The impact of kit, environment and sampling contamination on the observed microbiome of bovine milk

Contaminants can easily outnumber bacteria that originate within the milk itself, milk microbiome research currently suffers from a critical knowledge gap; namely, does non-mastitis bovine milk contain a native microbiome? In this study, we sampled external and internal mammary epithelium, stripped and cisternal milk, used numerous negative controls to identify potential sources of microbial contamination. Two algorithms were used to mathematically remove this contamination and to track potential movement of microbes among our samples. Our results suggest that majority (i.e., >75%) of the sequence data generated from bovine milk and mammary epithelium samples represents contaminating DNA. The contaminants in milk samples were primarily sourced from the DNA extraction kits and the internal and external skin of the teat, while the teat canal and apex samples were mainly contaminated during the sampling process. After decontamination, the milk microbiome displayed a more dispersed, less diverse and compositionally distinct bacterial profile compared with the teat skin samples. Similar microbial compositions were observed between cisternal and stripped milk samples, as well as between teat apex and canal samples. Staphylococcus and Acinetobacter were the predominant genera detected in the sequences of milk samples, and bacterial culture showed growth of Staphylococcus and Corynebacterium spp. in 50% (7/14) of stripped milk samples and growth of Staphylococcus spp. in 7% (1/14) of cisternal milk samples. Our study suggests that microbiome data generated from milk samples obtained from clinically healthy bovine udders may be heavily biased by contaminants that enter the sample during the sample collection and processing workflows. ImportanceObtaining a non-contaminated sample of bovine milk is challenging due to the nature of the sampling environment and the route by which milk is typically extracted from the mammary gland. Furthermore, the very low bacterial biomass of bovine milk exacerbates the impacts of contaminant sequences in downstream analyses, which can lead to severe biases. Our finding showed that bovine milk contains very low bacterial biomass, and each contamination event (including sampling procedure and DNA extraction process) introduces bacteria and/or DNA fragments that easily outnumber the native bacterial cells. This finding has important implications for our ability to draw robust conclusions from milk microbiome data, especially if the data have not been subjected to rigorous decontamination procedures. Based on these findings, we strongly urge researchers to include numerous negative controls into their sampling and sample processing workflows; and to utilize several complementary methods for identifying potential contaminants within the resulting sequence data. These measures will improve the accuracy, reliability, reproducibility, and interpretability of milk microbiome data and research.