Abstract
Chickens which had been inoculated orally with a nalidixic acid-resistant strain of Salmonella typhimurium were reared on a diet containing different concentrations of avoparcin in an attempt to explain the variation in response to commercial levels of this antibiotic observed by different workers. In one experiment small increases in faecal excretion of the inoculated salmonella occurred at 2.5 and 5.0 mg kg-1, with greater increases between 7.5 and 20 mg kg-1. In a second experiment there was a significant increase in excretion at 7.5 mg kg-1 and in a third experiment in which generally higher excretion rates were detected in all groups, significant increases were observed at 10 and 12.5 mg kg-1 only. In addition avoparcin significantly increased the faecal excretion of S. cholerae-suis, S. dublin and S. arizonae, serotypes not usually associated with poultry-derived food-poisoning in the United Kingdom. It did not increase faecal excretion of S. pullorum. Avoparcin at 10 mg kg-1 appeared to have little effect on the normal intestinal flora of S. typhimurium infected chickens despite the fact that when tested in vitro individual organisms were susceptible to this drug concentration. At 100 mg kg-1 viable counts of intestinal enterococci and Gram-negative anaerobic bacteria were considerably reduced while those of S. typhimurium and Escherichia coli increased. Antibiotic activity due to avoparcin was detectable in the alimentary tract and there was some increase in concentration of the antibiotic in the more distal regions.