Lack of influence of commonly used drugs on bioassay indicator organisms

Many commonly used pharmaceutical agents have been found to inhibit bacterial growth in vitro. Determinations of antimicrobial concentrations in sera of patients taking nonrecognized antibacterial agents could possibly be altered if bioassay systems are utilized for the determinations. We therefore attempted to determine the in vitro effect of commonly used drugs on bioassay indicator organisms. Fifty-one different agents (antihistamines, anticholinergics, central nervous system agents, cardiovascular agents, analgesics, steroids, muscle blockers, and other miscellaneous agents) were tested for inhibition or enhancement of the growth of Bacillus subtilis, Staphylococcus aureus, Micrococcus luteus, and Klebsiella pneumoniae. None of the agents tested exhibited any effect on standard in vitro bioassay organisms. Nortriptyline hydrochloride inhibited the growth of B. subtilis and M. luteus at a concentration of 500 micrograms/ml (zones of inhibition, 14 and 13 mm, respectively), but no inhibition was observed with concentrations of 50 micrograms/ml or lower.