Improved Selective and Differential Medium for Isolation of Escherichia coli O157:H7

Abstract

GMAC, a modified version of Sorbitol MacConkey medium (SMAC), was produced with a reduced quantity of selective agents and incorporated gentiobiose. GMAC supported a higher recovery rate of heat-or acidinjured Escherichia coli O157:H7 cells than SMAC with cefixime and tellurite (CT-SMAC), while differentiating E. coli O157:H7 from sorbitol-nonfermenting Hafnia alvei.

Recovery of E. coli O157 strains after exposure to acidification at pH 2

“…2011). Similar to the findings presented in this study, they reported only a slight decrease in recovery when plated to TSA postacid injury compared to CT‐SMAC, which resulted in a reduction of around 4 logs (Park et al. 2011).…”

“…2011). The present study demonstrated a reduction of around 2 logs between the two agars from HCl acidification, suggesting acid injury to cells may have been less severe than the Park et al. (2011) study.…”

“…Al‐Nabulsi and Holley (2007) also used CT‐SMAC (compared against all‐purpose Tween agar) to assess cell injury, but used exposure to the natural antimicrobial, lactoferrin, as the injury agent. An agar containing no cefixime or tellurite and bile salts reduced by a third (GMAC) was compared to CT‐SMAC and was reported to be less inhibitory to resuscitation of three EHEC O157:H7 strains (including ATCC 35150) after acid injury (BPW pH 2·26, 1% lactic acid, for 10 min) (Park et al. 2011).…”

“…Thus, survival of E. coli O157 strains was not impacted following exposure to pH 2 TSB when plated to this nonselective agar. As some STEC strains are sensitive to tellurite and other selective agents (de Boer and Heuvelink 2000;Park et al 2011), it is not surprising to observe reduced counts on CT-SMAC. Postacidification resulted in further decreases in recovery, suggesting acid treatment still caused cell injury.…”

Shiga‐toxin Producing Escherichia coli: Pathogenicity, Supershedding, Diagnostic Methods, Occurrence, and Foodborne Outbreaks

“…For the isolation of the strains present in the pre‐enrichment broth, the most traditional media used are methylene blue eosin (Levine) and MacConkey agars with sorbitol supplemented with tellurium and cefixime (TC‐SMAC). Other media have been developed with the inclusion of chromogenic substrates, such as Rainbow O157 agar, Biosynth culture media O157, Fluorocult E. coli O157:H7, HICrome EC O157, O157:H7 ID agar, and CHROMagar O157 (Manafi and Kremsmaier ; Park and others ; Tzschoppe and others ; Ngwa and others ). For the isolation of STEC O26, the use of Rhamnose‐MacConkey agar (RMAC) is preferred, due to the inability of this strain to ferment Rhamnose (Hiramatsu and others ).…”