Evaluation of the antibacterial, anti-biofilm, and anti-quorum sensing activities of plant extracts and essential oils against bovine mastitis causing pathogens

Bovine mastitis is the most common disease that leads to economic loss and is a major concern for the dairy cattle community worldwide. The most common mastitis-causing organisms are bacteria, that can be classified into contagious and environmental. Some common contagious pathogens are Staphylococcus aureus, Streptococcus dysgalactiae and Streptococcus agalactiae, while the environmental pathogens include mainly Streptococcus spp. and coliforms species. The development of antibiotic-resistant strains of pathogens has become a critical challenge in antibiotic treatment, thus, it is necessary to look for alternatives to antibiotic therapy, particularly those based on natural products such as plants. The present research aimed at evaluating the opportunity to use plant extracts and essential oils against bovine mastitis causing pathogens as a potential alternative or supplement to conventional antibiotics. The study included two in vitro projects. The first project aimed at assessing of the antibacterial and anti-quorum sensing activities of different extracts from six South African plants, as well as their cytotoxicity. Plant extracts were evaluated against ATCC strains of Staphylococcus aureus and Staphylococcus epidermidis, and two clinical isolates of S. aureus. The antibacterial investigations suggest the potential usefulness of the extracts of Vahcellia karroo and Terminalia sericea as antibacterial agents with good activity against staphylococci causing bovine mastitis and with anti-quorum sensing properties. The second project investigated the bactericidal activity of 11 essential oils and 2 blends against 5 strains responsible of bovine mastitis: S. agalactiae, S. dysgalactiae, S. uberis, S. aureus, and S. epidermidis. The assessment of both the reduction of biofilm formation and the inhibition of pre-formed biofilm, was carried out against biofilm-formative strains of S. aureus and S. epidermitis. Origanum vulgare and Cinnamomum zeylanicum resulted to be the most active essential oils against the above mentioned bacteria, inhibiting both the bacterial growth and the formation of biofilm.