|Scullen, Butch - Butch|
Submitted to: Food Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/21/2015
Publication Date: 3/3/2015
Citation: Sommers, C.H., Rajkowski, K.T., Scullen, O.J., Cassidy, J.M., Fratamico, P.M., Sheen, S. 2015. Inactivation of shiga toxin-producing Escherichia coli in lean ground beef by gamma irradiation. Food Microbiology. DOI: 10.1016/j.fm.2015.02/013.
Interpretive Summary: In recent years serogroups of Shiga Toxin-producing Escherichia coli (STEC) other than O157:H7 (O26, O121, O103, O45, O111, O145, and O104) have become increasingly associated with foodborne illness outbreaks. In 2011 the USDA Food Safety Inspection Service mandated testing of meat for these other serogroups (O26, O121, O103, O45, O111, and O145), collectively known as the “big/top six” non-O157:H7 STEC. Ionizing (gamma) irradiation is a sustainable US Food and Drug Administration and World Health Organization approved process which does not require the use of chemicals to improve the safety and shelf-life of foods. In this study we determined the radiation D-10 value, the dose needed to kill 90% of a microorganism, for 40 STEC isolates which carried various combinations of genes necessary to cause illness in humans. The STEC were inoculated into 80% lean ground beef and irradiated at refrigeration (4 deg C) temperature. We found that STEC which caused illness in humans (D-10 = 0.27 kGy), on average, were more sensitive to gamma radiation than those which were not (D-10 = 0.36 kGy). We also found that STEC which lacked a specific type of gene required for illness (intimin) were more resistant to radiation than those which contained it. One type of STEC which lacked the intimin gene, but can still cause illness is the O104 serogroup, was more radiation resistant on average (D-10 = ca 0.40 kGy) than the others that caused illness (D-10 = 0.27 kGy). The results of this study will allow regulatory agencies and the radiation and food processing industries to conduct risk analysis and provide safer meat to consumers. Consumers, especially those who are immuno-compromised (eg. cancer patients, diabetics, and the HIV/AIDS population) will benefit from having more information about foods treated with alternative processes which kill harmful bacteria such as the STEC.
Technical Abstract: Non-O157 serovars of Shiga Toxin-producing Escherichia coli (STEC) are now responsible for over 60% of STEC induced illnesses. The majority of illnesses caused by non-O157:H7 STEC have been due to serogroups O26, O121, O103, O45, O111, and O145, “the big/top six”, which are now considered adulterants in beef and beef products by USDA Food Safety Inspection Service. Unfortunately, relatively few nonthermal intervention technologies are available to inactivate foodborne pathogens in ground meat products. Ionizing (gamma) radiation is a safe, effective, and sustainable US Food and Drug Administration approved technology for improving the microbiological safety of foods including red meat. In this study the radiation resistance of 40 STEC isolates involved in illness outbreaks or non-outbreak isolates which contained various combinations of the Shiga Toxin 1 (stx1), Shiga Toxin 2 (stx2), intimin (eae), and hemolysin (ehx) genes were determined. The STEC were suspended in 80% lean ground beef and irradiated at 4 deg C. D-10 values, the radiation dose needed to inactivate 1 log (90%) of a microorganism, ranged from 0.16 to 0.48 kGy, with a mean of 0.31 kGy. Outbreak isolates had mean D-10 of 0.27 kGy (MinD-10= 0.16, MaxD-10=0.41), while non-outbreak isolates had significantly higher mean D-10 of 0.36 kGy (MinD-10= 0.16, MaxD-10=0.41), (p=0.0003). D-10 of the O104 serogroup which are involved in illness outbreaks had D-10 at the high end of the range (ca 0.4 kGy). The presence or absence of stx1, stx2, or both stx1 and 2 had no affect on D-10 (p=0.37). The presence (0.30 kGy) or absence (0.35 kGy) of the ehx gene had no affect on D-10 (p=0.07). However, the D-10 of isolates lacking the eae gene (0.37 kGy) were significantly higher than those containing eae (0.27 kGy) (p = 1.1 x 10-5). There was no difference in D-10 for isolates lacking eae regardless of whether or not they were associated with a foodborne illness outbreak (p=0.68).