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

Abstract

Seventy-one clinical isolates of 5 species of group D Streptococcus (Enterococcus) were tested for susceptibility to 15 antibiotics at p H 5.0, 7.4, and 8.5. Penicillin G, ampicillin, cephalothin, cephaloridine, and novobiocin were considerably more active against all strains at p H 5.0 than in the more alkaline media. On the other hand, lincomycin, clindimycin, erythromycin, and gentamicin were moderately to markedly more active at p H 8.5. No important differences were noted in the susceptibility of the strains to kanamycin and streptomycin, at the p H 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 p H. The size of the inoculum also had a variable effect on susceptibility, depending on the species of Enterococcus , the antibiotics, and the p H 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 p H in the urine, particularly in view of the fact that organisms of this group are often resistant to several antibiotics in the usual tests.