Diminished effect of gentamicin under anaerobic or hypercapnic conditions.

The in-vitro activity of gentamicin, judged by M.I.C. determinations, was much reduced when a normal aerobic atmosphere was replaced either by air +4% CO2 or by anaerobic conditions. The phenomenon was greatest for Staphylococcus aureus, where a decrease in activity of up to 20-fold was found. For Escherichia coli, Klebsiella aerogenes, Enterobacter spp., and Proteus spp. the factor of decrease was between 15-fold and 2-5-fold. Changes in medium pH, as a result of bacterial growth, can explain these findings for some, but not all, the species tested.