Swine pathogens cause significant economic losses for Canadian animal producers and there is an urgent need to identify novel antibacterial agents to combat infections. To identify new antimicrobials, researchers from Université Laval tested the efficacy of nine essential oils against six major swine respiratory bacterial pathogens: Streptococcus suis, Actinobacillus pleuropneumoniae, Actinobacillus suis, Bordetella bronchiseptica, Haemophilus parasuis, and Pasteurella multocida.
Essential oils with bacteriostatic and bactericidal effects. Cinnamon, thyme, and winter savory had the strongest antibacterial effects against the bacterial species tested. Specifically, these essential oils inhibited bacterial growth (bacteriostatic effect) and when their concentrations were doubled the bacteria were killed (bactericidal effect).
Effect on biofilms. Some of the bacterial species tested have the ability to grow as biofilms, which are multicellular bacterial communities that stick to surfaces and cover themselves with a protective gel. This gel protects bacteria growing in biofilms from antimicrobials, making the bacteria more resistant to antimicrobial treatments. Interestingly when applied at bactericidal concentrations, cinnamon, thyme, and winter savory essential oils were able to penetrate the protective layer of biofilm gel surrounding Streptococcus suis and Actinobacillus pleuropneumoniae to kill some bacteria without causing the biofilm to come off the surface.
In addition to essential oils, the researchers tested the antimicrobial effects of other products such as bacteriocins (toxins produced by bacteria that kill closely related species). Nisin is a bacteriocin that kills Streptococcus suis by destroying its membrane. This bacteriocin is already used commercially as a preservative for thermally processed food, including dairy products. When nisin was used in combination with thyme or winter savory essential oils, the researchers observed a highly synergistic effect against Streptococcus suis strains (synergistic effect: to have a greater antimicrobial effect than the sum of the antimicrobial effect of each product).
Swine safety. Finally, researchers tested whether these products cause damage to porcine respiratory cells by treating porcine tracheal epithelial cells with the three essential oils alone and in combination with nisin. When used at minimum bacteriostatic and bactericidal concentrations, the porcine respiratory cells were not damaged.
These findings are encouraging for work focused on developing therapeutics for swine medicine. For treatments to be both effective and economical, the administration method (Spray, patch or ointment provided as a therapy or as a prophylaxis) must be carefully selected. One must consider current aerosol treatment used, where some antibacterial applications require 15-20 hours of vapour diffusion in a small space, whereas some antiviral treatments only need a 15-minute application.
Therefore, the next steps will be to check the effectiveness of these treatments in more realistic conditions, such as testing the treatments during experimental animal infections.
Sources: Antibacterial activity against porcine respiratory bacterial pathogens and in vitro biocompatibility of essential oils. Geneviève LeBel, Katy Vaillancourt, Philippe Bercier, Daniel Grenier. Archives of Microbiology (2019) 201:833–840.
Essential oils in the treatment of respiratory tract diseases highlighting their role in bacterial infections and their anti-inflammatory action: a review. Horvath G, Acs K. Flavour Fragr J (2015) 30:331–341.