MICROBIAL ECOLOGY AND SAFETY OF FRESH ON-FARM ORGANICALLY GROWN PRODUCE
Title: Comparison of expression of E. coli O157:H7 virulence factors in ground beef and on fresh-cut lettuce
Submitted to: International Association for Food Protection Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: March 10, 2009
Publication Date: July 14, 2009
Citation: Sharma, M., Patel, J.R., Nou, X., Ferguson, S.E., Mudd, C.L., Donnenberg, M. 2009. Comparison of expression of E. coli O157:H7 virulence factors in ground beef and on fresh-cut lettuce [abstract]. International Association for Food Protection Program Book. p. 87.
Introduction: Several E. coli O157:H7 outbreaks associated with leafy greens resulted in high rates of hospitalization and incidences of hemolytic uremic syndrome in patients, indicating that E. coli O157:H7 may be more virulent on leafy greens than in ground beef.
Purpose: The objective was to determine if the expression of E. coli O157:H7 virulence factors is different on cut lettuce than in ground beef by measuring gene expression on both commodities.
Methods: E. coli O157:H7 was inoculated on to lettuce, ground beef, or in TSB and incubated at 37 deg C for 24 h. RNA from these cells was extracted. Reverse-transcriptase real-time PCR was used to evaluate expression of six virulence genes (stxII, eae, ehx, espA, ihaA, and rfbE) and one standard gene (gnd) in ground beef, on lettuce, or in TSB. Cycle threshold counts of each gene in TSB were used to construct standard curves, and expression of genes from cells grown on lettuce or ground beef were fitted to the curve. These values were measured against expression of gnd on lettuce or ground beef, and ratios were analyzed for significant differences (P<0.05) in expression of genes based on commodity.
Results: The expression of virulence factors rfbE (O157-antigen) and ehx (hemolysin) were significantly higher when E. coli O157:H7 was grown in ground beef than on cut lettuce. No differences in expression of stxII (shiga toxin 2), eae (intimin), espA (type III secretion filament), ihaA (adherence factor), were observed when E. coli O157:H7 cells were grown on lettuce or ground beef.
Significance: These data indicate that growth environment provided by iceberg lettuce supported statistically similar levels of expression of four of six virulence factors of E. coli O157:H7 as ground beef, indicating that growth environment may not be the sole basis for severe illness observed in previous leafy green outbreaks.