Rapid isolation of Yersinia spp. from feces

Abstract

Direct plating or cold enrichment or both have been used to isolate Yersinia spp. from feces. Freeze-shock double enrichment and KOH treatment have been recommended for recovery of Yersinia enterocolitica from surface waters and food, respectively. These techniques were evaluated as alternatives for rapid recovery of Yersinia spp. from feces. Stool samples were homogenized in buffered saline and autoclaved. Escherichia coli, Klebsiella pneumoniae, and Pseudomonas aeruginosa were each added to the suspension at a final concentration of 1.5 X 106 colony-forming units per ml. Yersinia cells were then added to a final concentration of 1.5 X 103, 1.5 X 104, 1.5 X 105, or 1.5 X 106 colony-forming units per ml. A total of 21 strains of Y. enterocolitica, 2 of Yersinia kristensenii, and 1 each of Yersinia intermedia and Yersinia fredriksenii were tested. For freeze-shock double enrichment, seeded stool samples were frozen overnight (-70° C), transferred sucessively to m-tetrathionate broth (6 h, 37°C) and selenite broth (2 h, 37° C), and plated on MacConkey, salmonella-shigella, and cellobiose-arginine-lysine agars for quantitation. For KOH treatment, seeded stool samples were mixed with 0.5% KOH at a ratio of 1:2 for 2 min and plated as described above. E. coli, K. pneumoniae, and P. aeruginosa were virtually eliminated after either method was used. All Yersinia strains were recovered after KOH treatment even at the lowest initial concentration (1.5 X 103 colony-forming units per ml). However, after freeze-shock double enrichment, not all strains were retrievable, and those isolates which were recovered were grown only from samples containing the highest number of Yersinia strains (1.5 X 106 colony-forming units per ml). KOH treatment of stool samples seems to be a viable substitute for more protracted methods of recovering Yersinia spp.