Effect of freezing of water samples on viable counts of environmental mycobacteria.

The effect of prolonged storage on mycobacteria and other heterotrophic bacteria in brook water samples was examined by determination of viable counts from fresh samples and again after water concentrates had been stored in nutrient broth at -75 degrees C for 15 months. The counts of mycobacteria were on average three times higher after storage (range of ratio 0.9-10.4). In contrast, the viable counts of other heterotrophic bacteria were reduced by 69%. The increase in mycobacterial counts was probably due to break-up of microcolonies or release of attached bacteria from particles. The possibility of cultivating mycobacteria from frozen samples is of practical help in large-scale field surveys.