Antimicrobial substances and chick growth promotion: The growth‐promoting activities of antimicrobial substances, including fifty‐two used either in therapy or as dietary additives

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Abstract
1. Fifty‐five antimicrobial substances were tested for their ability to promote growth when added to the diet of chicks. 2. Both cephalosporins and all the nine penicillins tested were active. 3. Of six aminoglycosides, streptomycin and gentamicin had the greatest activity and neomycin had none. 4. Growth rate was significantly improved by clindamycin, lincomycin, vancomycin, spectinomycin, rifampicin, oxytetracycline, chlortetracycline, erythromycin, tylosin, flavomycin, virginiamycin and zinc bacitracin. Chloramphenicol and nalidixic acid were inactive. Polymixin B, novo‐biocin, cycloserine, phosphonomycin, and sodium fusidate had little activity. Fusidic acid promoted growth at 250 mg/kg diet. 5. Trimethoprim was inactive alone and in combination with sulpha‐diazine. Of seven 5‐nitroimidazoles, only dimetridazole and metronidazole showed slight activity. Of the six 5‐nitrofurans, only nitrovin, the standard reference substance used, promoted growth. 6. Caprylohydroxamic acid, a urease inhibitor, had no beneficial effect on growth rate or on the efficiency of food conversion. 7. The growth‐promoting properties of the various substances could not be related with their known antimicrobial and absorption characteristics in mammals.