An organic substance of microbial origin (usually mold or actinomycete bacteria) that is either toxic or growth inhibiting for other organisms. Also with the advent of synthetic methods of production, a substance produced by a microorganism or a similar substance (produced wholly or partly by chemical synthesis) which, in low concentrations, inhibits the growth of other microorganisms. Penicillin, tetracycline, and erythromycin are examples of antibiotics.