Assessment of the tolerance of Lactococcus lactis cells at elevated temperatures

When Lactococcus lactis strains were exposed directly to the lethal tempera ture of 50 degrees C for 30 min, 0.1-31% of the cells survived. However, wh en pre-exposed to 40 degreesC, prior to exposure at 50 degreesC, 4-61% of t he cells survived. A plasmid carrying a unique heat shock gene from the the rmophile Streptococcus thermophilus was cloned into L. lactis. When the tra nsformed cells were cultivated at 30 degreesC the introduction of the plasm id had no obvious effect on the growth of L. lactis. However, when the temp erature was abruptly shifted from 30 degreesC to 42 degreesC at mid-growth phase the growth decreased by 50%.