Purpose: This study compared the survival and/or growth of STEC of the six major non-O157 serogroups to that of O157:H7 STEC in ground veal stored at 10°C for 96 h or 120 h.
Methods: Two strains of each non-O157 serogroup (all clinical isolates from the CDC) and five O157:H7 strains (strain ATCC 43895 and four cold-tolerant beef-trim isolates), were grown individually in Brain Heart Infusion Broth (BHIB) at 37°C for 24 h. Inocula, each strain separately or combined as multi-strain cocktails of non-O157 or O157:H7, were prepared, 1 ml of inoculum was transferred to packages of fresh ground veal (25 g), resulting to an initial population of ca. 4.00 log CFU/g, and incubated at 10°C for up to 120 h. At each sampling point, one package of inoculated ground veal was withdrawn from storage. Serial dilutions were made in BPD and 0.1 ml samples were spread-plated on modified Eosin Methylene Blue agar (MEMB) with added sorbitol and bile salts. Samples were taken at 0 h, 12 h, 24 h, 36 h, 48 h, 72 h, 96 h or 120 h. Control samples of uninoculated ground veal with 1ml of BHIB added were spread-plated on Brain Heart Infusion agar (BHIA) in order to enumerate native microflora (n = 3). Data analysis was done using ANOVA (a = 0.05).
Results: For individual-strain studies, growth did not exceed 0.33 log CFU/g after 120 h of storage at 10°C and no statistical differences (P > 0.05) were observed in growth patterns of all the STEC strains tested. When STEC were combined into multi-strain cocktails of O157:H7 or non-O157, strains grew, with growth of the O157:H7 > non-O157 STEC, but not significantly (P > 0.05). Native microflora exhibited exponential growth over 120 h at 10°C, reaching a final population of ca. 8.00 log CFU/g.
Significance: The results of this study show that individual STEC may behave differently than combined strains in ground veal stored at 10°C. Further research is needed to validate these results under different environmental conditions.