Effect of pH on survival, thermotolerance, and verotoxin production of Escherichia coli O157 : H7 during simulated fermentation and storage

Heat treatment is increasingly being introduced to fermented meat processin g, since the acid tolerance properties of Escherichia coli O157:H7 can perm it this organism to survive traditional processing procedures. This study i nvestigated the effect of growth pH and fermentation on the thermotolerance at 55 degrees C of E. coli O157:H7 in a model fermented meal system. E. co li O157:H7 (strain 380-94) was grown at pH 5.6 or 7.4 (18 h at 37 degrees C ), fermented to pH 4.8 or 4.4 in brain heart infusion broth, and stored for 96 h. Cells grown at pH 5.6 had higher tf values at 55 degrees C (D-55 deg rees C) than cells grown at pH 7.4 (P < 0.001), Cells fermented to pH 4.8 h ad higher D-55 degrees C than those fermented to pH 4.4 (P < 0.001). Cells fermented to pH 4.8 demonstrated an increase in D-55 degrees C during stora ge (P < 0.001), whereas cells fermented to pH 4.4 showed a decrease in D-55 degrees C during the same period (P < 0.001). The effect of growth pH on v erotoxin production by E. coli O157:H7 was assessed using the verotoxin ass ay. Cells grown at pH 5.6 had lower verotoxin production then cells grown a t pH 7.4, This effect was nor sustained over storage. These results indicat e that a lower growth pH can confer cross-protection against heat. This has implications for the production of acidic foods, such as fermented meat, d uring which a heating step may be used to improve product safety.