Submitted to: Journal of Food Protection
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/16/2001
Publication Date: 8/1/2001
Citation: Berry, E.D., Koohmaraie, M. 2001. Effect of different levels of beef bacterial microflora on the growth and survival of Escherichia coli O157:H7 on beef carcass tissue. Journal of Food Protection. 64:1138-1144.
Interpretive Summary: It has been hypothesized that it is possible for intervention treatments to make beef carcasses too clean. This is based on the idea that if most of the normal, nonpathogenic spoilage bacteria are removed and then the carcass becomes contaminated with a pathogen, then the pathogen may grow unchecked. Alternatively, if some "normal" level of nonpathogenic bacteria is present on the carcass, the growth of a contaminating pathogen may be inhibited by these competing microflora. The objective of this experiment was to examine the effects of different levels of these normal beef bacteria, including high, medium-high, medium-low, and low numbers, on the growth of Escherichia coli O157:H7 on beef carcass surfaces. Carcass tissue inoculated with the beef bacteria and E. coli O157:H7 was held at both 4 and 12 deg C, and both in air and vacuum-packaged. At 4 deg C. E. coli O157:H7 did not grow, regardless of the background beef microflora. At 12 deg C, which simulates temperature abuse, E. coli O157:H7 grew and was only slightly inhibited by higher background microflora. This indicates that interventions that reduce normal background microflora from carcasses do not allow this pathogen to grow unchecked. In addition, this experiment demonstrates the importance of proper refrigeration during meat handling and storage to prevent growth of E. coli O157:H7, and demonstrates that sanitation and processing practices that prevent and reduce contamination of carcasses with E. coli O157:H7 are essential, regardless of the numbers of natural beef bacteria present on carcasses.
Technical Abstract: The influence of various levels of endogenous beef microflora on the growth of Escherichia coli O157:H7 on bovine carcass surface tissue was investigated. Beef microflora inoculum was prepared by enriching and harvesting bacteria from pre-rigor bovine carcass tissue (BCT), and was inoculated onto UV-irradiated BCT at initial levels of 10**5, 10**4, 10**3, and <10**3 CFU/cm**2. E. coli O157:H7 was inoculated at an initial level of 10 cm**2 CFU/cm**2. Following 48 h incubation at 4 deg C, BCT was incubated up to 14 days at 4 or 12 deg C, either aerobically or vacuum-packaged. Regardless of microflora level, there was no substantial growth of E. coli O157:H7 on BCT during storage at 4 deg C under either aerobic or vacuum-packaged conditions. Instead, cell numbers at 4 deg C remained constant, with no reduction in numbers associated with the different beef microflora levels. E. coli O157:H7 grew on all BCT stored at 12 deg C, regardless of microflora level, reaching higher populations on aerobic samples than on vacuum- packaged samples in 10 days. However, the presence of the beef microflora did appear to slow the growth of the pathogen, and E. coli O157:H7 counts on BCT without added microflora were generally higher following 7 to 10 days of 12 deg C storage than those counts on BCT inoculated with beef microflora. These data demonstrate the importance of temperature control during meat handling and storage to prevent the growth of this pathogen, and indicate that proper sanitation and processing practices that prevent and reduce contamination of carcasses with E. coli O157:H7 are essential, regardless of background microflora levels.