Microbiota Characterization of the Cow Mammary Gland Microenvironment and Its Association with Somatic Cell Count

Simple Summary Subclinical mastitis is mostly caused by pathogenic infections, including bacteria, viruses, and fungi, with bacterial infections being the most serious. SCC is a relatively accurate indicator of the severity of mastitis and is widely used as a diagnostic indicator of mastitis in dairy cows. The purpose of this investigation is to explore the relationship between SCC, diversity in the microbiome, and SCM. Recent studies have suggested that SCM-related pathogens stay in isolation identification and drug resistance, and the relative lack of SCM is related to the change in microbiome diversity. Therefore, we aim to investigate the association between dysbacteriosis and the development of SCM using 16Sr RNA high-throughput sequencing and molecular diagnostic techniques to analyze the structure and abundance of microbiota in the mammary microenvironment in different SCC ranges. The roles of Staphylococcus spp. and Streptococcus spp., as well as Enterobacter spp., in various udder quarters and their relationship with inflammation and mastitis have been established. Bacillus spp. has also been associated with the development of mastitis. Among them, Pseudomonas, Serratia, and Acinetobacter can be used as landmark genera for the transition from the subclinical to the clinical state of bovine mastitis.Abstract Subclinical mastitis is a common disease that threatens the welfare and health of dairy cows and causes huge economic losses. Somatic cell count (SCC) is the most suitable indirect index used to evaluate the degree of mastitis. To explore the relationship between SCC, diversity in the microbiome, and subclinical mastitis, we performed next-generation sequencing of the 16S rRNA gene of cow's milk with different SCC ranges. The data obtained showed that the microbiota was rich and coordinated with SCC below 2 x 105. SCC above 2 x 105 showed a decrease in the diversity of microbial genera. When SCC was below 2 x 105, the phylum Actinobacteriota accounted for the most. When SCC was between 2 x 105 and 5 x 105, Firmicutes accounted for the most, and when SCC exceeded 5 x 105, Firmicutes and Proteobacteria accounted for the most. Pathogenic genera such as Streptococcus spp. were absent, while SCC above 2 x 105 showed a decrease in the diversity of microbial genera. SCC was positively correlated with the percentage of Romboutsia, Turicibacter, and Paeniclostridium and negatively correlated with the percentage of Staphylococcus, Psychrobacter, Aerococcus, and Streptococcus. Romboutsia decreased 6.19 times after the SCC exceeded 2 x 105; the SCC increased exponentially from 2 x 105 to 5 x 105 and above 1 x 106 in Psychrobacter. Analysis of the microbiota of the different SCC ranges suggests that the development of mastitis may not only be a primary infection but may also be the result of dysbiosis in the mammary gland.