Effect of suspending and plating media on the recovery of Salmonella gallinarum following freezing and thawing*

Summary Salmonella gallinarum suspended in distilled water was frozen at a rate of 22°C/min, 86% of the cells were inactivated, as determined from differences in numbers of colony‐forming units observed on TSA medium before and after freezing. About 35% of the viable survivors were injured, as determined by their failure to form colonies on a minimal plating medium (MA) but their success on a ‘complete’ medium (TSA). Following 1 hr incubation of the frozen‐thawed cells in water or in 0.3 mM monopotassium phosphate, the percentage survival was reduced from 14 to 8% whereas in selenite cystine or tetrathionate broths, the percentage of survivors was reduced to about 3%. Higher recoveries of the frozen‐thawed cells were obtained when nonselective suspending media such as nutrient broth and lactose broth were employed. The possibility of obtaining better recovery by the addition of specific compounds to plating media was investigated. Filter‐sterilized catalase or vitamin‐free hydrolyzed casein, when added to the minimal agar medium improved the recovery of frozen‐thawed S. gallinarum by 32 and 34% respectively. Sodium pyruvate, heat‐sterilized catalase or ethylenediaminetetraacetic acid had no significant effect, but the addition of haemin or cysteine reduced the rate of recovery by 48 and 57% respectively.