In vitro Activity of Gentamicin against Common Pathogenic Bacteria

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Abstract
It has repeatedly been observed that the results of sensitivity testing for gentamicin are influenced by both the pH of the medium and the size of the inoculum. In other reports the pH of the medium is generally kept within narrow limits but techniques used do allow for variations in the size of the inocula. In this study sensitivity testing for gentamicin in a number of strains of common pathogens was therefore performed under controlled conditions of both pH and inoculum size. When the MIC values were estimated under these experimental conditions it was found that 17 strains of E. coli, 33 strains of Proteus mirabilis, 28 strains of the indole positive Proteus species and 10 strains of Klebsiella showed a good sensitivity to the antibiotic. According to the MIC values 6 strains of Providencia were completely resistant and 15 strains of Pseudomonas were moderately sensitive. In all these strains the MBC of gentamicin was found to be only one to two times higher than the MIC. In 8 strains of Staphylococcus aureus and 6 strains of Staphylococcus albus low MIC values suggested a good sensitivity. In these cases, however, the corresponding MBC values were 3 to 4 times higher than the MIC values. The implications of this result for the efficacy of gentamicin in clinical therapy are discussed.

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