[Classification of sensitive and resistant strains based on the degree of antibiotic growth inhibition]

The course of growth suppression of sensitive strains depends on the type of antibiotic. Based on the kinetics of the early growth phase in the microculture when the inoculum is relatively concentrated it is possible to divide in the presence of the antibiotic the Gram negative fermenting rods into sensitive and resistant strains already within five hours. Data obtained by automated photometric assessment at selected intervals were processed by the aggregation method using a block distance as the coefficient of similarity. Ampicillin, ticarcillin, gentamicin, tetracycline and colistin readily discriminate sensitive and resistant strains, consistent with the minimal inhibitory concentration. Cefazolin, cefoxitin and chloramphenicol differentiate readily sensitive strains. Slowly growing sensitive strains of Proteus mirabilis and Serratia marcescens may appear to be resistant.