Action of Bacteriophage on Mixed Strain Starter Cultures. II. Relation to Acid Production of the Proportion of Resistant Bacteria

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Abstract
Although mixed strain starters, which may contain several strains of lactic streptococci, are used extensively in the manufacture of certain dairy products, retarded acid development caused by the action of bacteriophage has remained of primary importance. The use of mixtures has been considered to decrease the occurrence of starter failure. However, their use also has been found to complicate diagnosis of starter difficulty and to make the effect of bacteriophage action less predictable. Among those who have reported retarded development of mixed strain cultures due to the action of bacteriophage are Anderson and Meanwell (2), Johns (7) and Nelson el al. (10). In addition to retarded acid development, this laboratory has encountered starter failure caused by bacterio- phage, similar to that reported by Johns and Katznelson (8) and Babel (3), in which the retardation of acid production was as drastic and abrupt as that usu- ally encountered with single strain starters. In view of these different degrees of retarded acid development during the use of mixed strain starters, this study was undertaken to determine the proportion of susceptible bacteria necessary to permit the action of bacteriophage to appreciably change the amount of acid pro- duced by a starter composed of different strains of S. lactis. -METHODS Since a previous study (5) had shown that the "nascent phenomenon" would occur upon mixing certain bacteria-bacteri ophage combinations, such combina- tions were carefully excluded from mixtures. Cultures of S. lactis were propa- gated in litmus milk at 32 ° C. Fresh undiluted culture was added as inoculum for those experiments in which the amount of a culture required was 1 per cent or greater. For smaller amounts, cultures were diluted 1:3 in sterile distilled water to facilitate measurement of the inoculum. Bacterial populations were determined at 32 ° C. by the standard plate count (1) on tryptone-glucose -beef extract-milk agar. The limiting active dilution method (6) was used for estimating the most probable number of bacteriophage particles per milliliter of whey filtrate. Dilu- tions were made in sterile skimmilk, and bottles having the desired titer were used as inoculum. Experiments were run using 300-ml. quantities of sterile skimmilk. Culture and diluted filtrate were added to bottles of skimmilk which had been permitted to reach 32 ° C. in a water bath. Titratable acidity values and bacterial popula- tions were determined at hourly intervals following inoculation. Results of duplicate experiments were averaged.