Antimicrobial Activities of Commercial Essential Oils Against the Bovine Respiratory Pathogen Mannheimia Haemolytica, and Analysis of Their Chemical Composition and Cytotoxicity on Bovine Turbinate Cells

Recent studies have shown the emergence of bacterial pathogens associated with bovine respiratory disease (BRD) that are resistant to all classes of antibiotics used to treat BRD. Therefore, there is an impetus to develop alternatives to antibiotics for use in the beef industry. Essential oils (EO) may offer a potential mitigation strategy against BRD bacterial pathogens through direct inhibition and also attenuation of resistance to antibiotics. The purpose of this study was to determine the chemical composition of commercial EOs (N = 16), and their in vitro antimicrobial activities against the model BRD pathogen Mannheimia haemolytica, as well as any cytotoxicity in bovine turbinate (BT) upper respiratory cells. The EO chemical composition was determined using Gas Chromatography-Mass Spectrometry. Minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) of the 16 EOs against M. haemolytica serotype 1 were determined using the broth dilution method with serial dilutions ranging from 0.2% to 0.006% (v/v). Based on MIC and MBC, the most inhibitory EOs (N = 7; ajowan, cinnamon leaf, citronella, fennel, ginger grass, lavender, and thyme) were further characterized by evaluating their vapor-phase antimicrobial activities against M. haemolytica, impact on M. haemolytica morphology, and cytotoxicity against BT cells at concentrations between 0.0125% and 0.4% (v/v). A variety of volatile compounds were detected from the EOs, with some containing thymol and eugenol which are known to have antibacterial activities. Thymol was most abundant in ajowan and thyme EOs (> 46% of EO content), and eugonal was most predominant in cinnamon leaf EO (84% of EO content). The MIC ranged between 0.025% and 0.2 %. The lowest MIC (0.025%) was observed with ajowan, fennel and thyme EOs. Cinnamon leaf and citronella EOs showed MIC value of 0.05%. All MIC values were the same as MBC for all EOs. Ajowan, thyme and cinnamon leaf EOs completely or moderately inhibited M. haemolytica in their vapor-phase. Scanning electron microscopy revealed noticeable changes in cell structure between EO treated and non-treated cells. The cells treated with ajowan and thyme EOs displayed the greatest morphological cell damage compared to other EOs. None of the selected EOs exhibited noticeable cytotoxicity on BT cells within the tested concentrations. Our results indicate that ajowan, thyme and cinnamon leaf EOs may have potential to be used as antibiotic alternatives to control BRD bacterial pathogens.