Yogurt starter cultures

Yogurt starter cultures are communities of lactic acid bacteria, chiefly Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus, that ferment milk into yogurt. Their bacteriophages helped reveal CRISPR-mediated immunity.

Bacteriophage: A virus that infects bacteria or archaea and reproduces using their cellular machinery. Phages infecting yogurt bacteria supplied the selective pressure behind dairy cultures’ CRISPR defenses.
Streptococcus thermophilus: A lactic acid bacterium used with Lactobacillus delbrueckii subsp. bulgaricus to ferment milk into yogurt. It is one of yogurt’s principal starter organisms and a host of dairy phages.
Dairy fermentation: The use of microorganisms to transform milk into foods such as yogurt, cheese, and kefir. Yogurt starters are one specialized culture system within this broader food-making practice.
Francisco Mojica: A Spanish microbiologist who studied unusual repeated DNA sequences in archaea and helped establish their role in microbial immunity. His early work identified the repeated sequences later connected to phage defense in dairy bacteria.
CRISPR: An adaptive immune system in many bacteria and archaea that uses stored viral sequences to recognize and target invading genetic material. Phage-resistant yogurt strains helped establish that CRISPR sequences record past infections.
Lactobacillus delbrueckii subsp. bulgaricus: A lactic acid bacterium commonly paired with Streptococcus thermophilus in yogurt fermentation. Its partnership with S. thermophilus shapes fermentation and the culture’s vulnerability to phages.
Bacteriophage control in dairy production: Practices that reduce phage contamination and protect bacterial cultures during dairy fermentation. Phage outbreaks can slow or stop the starter activity needed for consistent yogurt production.
Philippe Horvath: A microbiologist whose research at Danisco helped connect CRISPR sequences with bacteriophage resistance in dairy bacteria. His group’s experiments with Streptococcus thermophilus provided direct evidence of sequence-specific resistance.
CRISPR adaptation: The stage of CRISPR immunity in which a cell acquires fragments of foreign genetic material and inserts them into its CRISPR array. This acquisition process explains how yogurt bacteria can gain resistance to newly encountered phages.
Lactic acid fermentation: A metabolic process in which microbes convert sugars into lactic acid, regenerating NAD⁺ without oxygen-dependent respiration. Yogurt starters acidify milk through this process, producing the conditions that define yogurt.

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