Injury and death of Streptococcus lactis due to freezing and frozen storage

Abstract

Cells of Streptococcus lactis were harvested in the early stationary phase, washed, and resuspended in either skim milk (10% nonfat milk) or buffered distilled water (0.0003 m dipotassium phosphate, pH 7.2). Samples of each suspension were frozen and stored at -20 C for intervals up to 28 days. Colony counts of the frozen culture were made using lactic agar and a "restricted" lactic agar medium (Tryptone reduced to 0.5%) to determine injury and death. Death was determined by the difference in plate counts on lactic agar before and after freezing. Injured cells were determined by the difference in plate counts on the two plating media. Greatest injury of the cells occurred during early stages of frozen storage and decreased with time, and death continuously increased. Injury and death were more pronounced when cells were frozen in water than when frozen in 10% nonfat milk solids. Certain cultures survived better when frozen rapidly, whereas with others survival was greater when freezing was slow. Successive freezing, thawing, and propagation of the culture gradually eliminated cells which showed injury by freezing.