Cation Reversal of Inhibition of Growth by Valinomycin in Streptococcus pyogenesand Clostridium sporogenes1

Study of the antimicrobial spectrum of valinomycin revealed that, in addition to the gram-positive bacteria reported in literature, Streptococcus pyogenesand Clostridium sporogenesare also susceptible to this antibiotic. The minimal inhibitory concentrations (MIC) of the antibiotic for S. pyogenesgrown aerobically and anaerobically did not differ markedly, negating the hypothesis that oxidative phosphorylation is involved in the mechanism of action of this antibiotic. This conclusion is further strengthened by the inhibition of growth of C. sporogenes, an obligate anaerobe. In a medium with a low K+concentration, the MIC for S. pyogeneswas 0.02 µg/ml, the lowest ever recorded for this antibiotic. The inhibition of growth of S. pyogenesand C. sporogeneswas readily reversed by addition of K+to the medium, indicating a compensation for net efflux of K+from the cells when the transmembrane potential reached equilibrium. In contrast to these bacteria, Bacillus subtiliswas less susceptible to the antibiotic when the potassium concentration of the medium was low. The addition of potassium in the presence of valinomycin increased the inhibition of growth, which appears to result from dissipation of metabolic energy as in the mitochondrial system.