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Appl Environ Microbiol. 1970 April; 19(4): 629-637
Copyright © 1970 American Society for Microbiology. All Rights Reserved.

Effect of pH of Medium and Size of Inoculum on Activity of Antibiotics Against Group D Streptococcus (Enterococcus)

Thorndike Memorial Laboratory (Channing Laboratory), Harvard Medical Unit, Boston, Massachusetts 02118
Department of Medical Microbiology, Boston City Hospital, Boston, Massachusetts 02118
Department of Medicine, Harvard Medical School, Boston, Massachusetts 02118

ABSTRACT

Seventy-one clinical isolates of 5 species of group D Streptococcus (Enterococcus) were tested for susceptibility to 15 antibiotics at pH 5.0, 7.4, and 8.5. Penicillin G, ampicillin, cephalothin, cephaloridine, and novobiocin were considerably more active against all strains at pH 5.0 than in the more alkaline media. On the other hand, lincomycin, clindimycin, erythromycin, and gentamicin were moderately to markedly more active at pH 8.5. No important differences were noted in the susceptibility of the strains to kanamycin and streptomycin, at the pH levels tested, but the organisms were quite resistant to them in these tests. The various species differed quantitatively in their susceptibility to the individual drugs and in the effects of pH. The size of the inoculum also had a variable effect on susceptibility, depending on the species of Enterococcus, the antibiotics, and the pH of the test medium. The data suggest that, in the antibiotic treatment of urinary tract infections caused by Enterococcus, attempts should be made to achieve the optimum pH in the urine, particularly in view of the fact that organisms of this group are often resistant to several antibiotics in the usual tests.