Evaluation of Enrichment and Transport Media for Detection and Enumeration of Shiga Toxin-producing Escherichia coli

Introduction: Food industry, research and regulatory interests in Shiga toxin-producing Escherichia coli (STEC) continue to increase, placing emphasis on optimization of cultural protocols that ensure detection (when target serotypes are present). Also important but often overlooked is the stability of STEC population levels in samples collected in the field and shipped to a laboratory for analysis, particularly in research settings where quantification of targets is an objective.  Much of the current cultural methodology currently applied to STEC analyses was developed for serotype O157, and growth/survival characteristics of other serotypes is not well established.

Purpose: This experiment was designed to characterize growth of seven beef adulterant STEC serotypes (STEC-7) in different media types for sample enrichment, and compare population level stability of STEC-7 in common transport media during short-term storage at refrigerated and slightly abusive temperatures sometimes encountered during sample shipment.

Methods: STEC-7 were individually inoculated into enrichment media (EC broth, Buffered Peptone Water, Universal Pre-enrichment Medium, and Tryptic Soy Broth; with and without novobiocin supplementation) and growth curves at 37°C were generated and compared.  STEC-7 were inoculated into three transport media (Cary-Blair, Maximum Recovery, and Buffered Peptone Water), stored at 4 and 10°C, and sampled over 72h to compare stability in relation to inoculation concentration. 

Results: EC broth supported rapid growth of all STEC-7 and was similar to non-selective media. Media containing novobiocin greatly restricted growth of most non-O157 serotypes. Cary-Blair transport medium maintained population level stability during storage at 10°C while other media allowed increases.  All transport media was acceptable at 4°C storage. 

Significance: Following common E. coli O157:H7 enrichment protocols to detect STEC-7 may lead to false-negative determinations, particularly if novobiocin is included as a selective agent.  When transporting field samples to a laboratory, Cary-Blair Medium should be considered to protect STEC-7 in the event that temperature abuse of samples occurs.