Synergistic lethal combination of nitrite and acid pH on a verotoxin-negative strain of Escherichia coli O157

This study was concerned with the possible consequences of reducing the nitrite concentration of a fermented sausage environment on the survival of the pathogen E. coli O157:H45, a verotoxin-negative relative of E. coli O157:H7. A liquid medium, FM, was constructed with a liquid phase, aw and pH similar to fermented sausage. Survival of E. coli O157:H45 in FM depended on both pH and nitrite concentration. In trials in which the pH was decreased by growing Pediococcus acidilactici in FM, survival of E. coli O157:H45 was clearly dependent on nitrite concentration; at least 100 ppm nitrite was required to inhibit growth and the number of survivors after 2 days with 200 ppm nitrite was 1000-fold less than in the absence of nitrite. In laboratory-scale sausage fermented with P. acidilactici, E. coli O157:H45 failed to grow in the absence of nitrite and the numbers slowly declined over 14 days. However, the rate of decline was much faster with nitrite present even at 50 ppm; at 200 ppm nitrite, the E. coli O157:H45 population declined 100 times faster than in the absence of nitrite.