Bulk tank milk microbiota (BTMM) is affected by various factors, including the characteristics of raw milk, microflora on teat surfaces, and the milking system. Clarifying the influence of these factors is important for producing high-quality dairy products. This longitudinal study describes the annual changes in BTMM at six dairy farms in northern Kanagawa Prefecture, Japan. Bulk tank milk samples were collected six times a year (in February, April, June, August, October, and December of 2022) to give a total of thirty-six samples. After bulk tank somatic cell counts (BTSCC) had been determined, we performed 16S rRNA gene amplicon sequence analysis to clarify the composition of the BTMM. Although no annual changes were observed in the BTSCC and alpha-diversity index, a significant difference in the beta-diversity index was observed between February and August (p = 0.0315). In February, the proportions of the psychrophilic genera Listeria and Enterococcus were significantly increased (p < 0.05). Similarly, in August, the proportion of commensal milk microbiota in the genera Catenibacterium and Acetobacter were significantly increased (p < 0.05). The results of this study suggest that the composition of BTMM in this region changed throughout the year, which may have been influenced by psychrophilic bacteria in winter.
