Short communication: Interaction of the isomers carvacrol and thymol with the antibiotics doxycycline and tilmicosin: In vitro effects against pathogenic bacteria commonly found in the respiratory tract of calves

Abstract

Bovine respiratory disease is the major problem faced by cattle, specially calves, leading to reduced animal performance and increased mortality, consequently causing important economic losses. Hence, calves must be submitted to antibiotic therapy to counteract this infection usually initiated by the combination of environmental stress factors and viral infection, altering the animal's defense mechanism, and thus allowing lung colonization by the opportunistic bacteria Mannheimia haemolytica and Pasteurella multocida. Essential oils appear to be candidates to replace antibiotics or to act as antibiotic adjuvants due to their antimicrobial properties. In the present study, we aimed to evaluate the 4 essential oil components carvacrol, thymol, trans-anethole, and 1,8 cineole as antibacterial agents or as adjuvants for the antibiotics doxycycline and tilmicosin against M. haemolytica and P. multocida. Bacteria were cultured according to standard protocols, followed by the determination of minimum inhibitory concentration (MIC) and minimum bactericidal concentration. A checkerboard assay was applied to detect possible interactions between components, between antibiotics, and between components and antibiotics. Doxycycline at 0.25 and 0.125 μg/mL inhibited the growth of P. multocida and M. haemolytica, respectively, whereas tilmicosin MIC values were 1.0 and 4.0 μg/mL for P. multocida and M. haemolytica, respectively. Carvacrol MIC values were 2.5 and 1.25 mM for P. multocida and M. haemolytica, respectively, whereas thymol MIC values were 1.25 and 0.625 mM for P. multocida and M. haemolytica, respectively. Trans-anethole and 1,8 cineole did not present any antibacterial effect even at 40 mM against the investigated pathogens. All minimum bactericidal concentration values were the same as MIC, except when thymol was tested against M. haemolytica, being twice the MIC data (i.e., 1.25 mM thymol). Based on fractional inhibitory concentration checkerboard assay, no interaction was observed between doxycycline and tilmicosin. Carvacrol and thymol presented an additive effect when one of them was combined with tilmicosin. Additive effect was also observed when doxycycline was combined with thymol. Synergism was observed when carvacrol was combined with doxycycline or with thymol. Although the antibacterial effects of the tested essential oil components were observed at high concentrations for in vitro conditions, the additive and synergic effects of carvacrol and thymol with antibiotics suggest the option to apply them as antibiotic adjuvants.

Essential oils inhibit the bovine respiratory pathogensMannheimia haemolytica,Pasteurella multocidaandHistophilus somniand have limited effects on commensal bacteria and turbinate cellsin vitro

“…This compound has previously been reported to confer antimicrobial activity against bovine respiratory pathogens (Kissels et al . ), and likely resulted in the inhibition of M. haemolytica in our study. Similar to ajowan and thyme, major compounds detected in fennel (anethole), cinnamon leaf (eugenol) and citronella (linalool and geraniol) EOs have been shown to inhibit either Gram‐negative or ‐positive bacteria previously (Lopez et al .…”

Biologically active compounds of plant origin in medicine

“…Animal studies have shown improvement of mucociliary clearance [Wienkötter et al 2007], anti-inflammatory properties [Vigo et al 2004], and antibacterial activity [Stojković et al 2013] in the respiratory system. As shown by Kissels et al [2017], both thymol and carvacrol have an additive effect in combination with tilmicosin against microorganisms isolated from the respiratory tract of calves. A similar effect was observed after administration of a combination of thymol with doxycycline.…”

Effects of mixed essential oils from eucalyptus, thyme and anise on composition, coagulation properties and antioxidant capacity of the milk of dairy cows

“…The major component in the anise EO is trans-anethole (Orav et al, 2008). In an in vitro experiment by Kissels et al (2017), trans-anethole and 1,8-cineole did not show any antibacterial effects whereas a synergistic effect was observed for carvacrol and thymol. Due to the synergistic and additive effects of EO compounds, the overall composition of different EO and a combination thereof may cause complex effects on rumen fermentation and animal metabolism, which makes it difficult to compare the results of studies using mixtures of different EO.…”