Fate of Escherichia coli O157:H7 during silage fermentation

The survival characteristics of Escherichia coli O157:H7 in silage derived from contaminated grass were investigated. The survival of other enteric bacteria was also investigated to determine if E. coli O157:H7 demonstrates enhanced acid tolerance in comparison. Samples of chopped grass were treated as follows: (i) no additive (control); (ii) inoculation with E. coli O15TH7 to a final concentration of log(10) 4.0 CFU g(-1); (iii) addition of an 85% solution of formic acid at 3.0 ml kg(-1) grass; and (iv) addition of both E. coli O157:H7 and formic acid, at the above concentrations. Treated 6-kg grass samples were packed into laboratory silos, sealed, and stored at 15degreesC for up to 180 days. Individual replicate silos were removed from storage periodically and subjected to microbiological and chemical analyses. Chemical analyses of the silage samples indicated that lactic acid-dominant fermentations, with a rapid drop in pH, occurred. Numbers of enteric bacteria decreased from log(10) 7.0 to 8.0 CFU g(-1) to undetectable levels within 19 days' storage. E. coli O157:H7 did not survive the silage fermentation process, with numbers declining from approximately log(10) 4.0 CFU g(-1) to undetectable levels within 19 days of ensiling. The pattern of decline in numbers of E. coli O157:H7 was the same as that for the enteric bacteria, indicating that under the conditions tested, the acid tolerance of E. coli O157:H7 was not significantly different from the acid tolerance of other enteric bacteria. This study found that E. coli O157:H7 did not survive a good silage fermentation process, indicating that properly ensiled grass that is correctly stored is unlikely to be a vector for the transmission of the pathogen among cattle.