Long-term storage of bacteriophages of lactic streptococci

Four phage strains representing phages of Streptococcus lactis, S. cremoris, and S. diacetilactis were selected for the observation of the effect of cold storage on their viability. Phages were stored at 4 C and at -18 C, or were frozen at approximately -70 C and stored at -18 C. They were found to display a high degree of stability with these storage methods. The same phage strains showed good stability to storage at room temperature for 3 weeks after thawing and also to alternate freezing and thawing eight times. Three series consisting of from 23 to 31 lactic streptococcal phage preparations were observed over periods extending up to 6 years, and with only a few exceptions were found to store satisfactorily at -18 C after quick freezing. Although the same phage preparations stored at 4 C were generally somewhat less stable, many were stable when stored by both methods.