Short communication: Salt tolerance of Lactococcus lactis R-604 as influenced by mild stresses from ethanol, heat, hydrogen peroxide, and UV light.

Lactococcus lactis is a culture widely used in salt-containing dairy products. Salt hinders bacterial growth, but exposure to environmental stress may protect cells against subsequent stress, including salt. The objective of this study was to evaluate the salt tolerance of L. lactis R-604 after exposure to various stresses. The culture was subjected to 10% (vol/vol) ethanol for 30 min, mild heat at 52°C for 30 min, 15 mM hydrogen peroxide for 30 min, or UV light (254 nm) for 5 min and compared with a control. Starting with 5 log cfu/mL for all treatments, growth was determined in M17 broth with 5 NaCl concentrations (0, 1, 3, 5, and 7% wt/vol). Plating was conducted daily for 5 d. Salt tolerance was enhanced with mild heat exposure before growth in M17 broth with 5% (wt/vol) NaCl on d 3, 4, and 5, and with exposure to hydrogen peroxide and ethanol stresses before growth in M17 broth with 5% (wt/vol) NaCl on d 4 and 5. Exposure of this culture to mild heat, hydrogen peroxide, or ethanol before growth in M17 broth containing 5% (wt/vol) salt can enhance its survival, which could be beneficial when using it in salt-containing dairy products.