Preparation and storage of high titer lactic streptococcus bacteriophages

Abstract

Lactic streptococci are of critical importance to the dairy fermentation industry because these bacteria supply the lactic acid for curd production and their metabolic products impart characteristic and desirable flavors. Bacteriophage infection of these starter cultures results in insufficient acid production and usually a failure of the fermentation. The economic and public health consequences of these failures are well known. Various approaches have been utilized in an attempt to minimize bacteriophage infection during dairy fermentations. The use of culture rotation, mixed strain starter cultures, and a phage inhibitory medium are currently in general use. These techniques never completely prevent failures, and constant precautions to prevent culture infections are advised. The use of mixed strains and starter rotation relies on the utilization of lactic streptococci that are resistant to a diversity of bacteriophage. Periodic examination of phage resistance patterns of starter cultures would be advisable. Previous studies dealing with bacteriophage propagation have not dealt specifically with the development of high titers and have not been concerned with extended survival times of the viruses. In the present study, two procedures are described for routinely obtaining bacteriophage titers in excess of 1010 plaque forming units. A simple storage procedure also is described which has proven suitable for maintaining these high titers during a 2 year study period.