The complex community structures and seasonal variations of psychrotrophic bacteria in raw milk in Heilongjiang Province, China

Psychrotrophic bacteria are the dominant organisms in refrigerated raw milk, which may have potentially adverse effects on the quality and shelf-life of dairy products. In this study, we investigated the concentration and microbial composition of psychrotrophic bacteria in raw milk from Heilongjiang Province (China) in winter and summer using traditional cultivation and single molecule real-time (SMRT) sequencing methods. The results showed that the mean counts of psychrotrophic bacteria were 3.73 log CFU/mL based on traditional cultivation method. The isolated psychrotrophic bacteria (45 genera, 120 species) showed high diversity. Pseudomonas, Lactococcus, Acinetobacter, Chryseobacterium and Staphylococcus were the most frequently isolated genera. Moreover, 2 potential novel genera and 3 unknown species were discovered. The SMRT sequencing method could accurately determine the relative abundance of psychrotrophic bacteria in raw milk. Brevundimonas (10.1%), Janthinobacterium (10.0%), Acinetobacter (8.9%), Sphingomonas (8.5%) and Enterococcus (8.1%), as the predominant genera, were present in all raw milk samples. The concentration and community structure of psychrotrophic bacteria varied significantly relying on the season. This work contributes to attracting dairy manufacturers' attention to the contamination of psychrotrophic bacteria in raw milk and providing support for the control of psychrotrophic bacteria.