P1-36 Growth of Escherichia coli O157:H7 in Common Pre-enrichment Broths

Monday, July 23, 2012
Exhibit Hall (Rhode Island Convention Center)
Emily Jackson, U.S. Food and Drug Administration, Bedford Park, IL
Annemarie Buchholz, U.S. Food and Drug Administration, Bedford Park, IL
Ravinder Reddy, U.S. Food and Drug Administration, Bedford Park, IL
Introduction: Pathogens are often present in foods at low levels and an enrichment step is typically required for detection using cultural or molecular techniques. Antibiotics and increased temperatures are common means of increasing enrichment selectivity.

Purpose: The growth of Escherichia coli O157:H7 in brain-heart infusion broth (BHI), buffered peptone water (BPW), BPW with vancomycin (BPW+V) and BPW with vancomycin, cefixme and cefsulodin (BPW+VCC) was compared at different temperatures.

Methods: An overnight culture of E. coliO157:H7 in tryptic soy broth was diluted to 3 log CFU/ml in each broth. Four hundred microliters were loaded into each well of a Bioscreen C Honeycomb Plate, with eight inoculated wells and two negative controls for each medium. Plates were incubated in the Bioscreen C at 30, 35 or 41°C for 10-15 hours, with optical density (OD) readings every 15 minutes. Three trials were performed at each temperature and the resulting growth curves were fitted to the Gompertz equation using DMFit.

Results: At 30°C, the growth rates in BHI, BPW, BPW+V and BPW+VCC were 0.291 ± 0.008, 0.051±0.005, 0.046 ± 0.002 and 0.037 ± 0.007 hr-1, respectively. At 35°C, these growth rates were 0.349 ± 0.004, 0.057 ± 0.002, 0.053 ± 0.002 and 0.049 ± 0.001 hr-1. Compared to BHI, only small increases in OD were observed in the BPW variations at the lower temperatures. At 41°C, the OD did not increase in the BPW-based enrichments, even when incubation was extended to 24 hours. The growth rate in BHI at 41°C was 0.326 ± 0.015 hr-1.

Significance: Increased temperatures and antibiotics are commonly used during enrichment for E. coli O157:H7. However, no growth was observed in BPW, BPW+V or BPW+VCC at 41°C. These conditions do not appear to be sufficient for the enrichment of this organism. Future experiments with the Bioscreen C will assess the growth of other strains and microorganisms in a variety of enrichment media.