Summary auto-generated
Sorbitol-MacConkey (SMAC) medium is widely used to isolate verotoxigenic E. coli O157, which causes serious infections including hemorrhagic colitis and hemolytic-uremic syndrome. However, SMAC lacks selectivity because other sorbitol non-fermenting organisms, particularly other E. coli serogroups and Proteus species, also grow on this medium and can be confused with E. coli O157. This study evaluated an improved selective medium by adding rhamnose and cefixime to SMAC. The researchers determined that E. coli O157 does not ferment rhamnose, while most other sorbitol non-fermenting E. coli do. Cefixime, a cephalosporin antibiotic, inhibits Proteus species more effectively than E. coli. Testing the new cefixime-rhamnose-sorbitol-MacConkey (CR-SMAC) medium on 1763 clinical samples showed it significantly reduced the number of false-positive colonies requiring further investigation from 411 (23.3%) on SMAC to 178 (10.1%) on CR-SMAC. All seven confirmed E. coli O157 isolates grew on both media, though CR-SMAC provided superior selectivity in mixed cultures. The improved medium offers diagnostic laboratories time and cost savings while maintaining reliable identification of this important pathogen.
Key findings
- CR-SMAC medium incorporating rhamnose and cefixime improves selectivity for E. coli O157 compared to standard SMAC medium
- E. coli O157 strains do not ferment rhamnose, while 60% of other sorbitol non-fermenting E. coli serogroups do ferment it
- Cefixime at 0.05 mg/L inhibits Proteus species (MIC 0.004-0.016 mg/L) while allowing E. coli O157 growth (MIC 0.125-0.5 mg/L)
- CR-SMAC reduced organisms requiring further investigation from 23.3% to 10.1% of samples, providing significant time and cost savings for diagnostic laboratories
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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.