Increased Detection of Acid-Injured Escherichia coli O157:H7 in Autoclaved Apple Cider by Using Nonselective Repair on Trypticase Soy Agar

Abstract

Three different acid-resistant strains of Escherichia coli O157:H7 were inoculated individually and as a cocktail into sterile apple cider (pH 3.2) at a level of approximately 105 cells per ml and incubated at 2°C. Samples were plated on Trypticase soy agar (TSA), violet red bile agar (VRBA), sorbitol MacConkey agar (SMA), and Petrifilm E. coli count plates (Petrifilm) at 24-h intervals. Repair of acid-injured cells was assessed by surface plating cider samples on TSA and allowing a 2-h room-temperature incubation period followed by overlaying with double-strength VRBA or SMA. Since SMA is a surface plate medium, the repair procedure was modified by overlaying SMA with Trypticase soy broth after 2 h of room-temperature incubation. Populations of all three strains and the cocktail of strains decreased rapidly in apple cider and approached undetectable levels within 72 h. At 24 and 48 h, 98.4% and >99% of the E. coli populations were injured, respectively. Repair procedures significantly (α = 0.05) increased detection of E. coli O157:H7. After 72 h E. coli O157:H7 was not detected by using SMA and Petrifilm; however, it was detected using repair procedures. Although detection levels were increased with resuscitation procedures, the levels detected were still lower than those obtained using nonselective TSA. This research confirms the need for special recovery steps when analyzing acidic food products suspected of containing E. coli O157:H7.