Simple combined agar-strip test for detection of enterococcal susceptibility to ampicillin, moderate- or high-level gentamicin resistance and potential additive activity of ampicillin plus gentamicin.

A simple combined agar-strip test (CAST), involving filter paper strips charged with 20 micrograms of ampicillin or 40 micrograms of gentamicin, accurately detected high-level gentamicin resistance among 43 of 146 isolates of Enterococcus faecalis and additive activity of ampicillin plus gentamicin against 101 of 103 (98%) isolates of E. faecalis with moderate-level gentamicin resistance (MLGR). All 15 MLGR Enterococcus faecium isolates were moderately to highly resistant to ampicillin; 5 isolates with moderate resistance to ampicillin (minimal inhibitory concentrations = 16 micrograms/ml) were additively inhibited by ampicillin plus gentamicin, whereas the highly ampicillin-resistant E. faecium isolates yielded an indifferent effect with the CAST procedure.