Genetic characterization of plasmid-encoded multiple antibiotic resistance in a strain of Listeria monocytogenes causing endocarditis.

One susceptible and two multiply resistant isolates of Listeria monocytogenes from a patient suffering from prosthetic valve endocarditis are described. They could not be distinguished by several typing methods. Two isolates were resistant to chloramphenicol, macrolide/lincosamide/streptogramin antibiotics and tetracycline. The resistance determinants were located on a 39 kb plasmid pWDB100 that was transferable by filter mating to several gram-positive bacteria. Evidence was obtained to support the hypothesis that the resistant variant had primarily infected the patient's blood and prosthetic valve, and later lost the resistance plasmid. The three resistance determinants showed homology to other known markers, cat221/cat223, ermB and tetM, which are frequently found in different gram-positive genera. Plasmid pWDB100 showed extensive homology to the Streptococcus agalactiae broad-host-range plasmid pIP501. It was also very similar to two listerial plasmids found in France. Thus, plasmid pWDB100 and the homologous plasmids from France, although isolated in geographically distant regions, may illustrate spread of a plasmid and its relatives.