Role of bacterial association and penetration on destruction of Escherichia coli O157 : H7 in beef tissue by high pH

This study was undertaken to determine if association with collagen enables Escherichia coli O157:H7 to resist high-pa treatments and to determine the effects of high pH on the survival of E. coli O157:H7 within different lay ers of beef tissue. E. coli O157:H7 was inoculated onto purified bovine typ e I collagen on 12-mm(2) circular grass coverslips, plain 12-mm(2) circular grass coverslips (control), and 12-mm2 irradiated (cobalt-60) lean beef ti ssue. The rates of destruction of E. coli O157: H7 inoculated on coverslips in pH 10.5 NaHCO3-NaOH buffer at 35 degrees C were determined at various s ampling times. E. coli O157:H7 cells associated with collagen and treated i n the same manner were also examined using scanning electron microscopy to determine if association with collagen enabled the organism to resist high- pa treatments. The inoculated tissue was treated in pH 13.0 NaHCO3-NaOH buf fer at 25 degrees C, and penetrating cells of E. coli O157:H7 were recovere d using a cryostat technique. There was no significant difference (P < 0.05 ) between the rates of destruction of collagen-associated E. coli O157:H7 a nd non-collagen-associated E. coli O157:H7 following exposure to high-pH tr eatments. Scanning electron micrographs showed that collagen-associated E. coli O157:H7 cells appeared physically damaged by exposure to high-pH treat ments, and association of E. call O157:H7 to collagen did not increase the resistance of the organism to destruction by high-pH rinses. No significant differences were seen between 20 ml of NaHCO3-NaOH buffer at pH 13.0 (trea tment) and 20 mi of distilled water at pH 7.0 (control) when E. coli O157:H 7 cells were recovered in beef tissue at depths of up to 2,000 mu m (P < 0. 05). The ability of E. coli O157:H7 to penetrate beef tissue may be an impo rtant factor in reducing the effectiveness of high-pH treatments in killing this organism on beef tissue. This funding should be considered in the fut ure when designing treatments to decontaminate beef carcasses.