Pseudo-outbreak of Enterococcus durans infections and colonization associated with introduction of an automated identification system software update

Abstract

Enterococci are an important cause of hospital-acquired infections. Since 1989, there has been an increase in the number of nosocomial enterococcal infections caused by strains resistant to vancomycin in the United States. Although many enterococcal species can colonize humans, only Enterococcus faecalis, E. faecium, E. raffinosus, and E. casseliflavus have been implicated in clusters of infection. In January 1996, the Centers for Disease Control and Prevention received a report of an outbreak of vancomycin-resistant enterococci in which 31 of 84 (36.9%) isolates were identified as E. durans. Twenty-nine isolates identified as E. durans were identified to the species level after the introduction of an automated identification system software update (Vitek gram-positive identification card, version R09.1) for the identification of species or gram-positive organisms. When seven isolates initially reported as E. durans were identified to the species level by alternate methods, they were found to be E. faecium. Subsequently, isolates identified as E. durans by the automated system were reidentified by using a rapid streptococcus test, and no further enterococcal isolate has been confirmed as E. durans. Automated microbial analysis is a potential source of error that is not easily recognized. When laboratory findings are discordant with expected clinical or epidemiologic patterns, confirmatory testing by alternate methods should be performed.