Between 1986 and 1988, the incidence of ampicillin-resistant enterococci increased sevenfold at a universityaffiliated hospital. Forty-three patients acquired nosocomial infections with ampicillin-resistant enterococci, most of which were also resistant to mezlocillin, piperacillin, and imipenem. An analysis of plasmid and chromosomal DNAs of isolates revealed that the increase was due to an epidemic of 19 nosocomial infections that yielded closely related strains of Enterococcus faecium and to a significant increase in the incidence of nonepidemic, largely unrelated strains of ampicillin-resistant enterococci. The nonepidemic strains were identified as E.faecium, E. raffinosus, E. durans, and E. gallinarum. A logistic regression analysis revealed that patients with nonepidemic resistant strains were 16 times more likely than controls to have received preceding therapy with imipenem. In our institution, the increase in the incidence of ampicillin-resistant enterococci appears to be due to the selection of various strains of resistant enterococci by the use of imipenem and to the nosocomial transmission of E. faecium and E. raffinosus.Enterococci have become an increasingly important cause of nosocomial infections (10, 14-16, 18, 25, 38, 43). They are now the second most common cause of nosocomial urinary tract and surgical wound infections and are the third most common cause of nosocomial bacteremias (10). Most of these infections are caused by Enterococcus faecalis, a species that is usually susceptible to ampicillin, mezlocillin, piperacillin, and imipenem. A few strains of E. faecalis produce P-lactamase, rendering them resistant to ampicillin, mezlocillin, and piperacillin (17, 20-22, 24, 29-33), but nosocomial transmission of such strains has occurred infrequently (17,24,33).Other species of enterococci, such as E. faecium, may be resistant to imipenem as well as to penicillin, ampicillin, mezlocillin, and piperacillin (11). Such species do not produce P-lactamase and are resistant by virtue of their producing penicillin-binding proteins with a low affinity for betalactams (19,41). Infections caused by these species are much less common than those caused by E. faecalis.In 1987 and 1988, we noted an increase in the frequency of enterococci resistant to penicillin, ampicillin, mezlocillin, piperacillin, and imipenem among patients at a universityaffiliated hospital. The increase was due to an epidemic of closely related strains of E. faecium which was superimposed on an increasing incidence of nonepidemic, largely unrelated strains of E. faecium, E. raffinosus, E. gallinarum, and E. durans. The emergence of nonepidemic strains of ampicillin-resistant enterococci was associated with an increase in the use of imipenem.
MATERIALS AND METHODSMicrobial identification and susceptibility tests. Isolates recovered from clinical specimens were identified as entero-* Corresponding author.
