Purpose: To assess, at transcriptional level, the effect of fat content and frozen storage duration of beef burgers on the thermotolerance of E.coli O157:H7.
Methods: Beef patties of 3 cm thickness were prepared from lean ground beef, with or without the addition of fat (30%), containing 0.5 g of inoculated ground beef with 109 CFU/g E.coli O157:H7 in the center and stored at -28°C. Thermal process took place on the 1st and 20th day of storage, without thawing, in a preheated (200°C) oven broiler to internal temperature of 50°C (n=2x2). Normalized relative quantification (NRQ) of heat/ cold shock- (groEL, dnaK, cspA), stress- (rpoS) and virulence- (stx1, stx2, eae) related genes was assessed along the i) freezing curve (20°C, 1°C, -1°C, -28°C), ii) heating curve (-28°C, -1°C, 1°C, 50°C) after 1 and 20 days (-28°C) using Real-time PCR (n=2x2).
Results: Freezing followed by thermal processing at 50°C had no significant effect (P>0.05) on E. coli O157:H7 population. Cryoprotective role of fat was demonstrated by the delayed transcriptional response of the pathogen regarding rpoS and cspA along the freezing curve. Particularly, cspA was upregulated (log2 NRQ=2) as soon as -28°C was reached, while in low fat burgers maximum NRQ (log2 NRQ=3) was observed after 1 day at -28°C. With regards to rpoS, upregulation (log2 NRQ=1.5) was observed after 1 or 20 days at -28°C in low and high fat burgers, respectively. During heating of patties the slight upregulation of dnaK, only after short-term storage of low fat burgers, could be indicative of the intensity of heat stress experienced by the pathogen and thus, may partially explain the increased heat tolerance of E.coli after prolonged freezing and in the presence of fat.
Significance: These results could contribute to further elucidate the correlation between frozen storage and subsequent thermotolerance of E.coli O157:H7.