The incidence, growth and survival of Diarrhoeagenic Escherichia coli in South African meat products
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Escherichia coli that cause diarrhoeal disease are classified into virotypes based on virulence properties, mechanisms of pathogenicity, clinical syndromes and distinct O:H serotypes. These virotypes include enteropathogenic E. coli (EPEC), enteroinvasive E. coli (EIEC), enterotoxigenic E. coli (ETEC), enteroaggregative E. coli (EAggEC), diffuse adhering E. coli (DAEC) and verocytotoxin-producing E. coli (VTEC) also known as Shiga toxin -producing E. coli (STEC) which includes the most virulent subgroup, the enterohaemorrhagic E. coli (EHEC). The most predominant EHEC is E. coli O157:H7. Cattle are the principal reservoir for diarrhoeagenic E. coli (DEC) and ground beef products are the major vehicle for their transmission. The aim of this study was to determine the incidence of DEC in minced beef and boerewors and to investigate the growth, survival and thermal inactivation of E. coli O157:H7 in boerewors. In the first part of this study, a survey was conducted on 30% of butcheries in the Bloemfontein District and isolated virotypes were identified by serotyping. More minced beef samples were positive for DEC (38.10%) compared to boerewors samples (28.57%). Enteropathogenic E. coli were the most frequently isolated virotype while VTEC were the most virulent virotype recovered from both samples, hence highlighting the potential of ground meat products as vehicles of transmission of DEC. An enrichment step enabled optimal recovery of DEC on chromocult™ chromogenic agar (Merck 1.10426). In the second part of this study, boerewors with and without sulphur dioxide preservative were manufactured following a typical commercial procedure and inoculated with E. coli O157:H7 at low (3.5 log cfu/g) and high (7.5 log cfu/g) inoculum size, and stored at 0 'C, 4 'C and 10 'C followed by evaluation of growth and survival every 48 hours for 10 days. Boerewors with preservative had significantly lower recoveries compared to boerewors without preservative at all the three storage temperatures. The low and high inocula with preservative had significant (p<0.001) reductions in recoveries at 0 'C, declining to below detectable limits at days 8 and 10 respectively. At 4 'C, the low and high inocula with preservative recoveries declined significantly (p<0.001) to below detectable limits and to 1.01 log cfu/g respectively at day 10. At 10 'C, significant (p<0.001) increases of 1.85 log cfu/g and 2.35 log cfu/g respectively were observed from day 0 to day 10. At 0 'C, the low and high inocula without preservative treatments had declines in recoveries of 1.21 log cfu/g and 1.45 log cfu/g respectively. Insignificant (p<0.001) increases of 0.19 log cfu/g and 0.26 log cfu/g respectively were noted at 4 'C while significant (p<0.001) increases of 1.97 log cfu/g and 1.84 log cfu/g respectively were noted at 10 'C from day 0 to day 10. The combination of sulphur dioxide preservative and low temperature (0 'C and 4 'C) demonstrated the best efficacy against the sur vival of E. coli O157:H7. It was established that thermal inactivation end point temperatures of E. coli O157:H7 inoculated into boerewors with preservative at 7.00 log cfu/g, was 60 minutes at 60 'C, 80 seconds at 65 'C and 60 seconds at 70 'C. This study demonstrated that E. coli O157:H7 can survive in boerewors with and without preservative, stored at 0 'C, 4 'C and 10 'C and is more sensitive to heat treatment at 70 'C.