Determining the Appropriateness of Established Escherichia coli Biotype I Surrogates as Predictors of Non-O157 Shiga Toxin-producing E. coli (STECs) Using Various Growth Characteristics

Introduction: Foodborne illness attributed to STECs has increased in the past decade, and the USDA-FSIS has declared the “Big 6” as adulterants in raw beef manufacturing.  Escherichia coli biotype I strains were previously isolated from cattle hides and determined to be appropriate for use as surrogates for E. coli O157:H7.

Purpose: To determine if surrogates can be used for non-E. coli O157 STECs by evaluating growth characteristics.

Methods: Non-O157 Shiga toxin-producing E. coli (STECs) (ATCC # 2192, 2193, 2196, 2215, 2217 and 2219) and E. coli biotype I surrogates (ATCC # BAA-1427, BAA-1428, BAA-1429, BAA-1430 and BAA-1431) were individually cultured in tryptic soy broth at 37°C for 18h.  Rifampicin-resistant surrogates were prepared and compared to the parent strains (16 bacterial strains x 3 replications).  Enumeration of cultures was performed on tryptic soy agar (TSA) with incubation at 37°C for 24h.  The GLM of SAS was used to identify differences in growth parameters.

Results: Growth curves of E. coli biotype I surrogates were similar to those of STECs throughout the evaluation, differing by 0.5 log or less at each 1-hour evaluation during the 24-hour growth period.  While some statistical differences (P < 0.05) were detected at intervals during growth from an initial population of approximately log 3.0 to log 9.0 over 24h, trends were clearly within ranges needed for use as surrogates.

Significance: These data suggest that E. coli biotype I surrogates may be used to predict growth of non-O157 Shiga toxin-producing E. coli (STECs) within a small amount of error.