Antimicrobial Susceptibility and Selection of Resistance Among Staphylococcus epidermidis Isolates Recovered from Patients with Infections of Indwelling Foreign Devices

Abstract

Twenty-seven isolates of Staphylococcus epidermidis from patients with prosthetic valve endocarditis or infected cerebrospinal fluid shunts were examined for susceptibility to antimicrobial agents. Subpopulations resistant to 20 and 100 ttg of methicillin per ml were present in 63% of the isolates (methicillin-resistant isolates). Subpopulations resistant to 20 jig of nafcillin and cephalothin per ml were found in every methicillin-resistant isolate but with frequencies (10-5°0± 0.5 and 10-64 * 0.9, respectively) which were not always detectable by susceptibility testing. Resistance to '1.6 jig of penicillin per ml was found in 80% of isolates.Cephalothin, cefazolin, and cefamandole were more active than cefoxitin or cephradine, and gentamicin was more active than tobramycin or amikacin; rifampin was the single most active agent against all isolates. There was no difference in susceptibility between prosthetic valve endocarditis and cerebrospinal fluid shunt infection isolates. Among methicillin-resistant isolates, the phenotypic expression of resistance to methicillin or nafcillin but not to cephalothin could be enhanced by 48 h of incubation with each drug. Isolates containing no methicillin-resistant subpopulations were killed by incubation with methicillin, nafcillin, or cephalothin. High-level resistance to rifampin emerged in both methicillin-resistant and methicillin-sensitive isolates after 8 to 24 h of incubation with this drug. The presence or absence of antibiotic-resistant subpopulations among S. epidermidis isolates and their selection during treatment should be considered when therapy is devised.

Vancomycin Resistance in Staphylococci

“…This relatively uniform susceptibility has been demonstrated in the in vitro prevalence studies in Table 2, in which only one group reported S. epidermidis isolates with reduced susceptibility to vancomycin (40). This finding seems to be supported in the laboratory, where some investigators have reported that vancomycin resistance in S. epidermidis has been difficult to induce (1,59).…”

Activity of cephalosporins against methicillin-susceptible and methicillin-resistant, coagulase-negative staphylococci: minimal effect of beta-lactamase

“…Although there was a wide range of MICs, we found more cross-resistance between methicillin and cephalosporins among methicillin-resistant, coagulase-negative staphylococci than has been reported previously (1) …”

Susceptibility and Synergy Studies of Methicillin-Resistant Staphylococcus epidermidis

“…In the other two strains, both the rifampin alone and all antibiotic combinations caused at least a 100-fold reduction in the number of CFU at both 24 and 48 h. DISCUSSION Vancomycin was the most consistently effective of the antibiotics tested against MR S. epidermidis, inhibiting all 100 strains at -3.12 ,ug/ ml. Other investigators (3,11) have also found uniform susceptibility of MR S. epidermidis to vancomycin. In this laboratory, Siebert et al…”