Non-aureus Staphylococci and related Mammaliicoccal species and their association with udder health in organic dairy cows

Abstract

Restrictions placed on antimicrobial use lead to important challenges for dairy farms to maintain udder health, which are especially challenging for organic dairy farmers. This dissertation aims to provide additional understanding of the epidemiology of mastitis in organic dairy cows and the potential of Non-aureus Staphylococci and related Mammaliicoccal species (NASM) to control mastitis on dairy farms. Multiple investigations were performed to: explore the intramammary infections (IMI) dynamics on organic dairies, investigate the relationship between IMI, udder health, and milk production and investigate the antimicrobial activity of NASM against mastitis pathogens. In the first and second chapter, an overview of mastitis significance and the main mastitis control strategies was provided. In these chapters, NASM epidemiology and potential probiotic properties of these microorganisms was described. The third chapter provides a description of IMI prevalence and persistence in first-lactation cows across 5 different organic dairy farms and showed a high prevalence at calving and persistence of IMI by Staphylococcus aureus and Streptococcus spp., two major mastitis pathogens. In the fourth chapter, we investigated the association between the presence and persistence of IMI in the first 35 DIM, udder health, and milk production in the first 180 days in milk (DIM). Our findings revealed that IMI caused by Staphylococcus aureus and Streptococcus spp., led to an increased SCC and, in the case of Streptococcus spp., a decreased in milk production. Our findings also suggested an association between IMI persistence and the number of times that a cow had high SCC throughout the first 180 DIM. The fifth chapter investigated the relationship between NASM in vitro inhibitory activity and the presence of IMI, encountering that the presence of high in vitro inhibitory activity was associated with a lower risk of IMI. The last experimental chapter (sixth chapter) had the main objective of investigating the genome of NASM isolates with a focus on their phylogeny, the presence of genes related to the production of antimicrobial peptides (AMPs), and in vitro antimicrobial activity. We encountered a high prevalence of genes related to the production of AMPs. However, their presence was not associated with the in vitro antimicrobial activity of NASM isolates. NASM in vitro antibacterial activity was not related to clade membership, apart from isolates classified as Staphylococcus succinus. This Ph.D. dissertation provides substantial knowledge about udder health on organic dairy farms and initial steps into the potential utilization of NASM to control mastitis in dairy farms. Nonetheless, further knowledge is needed to investigate the mechanisms leading to the antimicrobial activity of NASM against mastitis pathogens. Future studies should focus on identifying the mechanisms behind the antimicrobial properties of NASM and investigate if the inoculation of NASM isolates with high antimicrobial activity and/or their AMPs leads to a reduced risk of IMI and mastitis.