Lack of efficacy of alkali treatment for isolation of Yersinia enterocolitica from feces

The efficacy of KOH treatment to enhance isolation of Yersinia enterocolitica from stools containing fecal flora which is not adequately suppressed by selective media was tested by direct plating and enrichment techniques. Although a distinct difference in the level of alkali tolerance was observed between Y. enterocolitica and the fecal organisms included in the study, the selective suppression of the fecal flora from stools could not be consistently achieved on this basis. By direct plating, the isolation rates ranged from 4 to 32% for untreated control samples and 11 to 40% for KOH-treated samples, varying with the medium used for culture. The maximum overall recovery rate was 34% by overnight enrichment of fecal samples in Selenite F, tetrathionate, Trypticase soy, and gram-negative broths or by 2-day enrichment in modified Rappaport broth. Alkali treatment of fecal samples did not significantly enhance the isolation rates by these methods. One hundred percent recovery was achieved by prolonged cold enrichment, and alkali treatment made no difference in the recovery rate by this method. Cefsulodinirgasan-novobiocin agar generally yielded higher isolation rates of Y. enterocolitica than did cellobiose-arginine-lysine agar and MacConkey agar, both by direct plating of fecal samples and through subcultures of enrichments, regardless of KOH treatment.