Growth and Repair of Escherichia coli Non-O157:H7 STEC Strains in Selective Enrichment Broths

Introduction: Shiga toxin-producing Escherichia coli (STEC) are a group of pathogenic E. coli strains characterized by O157:H7 and non-O157 serogroups. The non-O157 infections have recently emerged as a significant concern in various countries. Within the E. coli non-O157 group, the Centers for Disease Control has identified six serogroups (O26, O111, O45, O121, O103 and O145) that are responsible for over 70% of the non-O157 STEC-associated illnesses. The resistance of these six serogroups to selective ingredients can be quite varied. Therefore, a selective enrichment broth must take this into account in order to repair and rapidly grow non-O157 cells with inhibition of its competing organisms.

Purpose: To develop and assess R & F^® non-O157 STEC selective enrichment broth (STEC-SEB).  

Methods: All bacterial strains were grown in brain heart infusion broth at 35 °C for 24-48 h. An isolate from each of the six non-O157 STEC serogroups were freeze injured in 10% skim milk and stored at -76 °C. Repair of injured cells were ascertained using recovery of cells on brain heart infusion agar versus MacConkey agar containing 0.6% bile salts #3 agar. Growth and/or repair of the isolates were evaluated in the USDA recommended modified tryptone soy broth (mTSB) with 8 mg/l of novobiocin and STEC-SEB compared with tryptic soy broth (TSB) incubated at 42 °C.   

Results: Repair of injured non-O157 STEC strains were similar in TSB and STEC-SEB, whereas, resuscitation of injured cells was delayed in mTSB. Also, mTSB inhibited the growth of 10% of the non-O157 STEC (includes 28 isolates) strains tested, whereas, STEC-SEB grew all the tested non-O157 STEC strains similar to TSB. Selectivity of competing bacterial strains in mTSB and STEC-SEB was similar.    

Significance: With the growth of all the tested non-O157 STEC strains to a titer of 10⁸/ml after 24 h at 42 °C coupled with a limited selectivity, the use of STEC-SEB will result in fewer false-negative isolations compared with the USDA recommended mTSB.