[Evaluation of xylose galactosidase (XG) culture medium for the isolation of enteropathogens].

Introduction: The chromogenic medium, XG, was evaluated and compared to conventional media for the isolation of Salmonella spp., Shigella spp., Yersinia enterocolitica and Aeromonas spp.
Methods: A total of 1226 human stool samples were inoculated on XG, MacConkey agar, Salmonella-Shigella agar (SS), selenite broth, blood-ampicillin agar and cefsulodin-Irgasan-novobiocin agar (CIN).
Results: The 235 positive cultures included the following: 229 Salmonella spp., 3 Shigella spp., 2 Yersinia enterocolitica and one Aeromonas spp. Among the 229 containing Salmonella spp., 100 were detected on both XG and conventional media and the 129 remaining were detected only on conventional media; recovery of Salmonella spp. on conventional media was significantly higher with respect to XG medium (p < 0.005). The 3 isolates of Shigella spp. were obtained on XG, the 2 isolates of Yersinia enterocolitica were recovered on CIN agar and the single isolate of Aeromonas spp. was obtained both on XG and blood-ampicillin agar. Colonies suspected to be some of the enteropathogens investigated were present in 791 of the negative stool samples. Among these false-positives 441 (35.9%) were obtained from XG, 142 (11.6%) after selenite enrichment, 132 (10.8%) from MacConkey agar and 76 (6.2%) from SS agar. Most of the false-positive isolates obtained on XG medium were consistent with Salmonella spp. (n = 408).
Conclusions: XG chromogenic medium showed low sensitivity (64%) and specificity (69%) for the detection of Salmonella spp. Recovery of Shigella spp. on XG medium in three samples may have been due to the immediate processing of the samples. We conclude that XG chromogenic medium can not be recommended as an alternative to currently used conventional media.