Alkatolerance of Yersinia enterocolitica as a basis for selective isolation from food enrichments

Abstract

Alkalotolerance of Y. enterocolitica measured in solutions of potassium hydroxide with 0.5% sodium chloride was influenced by the cell suspension medium, temperature, and growth phase. The rate of cell destruction (Δ log N per minute) was five times greater at 30°C than at 20°C. Differences in the degree of cell destruction at various concentrations of potassium hydroxide were related to pH and not to osmolarity. The addition of peptones to potassium hydroxide provided a protective effect that was greater for cells suspended in Trypticase soy broth than for those suspended in phosphate-buffered sorbitol-bile salts broth. Log-phase cells were less alkalotolerant than cells in the stationary phase of growth. A modified procedure for alkali treatment, using peptone-supplemented 0.5% potassium hydroxide-0.5% sodium chloride and the addition of a pH 6.6 buffer after treatment to prevent further cell destruction, was used to observe a marked difference in alkalotolerance between Y. enterocolitica and other gram-negative bacteria. Despite this difference, alkali treatment was not highly successful for recovery of Y. enterocolitica from enrichments of seeded foods in comparison with selective enrichment in bile-oxalate-sorbose broth.