A strictly anaerobic recovery and enrichment system for improved detection of heat-injured Escherichia coli O157:H7

The objective of this investigation was to evaluate the efficacy of various resuscitation broth systems for increasing the detection of heat-injured Escherichia coli 0157:H7 on Sorbitol MacConkey Agar (SMA). Trypticase Soy Broth (TSB) was inoculated with a three-strain mixture of E. coli 0157:H7 to give a final concentration of 1O7 CFU/ml, sealed in thermal death time (TDT) tubes, and heated at 570C. At 0, 6, and 8 minutes, aliquots of the heated cell suspensions were surface-plated directly onto Trypticase Soy Agar (TSA) and SMA or dispensed into tubes of TSB or Proteose Peptone Beef Extract Broth (PBEB), alone, or in a combination with 0.05% cysteine (cys), and/or purging with nitrogen gas (N2). The efficacy of each broth system to allow repair of injured E. coli 0157:H7 was assessed by plating aliquots of inoculated broth onto TSA and SMA after 2 hours of incubation at 250C. At 6 and 8 minutes of heating, 96.3% and 98.8% of the cell population, respectively, were injured. At 8 minutes of heating only 49.8% of E. coli 0157:H7 were detected by direct plating; whereas, TSB or PBEB alone increased detection to 63.2% and 67.0%, respectively (p\u3c0.05). Addition of cys + N2 to TSB or PBEB further increased detection to 75.3% and 76.1%, respectively (p\u3c0.05). The results of this research strongly supports the need for resuscitation procedures when testing for E. coli 0157:H7 in heat processed food products