Evaluar y comparar el medio cromogénico XG® con respecto a los medios habituales utilizados en coprocultivos para el aislamiento de Salmonella spp., Shigella spp., Yersinia enterocolitica y Aeromonas spp.
MétodosSe estudiaron un total de 1.226 muestras de heces que se inocularon en los medios XG®, MacConkey, Salmonella-Shigella (SS), caldo selenito, agar sangre-ampicilina y cefsulodina-irgasan-novobiocina (CIN).
ResultadosSe obtuvieron 235 cultivos positivos: Salmonella spp. (229), Shigella spp. (3), Yersinia enterocolitica (2) y Aeromonas spp. (1). De los 229 aislamientos de Salmonella spp., cien procedían del medio XG® como de los medios habituales y los 129 restantes se recuperaron sólo en estos últimos, encontrándose diferencias estadísticamente significativas (p < 0,005) para la recuperación de Salmonella spp. procedentes de los medios rutinarios con respecto al XG®. Los tres aislamientos de Shigella spp. se recuperaron únicamente del medio XG®, los dos de Yersinia enterocolitica del medio CIN y el de Aeromonas spp. tanto del medio XG®como del medio agar sangre-ampicilina. En 791 muestras negativas se trabajaron colonias compatibles con alguno los enteropatógenos valorados. Estos falsos positivos procedían del medio XG® 441 (35,9%), del pase a SS del selenito 142 (11,6%), del medio MacConkey 132 (10,8%) y del medio SS de la siembra directa 76 (6,2%). La mayor parte de los falsos positivos procedentes del medio XG® correspondieron a aislamientos compatibles con Salmonella spp. (n =408).
ConclusionesEl medio cromogénico XG® mostró baja sensibilidad (64%) y especificidad (69%) para el aislamiento de Salmonella spp. Los tres aislamientos de Shigella spp. recuperados del medio XG® pudieron deberse al inmediato procesamiento de la muestra. Consideramos que el medio cromogénico XG® no puede sustituir a los medios habituales utilizados en coprocultivo.
The chromogenic medium, XG®, was evaluated and compared to conventional media for the isolation of Salmonella spp., Shigella spp., Yersinia enterocolitica and Aeromonas spp.
MethodsA 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).
ResultsThe 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).
ConclusionsXG® 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.