Researchers: Dr. Maria Marco and Dr. Heidi Rossow, UC Davis

KEY TAKE-A-WAYS
• Current practices on dairy farms for antibiotic use when cows are dried off may reduce bacterial infections but may also reduce beneficial bacteria and white blood cells.

• Although data analysis are still ongoing, preliminary data from this research project suggest antibiotic treatment at dry-off does not appear to influence the bacterial diversity on the cow teat but does influence bacterial diversity in milk.

• Further research based on this study can direct recommendations for antibiotics that are effective at preventing intramammary infections, but also do not cause disruptions to somatic cell count function or enrich undesirable microorganisms on skin or in milk.

BACKGROUND AND OBJECTIVE
The goal of this study was to understand the influence of antibiotic teat treatments on the health of lactating cows and their subsequent milk production. Currently, California dairy industry practices involve injecting antibiotics into the teat when cows are dried off at the end of lactation. Although this practice is believed to treat current bacterial infections and prevent future infections and mastitis, it could instead reduce some populations of beneficial bacteria and the efficacy of white blood cells. The researchers hypothesized that different antibiotic teat treatments applied at dry-off would result in lasting shifts to the teat microbiome and alter the somatic cell count (SCC) as well as the quality of the milk produced in the next lactation period. Testing this hypothesis would allow for the identification of beneficial bacterial profiles and antibiotic profiles that do not impair somatic cell function, thereby improving the quality of milk produced in California.

METHODS, FINDINGS AND OUTCOMES
The researchers designed a study with four treatment groups that varied with respect to antibiotic use and low or high SCC (group 1: low SCC, no antibiotics; group 2: high SCC, no antibiotics; group 3: high SCC, given ceftiofur at dry-off; group 4: high SCC, given cephaphirin at dry-off). The pilot study was conducted using three cows for each treatment group on one dairy, but the larger study design had ten cows per treatment from five dairies. For each cow, milk, blood, and teat swabs (of two different teats) were collected at dry-off, seven days after dryoff, and finally at >40 days (in their next lactation). Results from the pilot study suggested a relationship between antibiotic use, teat and milk microbiota, and SCC. Antibiotic treatment at dry-off reduced the activity of mitochondrial Complex IV in SCC from cows in group 4 and in peripheral blood mononuclear cells (PBMC) from cows in group 3 (both at 7 days after dry-off). High SCC cows tended to have a higher bacterial diversity on the skin and a lower diversity in the milk, which included reduced Staphylococcus on the skin of cows receiving antibiotics.

Although only three cows were included, the pilot study results suggested greater microbial diversity among cows than between teats on individual cows. When the research team combined results from the pilot study with those from the first two dairies investigated, they found no direct effect of antibiotic use on the richness of the bacterial community on the cow teat. Moreover, due to the high variability in the teat microbiota among individual cows over time, there was no clear impact of antibiotic treatment although there was a slight effect of individual dairy farm factors. Further statistical analyses suggested that both dairy farm factors and time of sample collection impacted the microbial diversity on the cow teat, whereas antibiotic treatment remained a nonsignificant effect. The effect of dairy farm factors on the diversity and specific types of bacteria (measured at the level of the family) demonstrates the importance of the facility in shaping the teat microbiota.

The second part of the study plan focused on the effect of antibiotic treatment on the milk microbiome. The team developed a method that allowed them to have as pure of a milk microbiome sample as possible (no contamination from cow skin cells or environment). Pre-treatment with Cetrimonium Bromide followed by phenol-chloroform DNA extraction increased Polymerase Chain Reaction (PCR) success by 80%. Results from milk samples collected in the pilot study suggest milk from cows with higher SCCs have lower bacterial diversity. The microbiota from cows receiving ceftiofur were more diverse than those receiving Cephaphirin, suggesting an effect of antibiotic treatment on milk microbiome diversity. However, these data should be considered preliminary as they come from small sample sizes.

CONCLUSIONS
This study examined the effects of antibiotic teat treatment methods on the teat and milk microbiome and determined the impact of antibiotic treatment on the efficacy of somatic cells by measuring the energy status of milk and blood SCCs. First, this project lays the groundwork to better understanding of the short- and long-term impacts of antibiotics on udder health and the udder microbiome. Next, it can direct recommendations for antibiotics that are effective at preventing intramammary infections, but also do not cause disruptions to SCC function or enrich undesirable microorganisms on skin or in milk. The findings will help in identifying what a healthy cow and milk udder microbiome should look like and which individual microbial species promote cow health. And finally, the results can help reduce antibiotic use overall, thereby mitigating antibiotic resistance gene spread.