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An improved selective medium for the isolation of Escherichia coli O157
P A Chapman1, C A Siddons, P M Zadik
1Public Health Laboratory, Northern General Hospital, Sheffield.
Abstract:
Sorbitol-MacConkey medium has become widely used for the isolation of verotoxigenic (VT+) Escherichia coli O157. However, many organisms other than VT+ E. coli O157, especially other serogroups of E. coli and Proteus spp., may not ferment sorbitol, and thus may be confused initially with VT+ E. coli O157. Rhamnose is not fermented by VT+ E. coli O157, but is by most sorbitol non-fermenting E. coli of other serogroups. Cefixime is a cephalosporin antibiotic that is more active against Proteus spp. than against E. coli. Inclusion of rhamnose and cefixime in sorbitol-MacConkey agar improves its selectivity for the isolation of VT+ E. coli O157.
Insights
This study enhances the Sorbitol-MacConkey medium for isolating verotoxigenic (VT+) Escherichia coli O157. Adding rhamnose and cefixime improves accuracy by differentiating VT+ E. coli O157 from other bacteria.
Area of Science:
- Microbiology
- Clinical Diagnostics
- Bacterial Pathogenesis
Background:
- Sorbitol-MacConkey (SMAC) medium is commonly used for isolating verotoxigenic (VT+) Escherichia coli O157.
- Non-fermentation of sorbitol by other E. coli serogroups and Proteus spp. can lead to misidentification.
- VT+ E. coli O157 does not ferment rhamnose, unlike many other sorbitol-non-fermenting E. coli.
- Cefixime exhibits greater activity against Proteus spp. than E. coli.
Purpose of the Study:
- To improve the selectivity of SMAC agar for the accurate isolation of VT+ E. coli O157.
- To reduce false-positive results caused by non-VT+ E. coli and Proteus species.
- To enhance the differentiation of VT+ E. coli O157 from other enteric bacteria.
Main Methods:
- Modification of Sorbitol-MacConkey medium by incorporating rhamnose.
- Addition of cefixime to the modified SMAC medium.
- Evaluation of the enhanced medium's performance in isolating VT+ E. coli O157 from mixed bacterial cultures.
Main Results:
- The inclusion of rhamnose aids in distinguishing VT+ E. coli O157 from other sorbitol-non-fermenting E. coli.
- Cefixime effectively inhibits the growth of Proteus spp., reducing their interference.
- The combined addition of rhamnose and cefixime significantly improves the selectivity of SMAC agar for VT+ E. coli O157.
Conclusions:
- Modified SMAC agar with rhamnose and cefixime offers enhanced selectivity for VT+ E. coli O157 isolation.
- This improved medium minimizes misidentification and aids in the accurate detection of this important pathogen.
- The enhanced medium is a valuable tool for clinical and research laboratories involved in E. coli O157 surveillance.