Comparative Study of Selective Media for Recovery of Yersinia enterocolitica

Abstract

To determine the relative efficacy of pectin agar, cellobiose-arginine-lysine (CAL) agar, Y medium, cefsulodin-irgasan-novobiocin (CIN) agar, MacConkey (MAC) agar, and salmonella-shigella (SS) agar for the recovery of Yersinia enterocolitica , 35 strains of this organism representing the serotypes most commonly associated with human disease in Canada and the United States were inoculated on test media in pure cultures and mixed in fecal specimens. Randomly picked stool specimens were also cultured on these media to determine their selectivity. The ability of test media to support the growth of Y. enterocolitica strains varied markedly. Several strains failed to grow on Y medium, and a few failed to grow on SS agar. There were no significant differences in cultural characteristics and the recovery rates of Canadian and American strains. The combined recovery of the pure cultures of Y. enterocolitica on test media as compared with blood agar was MAC, 75%; SS, 48%; pectin, 70%; CAL, 62%; Y, 15%; and CIN, 85%. The selectivity of the test media expressed as the percent difference of the fecal colony counts obtained on blood agar and the selective media was MAC, 7%; SS, 50%; pectin, 4%; CAL, 65%; Y, ≥99.9%; and CIN, 95%. When stool suspensions containing 10 2 colony-forming units of the test strains were plated, CIN medium yielded 100% recovery of all 35 strains from all fecal samples. The combined recovery rate for other media was CAL, 63%; Y, 14%; and SS, 11%. With 10 1 colony-forming units, CIN yielded a 100% recovery of the test strains, whereas CAL, Y, and SS agar showed a 0% recovery. None of the test organisms was recovered on either MAC or pectin agar at either dilution. The inhibitory effect of Y medium for Y. enterocolitica could be overcome to some extent by reducing the sodium oxalate and sodium desoxycholate content of the medium. One such modified Y medium offered optimum selectivity and ensured greater recovery of Y. enterocolitica when compared with the Y medium. We found CIN agar by far the most effective medium for the recovery of Y. enterocolitica. This medium was highly selective and almost completely inhibited the fecal flora, while at the same time supporting luxuriant growth of Y. enterocolitica. On CIN agar, Y. enterocolitica colonies were distinctive in appearance and measured 1.5 to 4 mm in diameter within 20 to 40 h of incubation.