Evaluation of Culture Methods for Shiga Toxin-producing Escherichia coli

Introduction: Non-O157 Shiga toxin-producing Escherichia coli (STEC) are enzootic in cattle and constitute an emerging zoonotic threat.  A current research goal of the USDA-NIFA is the development of diagnostic tests for STEC O26, O45, O103, O104:H4, O111, O121, O145, and O157:H7. 

Purpose: The purpose of this study was to evaluate selected currently available culture media to detect STEC O26, O45, O103, O104, O111, O121, O145, and O157.

Methods: Selective broth enrichment and agar plating media were inoculated with 80 strains, 10 each of the targeted serogroups in pure culture and bovine feces.  A total of 22 non-STEC strains of varying genera were used as controls.  Possé selective enrichment broth and colorimetric selective differential agar were evaluated for detection of STEC O26, O45, O103, O104, O111, O121, O145, and O157:H7. In addition, we tested the utility of several other plating media for detection of STEC from these same serogroups.

Results: Enrichment conditions described by Possé et al. resulted in a trend of suppressed growth.  Possé selective differential agar provided a plating medium from which all 8 serogroups could be selected by means of 3 colony phenotypic colors (red-purple, blue-purple and green).  Modified Rainbow Agar^® O157 resulted in multiple colony colors in the purple-gray family, which did not allow for differentiation of non-O157 STEC from several other types of flora.  As expected, CHROMagar^TM O157 did not allow for differentiation of non-O157 STEC serogroups.  CHROMagar^TMSTEC tended to inhibit growth of the representative strains.

Significance: The enrichment conditions of Possé require further optimization to allow uninhibited growth across these 8 serogroups. The use of Possé differential agar plating allowed for the screening of all 8 serogroups of interest by 3 distinguishable colony colors and was more economical than other media tested.