The influence of various levels of endogenous beef bacterial microflora on the growth and survival of Escherichia coli O157:H7 on bovine carcass surface tissue was investigated. Bacterial beef microflora inoculum was prepared by enriching and harvesting bacteria from prerigor lean bovine carcass tissue (BCT) and was inoculated onto UV-irradiated prerigor BCT at initial levels of 10(5), 10(4), 10(3), and <10(3) CFU/cm2. Additional control BCT was inoculated with sterile H2O. E. coli O157:H7 was inoculated onto all tissues at an initial level of 10(2) CFU/cm2. Following a 48-h incubation at 4 degrees C, BCT was incubated up to 14 days at 4 or 12 degrees C, either aerobically or vacuum packaged. Regardless of the microflora level, there was no substantial growth of E. coli O157:H7 on BCT during storage at 4 degrees C under either aerobic or vacuum-packaged conditions. Instead, viable cell numbers at 4 degrees 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 degrees C, regardless of microflora inoculation treatment, reaching higher populations on aerobic samples than on vacuum-packaged samples in 10 days. However, the presence of the beef microflora did appear to delay the onset of growth or 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 degrees 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 outgrowth 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.