Pseudo-outbreak of listeriosis elucidated by pulsed-field gel electrophoresis.

Listeria monocytogenes was isolated from three patients hospitalized in two departments of the same hospital over a two-week period. A nosocomial outbreak of listeriosis was suspected. Patients presented with tenosynovitis, central venous catheter infection, and bacteremia. The first patient had a community-acquired infection, the second a nosocomial infection, and the source of the third patient's illness was uncertain. Epidemiological investigations failed to identify a common source of contamination within the hospital. The three strains were nontypeable by phage typing, but Smal macrorestriction analysis and pulsed-field gel electrophoresis yielded three distinct profiles. Therefore, the three cases seemed to represent a cluster of sporadic cases as opposed to an outbreak of listeriosis. Rapid typing of isolates is essential in the early investigation of potential outbreaks of listeriosis and may prevent the initiation of expensive and time-consuming epidemiological investigations.