A Network-Guided Narrative Review of Cross-Kingdom Associations Between Yeasts and Bacteria in Traditional Fermented Milks

In many industrial dairy products, yeasts are generally regarded as contaminants. However, in traditional fermented milks, they may contribute to distinctive sensory, technological, and functional properties through associations with bacterial partners, including lactic acid bacteria (LAB). Despite this, a structured synthesis of yeast–bacterium associations across fermented milk typologies is currently lacking. To address this gap, a PRISMA-informed literature search identified 42 studies across 24 traditional fermented milks reporting paired bacterial and fungal communities. A genus-level co-occurrence analysis was used to identify which yeast–bacterium pairs were most frequently co-detected across independently documented products. The main co-occurrence patterns selected for detailed bibliographical discussion were Kluyveromyces with Acetobacter and LAB, including Lactobacillus, Streptococcus, Lentilactobacillus and Lacticaseibacillus; Pichia with LAB; Saccharomyces with LAB, especially Lactobacillus; Kazachstania with Acetobacter; Candida with Leuconostoc and Enterococcus; and Geotrichum with Pseudomonas and Enterococcus. For the selected associations, possible interaction mechanisms and implications for sensory identity, technological potential, and microbiological safety were discussed by integrating evidence from milk co-cultures, controlled model systems, and related fermented foods. Overall, this review provides a structured synthesis of yeast–bacterium associations in traditional fermented milks and identifies candidate consortia for future experimental validation.