Mammary probiotics: a new jedi against bovine mastitis
Résumé
Bovine mastitis is an inflammation of the mammary gland and most often results from bacterial infection. Major etiologic agents of mastitis include Staphylocccus aureus, Escherichia coli and Streptococcus uberis. These intramammary infections cause huge economic losses in the dairy farming sector and dairy industry. As of yet, prevention of bovine mastitis is mostly based on vaccination, which proved elusive in field conditions, and by thorough hygienic procedures during milking, such as the application of systemic or local therapies (pre- and post-milking teat dipping). Despite these measures, mastitis is not fully under control. The treatment of bovine mastitis is predominantly based on antibiotics. However, they are not totally effective and this massive use contributes to the risk of antibiotic resistance dissemination, which, via animal-to-human transmission, can further impact both animal and human health. Milk production (as well as the entire farming activities) has to cope with an increasing societal demand for a more sustainable agriculture respectful of the environment (e.g. by reducing inputs and improving animal welfare). This demand prompts the need for alternative strategies that can be used as prophylactic or alternative or complementary curative treatments.
One alternative is the emerging concept of mammary probiotics. For this purpose, lactic acid bacteria (LAB) are good candidates due to their Generally Recognized As Safe (GRAS) status and their recognized technological and inhibitory properties. LAB have been investigated for many years for their beneficial effects on human health. Introducing selected bacterial strains such as LAB has been demonstrated to be efficient in human health to restore the microbiota homeostasis and has led to the development of vaginal and intestinal probiotics. Similarly, the barrier effect of LAB isolated from bovine mammary gland or from exogenous ecosystems against mastitis pathogens is being explored. LAB are able to interfere with the infectious cycle of mastitis pathogen by several means, including a direct inhibition of the pathogen growth, titration of the pathogen by co-aggregation, competition with pathogens for tissue colonization or modulation of the innate immune system. These mechanisms will be reviewed and illustrated by our own results as well as other studies. Among steps of the infectious cycle, tissue colonization deserves special attention. It is suspected to be used by some pathogens such as S. aureus to evade host defenses and persist in the mammary gland. We have notably shown that Lactobacillus casei was able to reduce S. aureus internalization into bovine mammary epithelial cells. This interesting property could help to counteract persistence of S. aureus infections.
The beneficial properties of LAB in mastitis context have been mainly explored in vitro, whereas only few studies have evaluated LAB potential in vivo. Nevertheless, these in vivo studies revealed the lack of adverse effects of LAB on mammary tissues and promising results compared to conventional antibiotic treatment against mastitis.
Altogether, these studies pointed out interesting properties of LAB that opens avenues to the development of alternative strategies to prevent or treat intramammary infections.