SUMMARY: Indole synthesis from glucose, ammonium chloride and serine by washed suspensions of 7-4 was inhibited by bacteriostatic concentrations of chloramphenicol and the tetracyclines (Aureomycin, Terramycin, and Achromycin). Indole synthesis was also inhibited by streptomycin but only by concentrations higher than those necessary for bacteriostasis. The inhibition by chloramphenicol and the tetracyclines does not appear to be due to inhibition of adaptive enzyme formation because these antibiotics still inhibited indole synthesis when added to suspensions which were forming indole at maximum rate. Streptomycin, in the same type of test, seemed to inhibit some adaptive process.

Indole synthesis by suspensions of antibiotic-resistant variants of 7-4 was not inhibited by concentrations of antibiotics which inhibited synthesis by the parent strain. Suspensions of organisms resistant to high concentrations of streptomycin formed at least the same amount of indole in the presence of any concentration of streptomycin tested, as they did in its absence. Streptomycin often stimulated indole synthesis by such organisms. Suspensions of organisms which required streptomycin for growth, after growth in suboptimal concentrations of streptomycin, did not produce appreciable amounts of indole. The addition of streptomycin restored the activity in organisms grown in a liquid defined medium but did not do so with organisms grown on heart infusion agar. Streptomycin-resistant and streptomycin-dependent strains resembled the parent strain in that they utilized malate + pyruvate, or oxalacetate, for indole formation in place of glucose. Strains resistant to various concentrations of chloramphenicol, Aureomycin and Terramycin showed a correlation between the amounts of antibiotic required to inhibit indole synthesis and to inhibit growth in a glucose + tryptophan + inorganic salts medium. The results indicate that some of the antibiotics interfere, either directly or indirectly, with indole synthesis.


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