Listeria monocytogenes infections: laboratory investigation of an unexpected increase in incidence.

Listeria monocytogenes is a facultative intracellular pathogen causing mainly meningitis and septicemia in immunocompromised hosts. From July 1988 through December 1989, 16 patients at The Mount Sinai Hospital were diagnosed as having listeriosis shortly after admission, 14 within a one-year period (July 1988-July 1989). Because this incidence was almost double the incidence in previous years (< 8 annually), an epidemiologic and microbiologic investigation was undertaken to determine a potential route of acquisition of L. monocytogenes. On the basis of plasmid profile, bacteriocin (enterocin) susceptibility pattern, and serotype, no single epidemic strain could be identified. Although direct evidence was lacking, we concluded that our patients may have acquired L. monocytogenes through transient contamination of food.