Impact of Growth Temperature and Agar Versus Liquid Media on Freeze–Thaw Tolerance of Yersinia enterocolitica

Abstract

Yersinia enterocolitica is a foodborne pathogen well known for its ability to grow at low temperatures. Recent studies with another psychrotrophic foodborne pathogen, Listeria monocytogenes, revealed that temperature of growth had pronounced impact on survival following repeated freezing and thawing (cryotolerance). Listerial cryotolerance was significantly more pronounced when bacteria were grown at 37°C than following growth at either 4°C or 25°C. However, it is not known whether such impact of growth temperature is a general adaptation shared with other foodborne pathogens. In this study, we investigated the impact of growth temperature (4°C, 25°C, and 37°C) on cryotolerance of Y. enterocolitica. In strong contrast to findings previously obtained with Listeria spp., cryotolerance of Y. enterocolitica was impaired following growth in liquid media at 37°C, with cell concentration dropping to undetectable levels (<10¹ colony forming unit/mL) following as few as six freeze–thaw cycles. On the other hand, when the bacteria were grown at 4°C, cryotolerance was significantly higher (p < 0.05), and substantial survival was maintained even after 18 cycles (2–5 log reduction, depending on strain). Enhanced cryotolerance was also observed with cultures grown at 25°C. Bacteria grown at 37°C on agar were significantly more cryotolerant than following growth at 37°C in liquid media (p < 0.05). The data suggest species-specific impact of growth temperature and liquid versus agar medium on cryotolerance of cold-tolerant bacteria.