We determined that resistance to ciprofloxacin has emerged in enterococci over the last 5 years in our hospital, mainly in strains demonstrating the phenotype of high-level gentamicin resistance. All high-levelgentamicin-resistant isolates from 1985 and 1986 were susceptible, whereas 24% of isolates from 1989 and 1990 were resistant to ciprofloxacin. Plasmid and genomic DNA typing showed at least six unique strains exhibiting resistance, but one type accounted for 80%o of recent resistant isolates, suggesting a role for cross infection in the emergence of resistance.Enterococci are a common cause of nosocomial infection and are notable for their resistance to many antimicrobial agents (2, 9). In recent years, increasing resistance to aminoglycosides (22), penicillins (1), and.vancomycin (18) has been noted. Ciprofloxacin has been shown to have activity in vitro against enterococci (6,7,15) and has been used in treating enterococcal infections (3, 5). Although relatively high MICs and the existence of an inoculum effect have been noted, resistance to ciprofloxacin has been uncommon (6,7,11,15,21).In both the acute-care and long-term-care facilities of the Ann Arbor Veterans Affairs Medical Center, the majority of enterococci have high-level resistance to aminoglycosides (22,23). We determined whether resistance to ciprofloxacin among enterococcal isolates had increased from 1985 and 1986 to 1989 and 1990. Additionally, we determined whether resistance was predominantly in high-level-aminoglycosideresistant strains and whether one strain or multiple strains of ciprofloxacin-resistant enterococci had emerged in our facility. Enterococci were screened for resistance to ciprofloxacin, as described previously (4), by dotting an inoculum of 104 CFU/ml of an overnight culture suspended in saline onto Mueller-Hinton agar containing 4 ,ug of ciprofloxacin per ml.
Strains of enterococci causing infection in patients inMICs for infecting isolates from 1989 and 1990 were determined in the Clinical Microbiology Laboratory by the API Uniscept microdilution broth method (Analytab); the MICs for susceptible isolates were < 1.0 ,ug/ml, while those for resistant strains were >4.0 ,ug/ml. Thirty randomly chosen high-level-gentamicin-resistant, ciprofloxacin-resistant isolates from 1989 and 1990 were selected for extended susceptibility testing by microtiter broth dilution and both plasmid DNA and genomic DNA analyses. Susceptibility testing showed all these isolates to require ciprofloxacin MICs of >25 p,g/ml and gentamicin MICs of >2,000 ,g/ml. For 16 aminoglycoside-resistant isolates from 1985 and 1986, gentamicin MICs were also found to be >2,000 ,ug/ml but ciprofloxacin MICs ranged from 0.4 to 1.2 ,ug/ml. For plasmid DNA analysis, isolates were lysed and plasmid DNA was isolated according to the method described by Zervos et al. (20), with the added step that the plasmid DNA samples were incubated with 1 ,ul of 5-mg/ml RNase for 1 h at 37°C prior to electrophoresis on 0.7% agarose gels. Lambda DNA (GIBCO-BRL, Grand Island, N...
