A new medium, M17, was developed to improve the growth of lactic streptococci and their bacteriophages. M17 contains β-disodium glycerophosphate (GP), which enhances buffering capacity, allowing better growth of lactic streptococci at 30°C. The medium also supports the isolation of bacterial mutants that cannot ferment lactose, as these mutants form smaller colonies compared to wild-type cells. GP added to skim milk enhances acid-producing activity by lactic streptococci when inoculated into GP-free milk. M17 is also superior to other media in demonstrating and distinguishing between lactic streptococcal bacteriophages, producing larger plaques and showing lysogeny through turbid plaques. Lactic streptococci are nutritionally fastidious and require complex media for optimal growth. In synthetic media, they need at least six amino acids and three vitamins. Their acid-producing nature requires well-buffered media for reasonable growth. Previous media like M16 and T5 had limitations, but M17 improved growth and buffering. The addition of GP to M17 improved pH stability, preventing it from dropping below 5.70 after 24 hours of growth at 30°C. This buffering capacity also helped in maintaining the acid-producing ability of lactic streptococci in milk. M17 was also effective in demonstrating bacteriophages, allowing the observation of large clear plaques, turbid plaques, and phage lysin diffusion. The medium supported better host growth, enabling the demonstration of phenomena previously not observed in lactic streptococcal phages. The use of M17 in selecting carbohydrate-requiring mutants was also beneficial, as wild-type colonies grew larger, making differences between mutants and parent cells more apparent. The study also showed that GP addition to milk protected cells from freezing damage, which is important for the storage and use of lactic starter cultures. M17 medium is likely to be useful for other acid-producing bacteria, especially lactobacilli. The findings suggest that GP can be used to buffer milk, improving acid production and potentially being an economical step due to GP's low cost and wide use in food and medicine. The medium is also valuable for isolating and studying mutants, and for demonstrating lysogeny in lactic streptococci.A new medium, M17, was developed to improve the growth of lactic streptococci and their bacteriophages. M17 contains β-disodium glycerophosphate (GP), which enhances buffering capacity, allowing better growth of lactic streptococci at 30°C. The medium also supports the isolation of bacterial mutants that cannot ferment lactose, as these mutants form smaller colonies compared to wild-type cells. GP added to skim milk enhances acid-producing activity by lactic streptococci when inoculated into GP-free milk. M17 is also superior to other media in demonstrating and distinguishing between lactic streptococcal bacteriophages, producing larger plaques and showing lysogeny through turbid plaques. Lactic streptococci are nutritionally fastidious and require complex media for optimal growth. In synthetic media, they need at least six amino acids and three vitamins. Their acid-producing nature requires well-buffered media for reasonable growth. Previous media like M16 and T5 had limitations, but M17 improved growth and buffering. The addition of GP to M17 improved pH stability, preventing it from dropping below 5.70 after 24 hours of growth at 30°C. This buffering capacity also helped in maintaining the acid-producing ability of lactic streptococci in milk. M17 was also effective in demonstrating bacteriophages, allowing the observation of large clear plaques, turbid plaques, and phage lysin diffusion. The medium supported better host growth, enabling the demonstration of phenomena previously not observed in lactic streptococcal phages. The use of M17 in selecting carbohydrate-requiring mutants was also beneficial, as wild-type colonies grew larger, making differences between mutants and parent cells more apparent. The study also showed that GP addition to milk protected cells from freezing damage, which is important for the storage and use of lactic starter cultures. M17 medium is likely to be useful for other acid-producing bacteria, especially lactobacilli. The findings suggest that GP can be used to buffer milk, improving acid production and potentially being an economical step due to GP's low cost and wide use in food and medicine. The medium is also valuable for isolating and studying mutants, and for demonstrating lysogeny in lactic streptococci.