Selected culturable enteric bacterial populations are modified by diet acidification and the growth promotant Tylosin

Aims: To determine the effect of diet acidification and an in-feed antibiotic growth promotant (Tylosin, Ty) on selected culturable bacterial populations in the gastrointestinal tract (GIT) of mice. Methods and Results: Female C57B1 mice were given a standard diet supplemented with Acid Pak (AP) or Ty in the drinking water. After 21 days, lumen and adherent populations of Enterobacteriaceae, enterococci/streptococci, and lactic acid bacteria (LAB) from the ileum, caecum, colon and faeces were enumerated. General intestinal health was assessed by the frequency of haemolytic bacteria in the different intestinal compartments. Contrary to expectations, AP and Ty significantly increased haemolytic bacteria in the lumen of the caecum and colon (P < 0.05). The small but significant growth-enhancing effect of Ty (P < 0.05) was associated with decreases in enterococci/streptococci and surprisingly, LAB, as well as increases in coliforms. AP, which failed to improve growth rates, reduced coliforms, had limited effects on enterococci/streptococci, and specifically failed to promote the growth of LAB populations in all intestinal compartments. Ty supplementation was also associated with a significant increase in macrolide-resistant enterococci throughout the GIT. Conclusions: Dietary acidification is less effective than Ty in modulating the population dynamics of selected culturable populations of enteric bacteria. Significance and Impact of the Study: The mouse can provide a useful experimental model to examine the effects of new dietary supplements, formulations or regimes on changes in microbial population dynamics, including monitoring for antibiotic resistance.