C-180. Comparison of CefiximeTellurite Sorbitol MacConkey Agar, Sorbitol MacConkey Agar, and CHROMagar O157 for Isolation of Escherichia coli O157:H7

J. Fernandez, K. G. Van Horn;
Focus Diagnostics, Inc., Cypress, CA.

Diagnosis of E. coli O157:H7 is often achieved by selective culture with Sorbitol MacConkey agar (SMAC) followed by serotyping or toxin testing. In this study, we compared Cefixime-Tellurite Sorbitol MacConkey agar (CTSMAC, Becton Dickinson, Sparks, MD) and CHROMagar O157 (Becton Dickinson) to SMAC, which is often overgrown with normal fecal enterics, for selective isolation of E. coli O157:H7 which grows as colorless colonies on CTSMAC and SMAC and as mauve colored colonies on CHROMagar O157. Ten isolates of known E. coli O157:H7 (conventional identification and serotyping), including E. coli O157:H7 ATCC 35150, were suspended in sterile saline at concentrations of 100 to 106 CFU/mL and 0.1 mL was plated onto all 3 media, streaked for colony counts and incubated at 37oC for 24-48 h. Clinical stool samples (n=41) received for E. coli O157:H7 were also tested with all 3 media. Selectivity of the media was tested with 18 non-E. coli O157:H7 isolates (8 Gram-positives and 10 Gram-negatives) inoculated at 1.0 x105 CFU/mL. For organism suspensions, optimal recovery of was achieved at 104 CFU/mL on all 3 media while isolates were consistently detected at the 102 CFU/mL concentration. Of the 41 clinical specimens, 3 (7%) were positive for E. coli 0157:H7 on all 3 media; there were no discrepant results. Gram-negative enteric flora from all stool samples was partially to completely inhibited on CTSMAC and on CHROMagar while most enterics grew on SMAC. Yeast and enterococci grew only on CHROMagar. In selectivity tests, no growth of the 8 Gram-positives was observed on any of the three media while 9 of 10 Gram-negatives grew well on SMAC, but were partially to completely inhibited on CT-SMAC and CHROMagar (2 Proteus grew well as non-mauve colonies). In conclusion, the 3 media provided equivalent isolation and differentiation of E. coli 0157:H7, but CTSMAC and CHROMagar were considered superior due to inhibition of contaminants. CHROMagar was considered superior to CTSMAC due to the production of distinct E. coli O157:H7 mauve-colored colonies allowing more rapid visual identification.