Effects of Intramammary Antimicrobial Treatment on the Milk Microbiota Composition in Mild Clinical Bovine Mastitis Caused by Gram-Positive Bacteria

SIMPLE SUMMARY: Because bovine mastitis is usually a result of bacterial infection, treatment with antimicrobial agents is widely used, and mastitis is the main reason for the use of antimicrobial agents in dairy cows. Although it is generally understood that antimicrobial treatment affects various microbiota in the body, the effects of intramammary administration of antimicrobials for mastitis on the milk microbiota in cows remain unclear. This study is the first to examine the changes in the milk microbiota during intramammary administration of antimicrobials for mastitis caused by Gram-positive bacteria. We found that the effect of the antimicrobial drug treatment on the milk microbiota composition became evident after the second day of antimicrobial administration, and similarities in the milk microbiota composition among the antimicrobial treatment group were discovered on the seventh day. This is the first study to report the changes in the milk microbiota during intramammary antimicrobial treatment for mild clinical mastitis caused by Gram-positive bacteria in dairy cattle. ABSTRACT: The purpose of this study was to clarify the effects of antimicrobial treatment for mild mastitis caused by Gram-positive bacteria on the milk microbiota in dairy cattle. Sixteen quarters of sixteen cows with mild clinical mastitis from the same herd were included in the study. On the day of onset (day 0), the cows were randomly allocated to a no-treatment (NT; n = 10) group or an intramammary antimicrobial treatment (AMT) group that received AMT starting on day 0 (AMT-AMT group; n = 6). The next day (day 1), the cows in the NT group were randomly allocated into an NT group (NT-NT group; n = 3) that received no treatment or an AMT group that received AMT starting on day 1 (NT-AMT group; n = 7). Milk samples were collected on days 0, 1, 3 and 7, and the milk microbiota of each sample was comprehensively analyzed via 16S rRNA gene amplicon sequencing of the milk DNA. During the treatment period, the milk microbiota of the NT-NT group did not change, but those of the NT-AMT and AMT-AMT groups changed significantly on days three and seven. Thus, the use of antimicrobials for mild mastitis caused by Gram-positive bacteria changes the milk microbiota composition.