IN VITRO ACTIVITY OF GENTAMICIN, ALONE AND IN COMBINATION WITH OTHER ANTIBIOTICS

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Abstract
A number of in vitro investigations have been carried out with gentamicin. Kinetic experiments were performed on the antibacterial effect of gentamicin alone and in combination with seven other antibiotics. Chloramphenicol and tetracycline inhibited the bactericidal effect of gentamicin, whereas the combination of gentamicin with three penicillins, sul‐phathiazole, polymyxin B, and streptomycin showed an additive effect by inhibiting regrowth of the test organism. The antibacterial spectrum of gentamicin was examined, based on IC50 determinations of 158 gram‐positive and 266 gram‐negative strains. In addition, 519 Staphylococcus aureus strains were tested, 516 of which were regarded as being fully sensitive. Comparison was made of the relationship between the sensitivity of gentamicin and streptomycin, kanamycin, and neomycin. A high degree of concordance between gentamicin and neomycin was found, although this was not complete. Streptomycin and kanamycin showed less correlation with gentamicin. In three strains which had been made resistant to gentamicin, a similar reduction in sensitivity to the other antibiotics developed. A method is described for routine sensitivity testing, using prediffusion and discs containing 25 μg gentamicin sulphate.

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