The bovine milk microbiome – an evolving science

• Published in: Domestic animal endocrinology Vol. 79; p. 106708
• Main Author: Ruegg, Pamela L.
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 01.04.2022
• Subjects: Animals; Bacteria; Bacteria - genetics; bacterial biomass; Bovine; Cattle; Cattle Diseases; DNA; dysbiosis; endocrinology; farms; Female; genome; genomics; keratin; mammary glands; Mammary Glands, Animal; Mastitis; Mastitis, Bovine; Microbiome; Microbiota - genetics; Milk; sanitation; Skin; Udder health; udders; viability; Bacteria; Mastitis; Bovine; Udder health; Milk; Microbiome
• ISSN: 0739-7240; 1879-0054; 1879-0054
• DOI: 10.1016/j.domaniend.2021.106708

•Progress in our understanding of the relevance and functioning of a microbiota of the bovine mammary gland and milk has been slow due to methodological issues associated with the low bacterial biomass of bovine milk.•To date, there is no consensus that a core commensal microbiome exists in the mammary gland and environmental factors have been shown to strongly impact microbial communitie's of bovine milk.•The concept of dysbiosis of microbial communities as part of a causal pathway for disorders of the mammary gland has not yet been established and remains a controversial topic.•Considerable variation in outcomes among studies has precluded the ability to make conclusions about the impact of host, environmental and microbial factors on the bovine milk microbiota.•Standardization of collection and processing methods and replication of experiments are critically needed to solidify our understanding of this emerging topic. Improved access to genome based, culture independent methods has generated great interest in defining the bovine milk microbiome. Several comprehensive reviews of this subject have recently been published and the purpose of this short review is to consolidate current understanding of the relevance and biological significance of this emerging topic. In contrast to mucosal organs that contain rich and well-characterized culturable and nonculturable microbial communities, milk obtained from the healthy bovine mammary gland usually contains few or no viable bacteria. The low bacterial biomass of milk has created methodological challenges that have resulted in considerable variability in results of studies that have used genomic methods to define the microbiota of milk obtained from healthy or diseased mammary glands. While genomes from several bacterial genera are routinely identified from samples of milk, teat skin and the teat canal, the viability, origin, and function of these organisms is uncertain as environmental factors have been shown to strongly influence the composition of these bacterial populations. Possible sources of microbial DNA include bacteria introduced from skin or the environment, bacteria trapped in teat canal keratin or bacteria engulfed by phagocytes. Researchers have not achieved consensus about key concepts such as the presence of a core commensal milk microbiome or dysbiosis as part of a causal pathway disrupting udder health. Understanding of the bovine milk microbiome has been greatly impeded by a lack of standardized methods used to collect, process, and assess bovine milk samples. Sample collection is a critical first step that will determine the validity of results. To minimize contamination with external sources of bacterial DNA, teat sanitation methods used for collection of milk samples that will be subjected to extraction and amplification of bacteria DNA should far exceed aseptic techniques used for collection of milk samples that will be submitted for microbiological culture. A number of laboratory issues have yet to be resolved. Contamination of low biomass samples with bacterial DNA from laboratory reagents is a well-known issue that has affected results of studies using bovine milk samples and results of sequencing of negative controls should always be reported. Replication of experiments has rarely been performed and consistency in results are lacking. While progress has been made, standardization of methods and replication using samples originating from differing farm conditions are critically needed to solidify knowledge of this emerging topic.