Routine culturing of stool specimens for Yersinia enterocolitica.

The sweeping conclusion of Kachoris et al. (3) that routine culturing of stool specimens for Yersinia enterocolitica is not a cost-effective procedure concerns us. We wish to recommend that the word procedure be followed by "from our patient population in Boston, Mass." The authors mentioned the variability of isolation rates reported by three groups of investigators and referred to Weissfeld's comments that geographical variations of Y. enterocolitica isolation rates do occur but failed to elaborate upon this phenomenon in their population. Since June 1982, our laboratory has used cefsulodinIrgasan-novobiocin (CIN) agar (Oxoid Ltd., Basingstoke, England; or Prepared Media Laboratories, Richmond, British Columbia, Canada) and cold enrichment to isolate Yersinia spp. After examining 6,209 fecal specimens, 415 Yersinia spp. have been recovered. More than 7% of the samples and 10.8% of symptomatic patients have been positive. On a yearly basis, our isolation rate has been consistent since we began culturing. Although the majority of our isolates have come from cold enrichment, many of them appear to be clinically relevant (2, 3a-5). In comparison, 4% of all specimens and 5% of all patients have been culture positive with Campylobacter spp. Our methodology differs from that of Kachoris et al. (3) in that CIN plates are incubated at 35°C, not at 28°C; and for only 18 to 24 h, not 48 h or longer. After 18 to 24 h of incubation our technologists have no difficulty in picking small (1to 1.5-mm) typical red bull's-eye colonies for identification. Other enteric organisms are larger at this temperature and generally would not be picked. All of our biochemical identification tests are incubated at 28°C. Yersinia spp. are more active metabolically between 22 and 28°C; thus, biochemical interpretations are most reliable when incubation is at 28°C rather than at 37°C (1). Although we have no firsthand experience with the Vitek system, as the authors do, we suspect that any system which requires a 37°C incubation for identification may possibly yield unreliable results for Yersinia spp. Prior to the development of CIN medium, only 48 isolates of Yersinia spp. were reported from British Columbia, Canada, from 1966 to 1978 (6). In fact, regional differences in the isolation rate of Yersinia spp. still exist, both within the province of British Columbia and even the city of Vancouver, despite a uniformity of technique. For our primarily adult patient population, we can only conclude that the routine culturing of feces specimens for Y. enterocolitica is both clinically relevant and cost-effective. Other laboratories should be encouraged, not discouraged as Kachoris et al. (3) suggest, to search for this organism in their own populations and draw their own conclusions as to the costeffectiveness of a CIN plate in their routine stool-screening procedure.