Comparison of Salmonella chromogenic medium with DCLS agar for isolation of Salmonella species from stool specimens

Abstract

Salmonella chromogenic medium (SCM; Oxoid, Basingstoke, United Kingdom), a new selective chromogenic medium, was compared to DCLS agar (Oxoid) for the detection and presumptive identification of Salmonella species from stool samples. This medium contains two chromogenic substrates, Magenta-cap (5-bromo-6-chloro-3-indolylcaprylate), which is hydrolyzed by Salmonella species to give magenta colonies, and X-Gal (5-bromo-4-chloro-3-indolyl-beta-D-galactopyranoside), which is incorporated to visualize beta-D-galactosidase-producing organisms as blue colonies. Thus, non-Salmonella organisms appear blue or are not stained by any of the chromogens of the medium. A total of 500 stool samples were investigated by plating them directly and after selenite enrichment on DCLS agar and SCM. A total of 44 Salmonella-positive stool samples were detected. The sensitivities for direct plating and after enrichment were 22.7 and 81.8%, respectively, for DCLS agar, and for SCM these values were 34.1 and 100%, respectively. The specificities for direct plating and after enrichment were 82.5 and 72.8%, respectively, for DCLS agar and 98.5 and 95.8%, respectively, for SCM. According to these results, the sensitivities of SCM and DCLS agar were comparable on primary plating. However, the sensitivity of SCM was significantly higher after enrichment. In addition, the specificity of SCM was also significantly higher than that of DCLS agar both before and after enrichment. On the basis of these results, SCM can be recommended for the isolation of Salmonella species from stool samples in preference to DCLS agar.