The effect of feeding diets containing permitted antibiotics on the faecal excretion of Salmonella typhimurium by experimentally infected chickens.

Groups of 45 chickens were fed continuously on diets containing 10 or 100 mg./kg. of different 'growth-promoting' antibiotics and infected orally with a nalidixic acid-resistant mutant of Salmonella typhimurium. The amount of S. typhimurium organisms excreted in their faeces was estimated by culturing them at weekly intervals and in a standard manner on plates of brilliant green agar containing sodium nalidixate, both direct and after enrichment in selenite broth. All of four groups fed diets containing 100 mg./kg. of nitrovin in three different experiments excreted much larger amounts of S. typhimurium than did groups fed antibiotic-free diets. In some, but not all, experiments, larger amounts were also excreted by groups fed diets containing 10 mg./kg. of nitrovin or 10 or 100 mg./kg. of flavomycin or tylosin. Feeding diets containing 10 or 100 mg./kg. of virginiamycin or bacitracin either did not influence or slightly increased the amount of S. typhimiurium excreted. Two groups fed continuously on diets containing 100 or 500 mg./kg. of sulphaquinoxaline for 44 days excreted smaller amounts of the S. typhimurium organisms that did groups fed antibiotic-free diets; no sulphonamide-resistant organisms of the S. typhimurium strain were isolated from the faeces.