An approach to quantitative antibiotic sensitivity testing for some Gram-negative pathogens

Abstract

The twofold dilution method for quantitative antibiotic sensitivity testing in agar is applicable where each antibiotic concentration in the series is the minimal inhibiting one for a significant proportion of cultures to be tested. The method is of much less relevance where cultures show a biphasic distribution consisting of very sensitive and highly resistant components, or where the margin between the minimal inhibiting concentration (m.i.c.) and attainable blood concentration is small, so that response to a single critical concentration becomes of prime importance.Experiments indicated (a) that kanamycin sensitivity of most Enterobacteria could be measured adequately by their qualitative responses to a single drug concentration (8 μg/ml); (b) that sensitivity of Pseudomonas aeruginosa to gentamicin could be measured in terms of log reduction in viable population by a concentration of 4 μg/ml; and (c) that the classical twofold dilution method was suitable for measuring the sensitivity of P. aeruginosa to carbenicillin.The principle established is that the method of quantitative testing should be adapted to the antibiotic–bacterium system under study.