Prevalence and evaluation of a choromogenic medium for isolation of Escherichia coli O157 from children with acute gastroenteritis.

Abstract

Objective: Comparative performance status of CHROMagar O157 (CHROM) sorbitol-MacConkey (SMAC) media for the detection of Escherichia coli (E.coli) O157 in stool specimens isolated from 339 children under 5 years of age who presented with acute gastroenteritis between September 2008 and September 2010 was determined. Methods: Stool specimens were inoculated onto Sorbitol-MacConkey agar (SMAC), CHROMagar O157, Selenit F, Salmonella-Shigella (SS) and MacConkey agars. All plates were incubated aerobically for 24 to 48 h at 35°C. Colorless colonies on the SMAC plate and mauve colonies on the CHROM plate were selected for further identification by conventional biochemical tests as well as by semi-automated system. Colonies confirmed to be E.coli were screened for O157 antigen by Dry spot E.coli O157 latex particle agglutination test. Results: In 339 stool samples examined, Salmonella spp was isolated in 14 (4.1%), and Shigella spp. in 11 (3.2%), while Escherichia coli O157 was detected in only 1 (0.3%) sample. Suspect E.coli O157 stains grew on 8 CHROMagar (2.1%; 8/339) and 14 SMAC (14/339; 3.8%) plates. Rate of false positivity for colony picks from SMAC (n=13; 65%) media was almost 2-fold higher than that for CHROM (n=7; 35%). Conclusion: Routine use of chromogenic media for the investigation of E.coli O157’nin in the selected cases with bloody diarrhea is deemed appropriate.