Postantibiotic effect of ampicillin/sulbactam against mycobacteria

The postantibiotic effect (PAE) is an important pharmacodynamic property of antibiotics. Most drugs continue to exert a suppressive effect on the grow th of bacteria, both in vitro and in vivo, even after the drug concentratio ns have fallen below detectable levels. Only limited information is availab le on the PAE of slow-growing organisms like mycobacteria. The PAE of ampic illin/sulbactam (Unasyn(R)) was investigated against six species of mycobac teria, viz Mycobacterium avium, M. africanum, M. bovis BCG, M. simiae, M, s crofulaceum and M. tuberculosis H37Ra, by spectrophotometry. The cell count er method was also used in one set of experiments. The bacteria were expose d to ampicillin/sulbactam for 2 h, 24 h, 72 h or 7-10 days. Five concentrat ions, 5, 10, 50 or 100 mu g/ml, of the drug were tested. Afterwards, the ba cteria were washed free of Unasyn(R) and allowed to multiply. Treatment of the mycobacteria for 2 h did not produce any PAE, although 100 mu g/ml of t he drug caused slower growth. Exposure to 50, 60, or 100 mu g/ml, resulted in a prolonged PAE of similar to 3 days. The data on the PAE of Unasyn(R) m ay be of clinical relevance in determining dosage regimens of the drug.