Membrane filtration in the study of antimicrobial drugs.

A membrane filtration apparatus of the type employed in water bacteriology was used in the assessment of some antimicrobial drugs. Samples of liquid nutrient medium inoculated with a small number of bacteria, to which dilutions of a drug had been added before incubation and again later, were filtered and the membranes washed and incubated on solid medium. By this means differences between bacteriostatic and bactericidal action were clearly demonstrated. It was shown that staphylococci which had survived exposure to penicillin or gentamicin at levels below their minimum bactericidal concentrations remained fully responsive to the subsequent addition of more of the same drug, while those which had survived exposure to chloramphenicol or clindamycin were not significantly affected by further addition, so that neither of the latter two drugs was bactericidal against the strain of organism used. Bactericidal effects were also demonstrated by benzyl penicillin against Clostridium welchii and ampicillin against Escherichia coli.