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.