Effects of rumen and gut microorganisms and their metabolites on the growth and survival of Escherichia coli O157
Escherichia coli O157 can cause serious gastrointestinal disease and ruminants reared for meat and milk are major reservoirs of this bacterium. This study attempted to identify factors that might reduce the growth and survival of E. coli O157 in the ruminant gut. Several plant metabolites were also found to be inhibitory to E. coli O157 including the essential oil carvacrol. The most striking inhibitor of those tested was the plant metabolite esculetin that inhibited the growth and survival of E. coli O157 under both aerobic and anaerobic conditions. The combination of esculetin and short chain fatty acids (SCFA) resulted in an additive inhibitory effect in batch culture while there was little effect against predominant rumen anaerobes. Esculetin also reduced the survival of E. coli O157 in continuous flow fermentors. Terminal labelled restriction fragment length polymorphism (T-RFLP) analysis of fermentor samples proved to be a useful method for monitoring microbial diversity and showed that the addition of esculin, which was rapidly hydrolysed to esculetin, to a fermentor system resulted in some banding shifts in the major bacterial groups. The profile closely resembled that at the beginning of the experiment once the plant metabolite was withdrawn. Non-verocytotoxic E. coli O157 strains 12900 and EC22 did not possess superior acid tolerance when compared to "wild type" E. coli strains but were found to be more tolerant of SCFA than E. coli K 12 strain 11300. Of the SCFA tested singly propionate was most inhibitory to the growth of E. coli O157 strains. E. coli O157 strain 12900 did not grow in unamended rumen contents but grew in contents supplemented with either glucose or yeast extract (a rich source of amino acids) suggesting that competition for nutrients with the autochthonous microflora may limit the growth and survival of E. coli O157 in the rumen.