatisfactory, mainly because of extensive serological cross-reactions with other bacterial pathogens. As a first step in the development of a specific serological assay, monoclonal antibodies directed against L monocytogenes were produced by fusion of a myeloma cell line with splenic lymphocytes from mice immunized with an ultrasound/EDTA extract of the organism. Eighteen hybridoma clones were selected all of which secreted antibody strongly reactive with all Listeria species, except L denitrificans, but not with Staphylococcus aureus, Streptococcus faecalis, Bacillus subtilis and Escherichia coli. These putative Listeria-specific antibodies, which show at least 4 different antigen specificities in an immunoblot, should prove helpful in the identification and purification of diagnostically useful reagents. The taxonomic relationships of L monocytogenes, L ivanovii, L innocua, L welshimeri and L seeligeri are not fully understood. To investigate their relationships, whole-cell proteins of these species were examined by SDS-PAGE. The polypeptide profiles of L monocytogenes (40 strains), L ivanovii (5 strains), L innocua (4 strains), L welshimeri (1 strain) and L seeligeri (4 strains) were all very similar. Although some differences between the species were observed, they were not greater than the level of intraspecies variation. The classification of these 5 species may, therefore, require further examination. Foetal infection with L monocytogenes may originate from carriage of the organism in the GI tract, the lower genital tract and the pharynx. To investigate the effect of pregnancy on listerial carriage, faecal samples along with cervico-vaginal and oropharyngeal swabs were collected from 54 healthy pregnant women and 60 healthy non-pregnant women, and examined for listeriae by cold-enrichment and repeated subculture on selective media containing acriflavin, nalidixic acid and potassium thiocyanate. L monocytogenes was isolated from the faeces of 1 pregnant woman (2%) and 2 non-pregnant women (3.4%). L innocua was isolated from the faeces of 1 pregnant woman (2%) and 1 non-pregnant woman (1.7%). L seeligeri was isolated from the faeces of 1 non-pregnant woman (1.7%). No listeriae were isolated from cervico-vaginal or oropharyngeal samples. The results indicate that pregnancy does not predispose to colonization, and thus infection, with L monocytogenes, and that faecal, but not cervico-vaginal or oropharyngeal, carriage of pathogenic and non-pathogenic Listeria species occurs in a small percentage of healthy women.