Effect of Bile Salts upon the Activity of Antibiotics

Abstract

IN the course of studies on the effect of bile salts upon the activity of a variety of antibiotics it was observed that the susceptibility to neomycin of a series of recently isolated strains of staphylococci (which are pigmented, coagulase positive, hæmolytic, and ferment mannitol) and of enterococci was increased when the in vitro tests were performed in a medium to which 0.5 per cent bile salts (Difco) had been added, as shown by the significant and consistent reduction in the minimum amount of antibiotic required to inhibit the growth of standard inocula of these strains in bile-salts medium as compared with the same medium without adjuvant (Table 1). The same concentration of bile salts also resulted in enhanced activity of penicillin against staphylococci resistant to penicillin, but had little or no effect upon the efficacy of this antibiotic against penicillin-sensitive strains of the same species or strains of enterococci tested. No such consistent potentiation against either resistant or sensitive strains was observed with bacitracin, chloramphenicol, erythromycin, nitrofurantoin, novobiocin, oleandomycin, streptomycin and tetracycline. With polymyxin, ristocetin and vancomycin, however, an opposite effect to that noted above was observed, namely interference, since significantly greater amounts of each of these antibiotics were required to inhibit similar inocula of a number of strains of both Gram-positive species in bile salts medium than was required in plain medium.