Growth and verotoxin 1 production by Escherichia coli O157:H7 in ground roasted beef

Abstract

Production of verotoxin 1 (VT1) by hemorrhagic Escherichia coli O157:H7 in food has received limited research attention. A study was therefore conducted to determine the effect of temperature and pH, as achieved using two different acidulants, on VT1 production in ground roasted beef slurry. Slurries (pH 5.9) containing 33% beef and 67% water were incubated at 21 degrees C or 37 degrees C. Transfers were made at 48-h intervals for up to 18 days to determine if repeated subculturing influenced VT1 production. Populations of E. coli O157:H7 ranged from 8.9 x 10(8) to 1.9 x 10(9) CFU/ml within 48 h of transfer, regardless of incubation temperature. Maximum VT1 concentrations ranged from 61 to 63 ng/ml and 63 to 85 ng/ml of slurries incubated at 21 degrees C and 37 degrees C, respectively. The amount produced within 48 h at 37 degrees C was 15-24% higher than the amount produced at 21 degrees C and did not change after ten successive 48-h transfers. Upon changing the incubation temperature of slurry cultures adapted to 21 degrees C to an incubation temperature of 37 degrees C, VT1 production increased within 48 h to the level of cultures which had been previously adapted to 37 degrees C. A shift in temperature from 37 degrees C to 21 degrees C resulted in an initial reduction of about 55% in the amount of VT1 produced within 48 h. For studies on the combined effect of incubation temperature, pH and acidulant, slurries were adjusted to pH 5.4 with acetic and citric acids. Growth and production of VT1 in slurry acidified with acetic acid was markedly reduced compared to that in the control slurry (pH 5.9). The amount of VT1 detected in slurries receiving the second and third 24-h transfer of culture incubated at 21 degrees C was essentially nil. Growth and VT1 production was reduced in slurry acidified with citric acid compared to that observed in the control slurry but not as drastically as that observed in slurry acidified with acetic acid. VT1 concentration in unacidified beef slurry (pH 5.9) and in beef slurry acidified at pH 5.4 with citric acid reached 21 and 16 ng/ml, respectively, within 24 h at 21 degrees C. Results emphasize the need for proper sanitation procedures in beef processing and preparation facilities to reduce the risk of cross contamination of roasted beef and subsequent growth of E. coli O157:H7 and VT1 production.