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Fate of Escherichia coli O157:H7 during Silage Fermentation

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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 O157:H7 to a final concentration of log10 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 15°C 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 log10 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 log10 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.


Document Type: Research Article

Affiliations: 1: Teagasc, The National Food Centre, Dunsinea, Castleknock, Dublin 15, Ireland 2: Teagasc, Grange Research Centre, Dunsany, Co. Meath, Ireland 3: Food Studies Research Unit, Faculty of Science, The University of Ulster, Jordanstown, Newtownabbey, Co. Antrim, BT37 OQB, Northern Ireland

Publication date: December 1, 2002

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