Adaptive laboratory evolution of heat tolerance in Streptococcus thermophilus BIOPOP-1 and BIOPOP-2 under gradually increasing heat temperature

Background Streptococcus thermophilus is one of the lactic acid bacteria (LAB), as such, has been commonly used in the production of fermented dairy foods during past decades. However, LAB including S. thermophilus struggle to survive at temperatures above 60°C, which are applied during manufacturing processes, such as pasteurization that can inhibit cell proliferation and promote cell death. Although there are many studies on S. thermophilus , no research regarding its heat tolerance in over 60°C has been done yet. In this study, a method of adaptive laboratory evolution (ALE) was implemented in an attempt to increase the upper thermal threshold of two S. thermophilus strains of LAB found in South Korea. Results To develop heat tolerant LABs, heat treatment was continuously applied to bacteria by increasing temperature from 60°C until the point of not being detected. The results indicated significant increase in temperature tolerance of ALE strains compared to the wild type (WT) strains. In addition, ALE strains acquired cross protection from various stress conditions like acid, bile salts and salinity. The improved heat tolerance of ALE strains showed higher survival rate during long term storage at sub-zero temperatures compared to the wild type strains. Fatty acid analysis indicated that saturated fatty acid ratio of ALE strains was higher than that of WT cells. Conclusions The results of this study demonstrated that this ALE method can be utilized to improve bacterial tolerance against various environmental stresses. Adaptive evolution under heat stress might be applied in the various industries.