Various body sites of vertebrates provide stable and nutrient-rich ecosystems for a diverse range of commensal, opportunistic, and pathogenic microorganisms to thrive. The collective genomes of these microbial symbionts (the microbiome) provide host animals with several advantages, including metabolism of indigestible carbohydrates, biosynthesis of vitamins, and modulation of innate and adaptive immune systems. In the context of the bovine udder, however, the relationship between cow and microbes has been traditionally viewed strictly from the perspective of host-pathogen interactions, with intramammary infections by mastitis pathogens triggering inflammatory responses (i.e., mastitis) that are often detrimental to mammary tissues and cow physiology. This traditional view has been challenged by recent metagenomic studies indicating that mammary secretions of clinically healthy quarters can harbor genomic markers of diverse bacterial groups, the vast majority of which have not been associated with mastitis. These observations have given rise to the concept of “commensal mammary microbiota,” the ecological properties of which can have important implications for understanding the pathogenesis of mastitis and offer opportunities for development of novel prophylactic or therapeutic products (or both) as alternatives to antimicrobials. Studies conducted to date have suggested that an optimum diversity of mammary microbiota is associated with immune homeostasis, whereas the microbiota of mastitic quarters, or those with a history of mastitis, are considerably less diverse. Whether disruption of the diversity of udder microbiota (dysbiosis) has a role in determining mastitis susceptibility remains unknown. Moreover, little is known about contributions of various biotic and abiotic factors in shaping overall diversity of udder microbiota. This review summarizes current understanding of the microbiota within various niches of the udder and highlights the need to view the microbiota of the teat apex, teat canal, and mammary secretions as interconnected niches of a highly dynamic microbial ecosystem. In addition, host-associated factors, including physiological and anatomical parameters, as well as genetic traits that may affect the udder microbiota are briefly discussed. Finally, current understanding of the effect of antimicrobials on the composition of intramammary microbiota is discussed, highlighting the resilience of udder microbiota to exogenous perturbants.

脊椎动物的各个身体部位为各种常见，机会性和致病性微生物的繁衍提供了稳定且营养丰富的生态系统。这些微生物共生体（微生物组）的集体基因组为宿主动物提供了许多优势，包括难消化的碳水化合物的代谢，维生素的生物合成以及先天性和适应性免疫系统的调节。然而，在牛乳房的背景下，传统上严格从宿主-病原体相互作用以及乳腺炎引起的乳房内感染的角度严格看待牛与微生物之间的关系。引发炎症反应（即乳腺炎）的病原体，通常对乳腺组织和母牛的生理有害。传统的观点受到最近的宏基因组学研究的挑战，该研究表明，临床上健康的地方的乳腺分泌物可以带有多种细菌群的基因组标记，其中绝大多数与乳腺炎无关。这些观察结果引起了“共乳腺微生物群”的概念，其生态特性可能对理解乳腺炎的发病机理具有重要意义，并为开发新型的预防性或治疗性产品（或两者）提供了替代抗菌剂的机会。迄今为止进行的研究表明，最佳的乳腺菌群多样性与免疫稳态有关，乳腺区或有乳腺炎病史的菌群的多样性要小得多。乳房微生物群多样性的破坏（生物异质化）是否在确定乳腺炎易感性中起作用尚不清楚。此外，关于各种生物和非生物因素在塑造乳房微生物群整体多样性方面的贡献知之甚少。这篇综述总结了当前对乳房各个生态位内微生物群的了解，并强调了需要将乳头，乳头道和乳腺分泌物的微生物群视为高度动态的微生物生态系统的相互关联的生态位。此外，还简要讨论了与宿主相关的因素，包括生理和解剖学参数以及可能影响乳房微生物群的遗传特性。最后，