In vitro evaluations of aminopenicillin/beta-lactamase inhibitor combinations

Sulbactam/ampicillin (Unasyn) possesses the antimicrobial activity of each drug plus the synergistic action and qualities of sulbactam used as a beta-lactamase inhibitor. The combination has a spectrum of activity against many Enterobacteriaceae, Haemophilus species, Branhamella catarrhalis, pathogenic Neisseria species, Acinetobacter anitratus, some pseudomonads, nearly all anaerobes (including Bacteroides), Staphylococcus species, streptococci, and the enterococci. Sulbactam/ampicillin was found to be bactericidal, and its minimum inhibitory concentrations (MICs) affected only by high inoculum concentrations (greater than 10(6) CFU/ml). Fixed ratio in vitro susceptibility tests appear to be preferred, since they best approximate clinical formulations and drug pharmacokinetics. The ratio of 1:1 has been studied, but other ratios such as 1:2 (sulbactam/ampicillin) should be evaluated to promote greater test accuracy. Separate interpretive criteria for most Gram-negative bacteria, Haemophilus, and staphylococci appear unnecessary for some tests.