Minimum inhibitory concentration of penicillin as a surrogate for in vitro piperacillin susceptibility of ampicillin-susceptible Enterococcus faecalis.

This study evaluated whether penicillin MICs can predict piperacillin susceptibility in Enterococcus faecalis. Among 427 strains, 10 were resistant to piperacillin. Even strains with penicillin MICs of 4-8 µg/mL may exhibit resistance. These findings suggest that even penicillin MICs within the susceptible range may indicate piperacillin resistance, challenging CLSI guidelines.