Evaluation of five new plating media for isolation of Salmonella species

Abstract

A three-phase study was conducted to compare Hektoen enteric agar (HE), Rambach agar (Ra), SM-ID medium (SM), xylose-lysine-Tergitol 4 agar (XLT4), novobiocin-brilliant green-glycerol-lactose agar (NBGL), and modified semisolid Rappaport-Vassiliadis medium (MSRV) for the recovery of nontyphoid salmonellae from stool specimens. After evaluation of the first two phases, which resulted in the elimination of Ra, SM, and NBGL, 593 consecutive stool samples were investigated by plating them directly and after tetrathionate enrichment at 37°C on HE, XLT4, and MSRV. A total of 82 Salmonella-positive stool specimens were detected (positivity rate, 13.8%). Sensitivities for direct plating and after tetrathionate enrichment were 32.9 and 86.6%, respectively, for XLT4, 63.4 and 100.0%, respectively, for MSRV, and 34.1 and 79.3%, respectively, for HE. Specificities (percentage of morphologically suspicious colonies that were indeed salmonellae) were 100.0 and 99.8%, respectively, for XLT4, 99.0 and 98.8%, respectively, for MSRV, and 67.9 and 75.0%, respectively, for HE. The use of MSRV instead of HE increased the isolation rate of salmonellae by 26.2% (65 versus 82 strains isolated from HE and MSRV, respectively). We conclude that MSRV is the most sensitive medium tested and is a very specific medium for the isolation of nontyphoid salmonellae from stool specimens. However, its semisolid nature is a disadvantage and requires careful handling in the laboratory, especially when salmonellae are present. XLT4 had a sensitivity comparable to that of HE and a nearly 100% specificity and can be regarded as an alternative for the isolation of nontyphoid salmonellae from stool samples.