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 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 degrees 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.
Byrne
C M
CM
Teagasc, The National Food Centre, Dunsinea, Castleknock, Dublin 15, Ireland. cbyrne@arserrc.gov
O'Kiely
P
P
Bolton
D J
DJ
Sheridan
J J
JJ
McDowell
D A
DA
Blair
L S
LS
D019453:Escherichia coli O157 / Q000254:growth & development*; D005285:Fermentation; D005506:Food Contamination / Q000517:prevention & control; D005511:Food Handling / Q000379:methods*; D005516:Food Microbiology; D005519:Food Preservation; D006863:Hydrogen-Ion Concentration; D007778:Lactobacillus / Q000378:metabolism; D012820:Silage / Q000032:analysis / Q000382:microbiology*; D013997:Time Factors