Interaction of essential oil isomers carvacrol and thymol with the antibiotics doxycycline and tilmicosin against pathogenic respiratory tract bacteria.

Essential oils are candidates for adjuvant or replacement for antibiotics due to their antimicrobial properties¹. This study evaluates 4 essential oil components as antibacterial agents or adjuvants for antibiotics against major respiratory pathogens. Respiratory disease is a problem in calves leading to reduced animal welfare and economic losses. Often antibiotic therapy is necessary. Bacterial resistance against antibiotics is an increasing problem. The current policy focusses on resistance prevention and reduction of antibiotic usage. Combined treatment of antibiotics with essential oils might offer a solution.Objective was to evaluate 4 essential oil components carvacrol, thymol, trans-anethole, and 1,8 cineole as antibacterial agents or adjuvants for doxycycline and tilmicosin against M. haemolytica and P. multocida. Bacteria were cultured according to standard protocols and determined for minimum inhibitory concentration (MIC) and minimum bactericidal concentration. A checkerboard assay was applied to detect interactions between components, between antibiotics, and between components and antibiotics. The fractional inhibitory concentrations (FIC) were calculated.

Tab. 2: Fractional inhibitory concentration (FIC) values of the components against Pasteurella multocida and Mannheimia haemolytica

Combination	Bacteria strains	FIC	FIC index	Remark
Dox1 + Til2	P. multocida	0.25/1.0	1.25	Indifferent
	M. haemolytica	0.5/0.5	1.0	Indifferent
Dox + Cvc3	P. multocida	0.125/0.125	0.25	Synergy
	M. haemolytica	0.0625/0.0625	0.125	Synergy
Dox + Thy4	P. multocida	0.5/0.0625	0.5625	Additive
	M. haemolytica	0.5/0.0625	0.5625	Additive
Til + Cvc	P. multocida	0.5/0.0625	0.5625	Additive
	M. haemolytica	0.0625/0.5	0.5625	Additive
Til + Thy	P. multocida	0.0625/0.5	0.5625	Additive
	M. haemolytica	0.5/0.25	0.75	Additive
Cvc + Thy	P. multocida	0.25/0.125	0.375	Synergy
	M. haemolytica	0.25/0.125	0.375	Synergy
1Dox: doxycycline. 2Til: tilmicosin. 3Cvc: carvacrol. 4Thy: thymol.

Only carvacrol and thymol were selected for checkerboard assay because no antibacterial effect of trans-anethole or 1,8-cineole was observed. MIC values for P. multocida and M. haemolytica, were 2.5 and 1.25 mM for Carvacrol and 1.25 and 0.625 mM for thymol respectively. Carvacrol and thymol presented additive effect when each was combined with tilmicosin. Additive effect for thymol was observed when combined with doxycycline. Synergism was observed when carvacrol was combined with doxycycline or with thymol. The study shows additive and synergistic effect of carvacrol and thymol when combined with each other, or with doxycycline or tilmicosin against P. multocida and M. haemolytica. These findings indicate both components as potential adjuvants during antibiotic therapy. Economic justification for selection of essential oil components can be subject for further studies.

[1] Kissels, Wu, Santos.Journal of Dairy Science, 2017