Several European animal nutrition companies have incorporated essential oils (EOs) into animal feed as a result of the prohibition of antibiotics to promote animal growth. Previous studies of EOs have highlighted the absence of bacterial resistance for these substances, although most of the published works focus on studying their tolerance to subinhibitory doses. For this study, oregano essential oil (OEO) was chosen for its proven inhibitory and bactericidal activity. This study is an in vitro assay of the possible induction of Salmonella enterica serovar Typhimurium strains with reduced susceptibility to OEO by mutation, seeking to calculate the mutant prevention concentration (MPC) since this is an important measurement for the control Salmonella's resistance to fluoroquinolones such as enrofloxacin (ENR), the treatment of choice for this infection. To establish the MPC, we used a bacterial inoculum ≥109 colony-forming unit (CFU)/mL and examined the bases for points of resistance to ENR and mutations of target genes of the quinolone resistance determining region (QRDR). The three strains of Salmonella Typhimurium used in this study showed an MPC of four times the minimum inhibitory concentration (MIC) for ENR. In all cases, strains with reduced susceptibility to ENR were obtained, although none reached the point of resistance. The QRDR characterization region was in all cases of wild type (wt). Two of the strains tested with OEO grew at a concentration of 1 × MIC, which could be strains with reduced susceptibility, associated with mutation or not. In this case, the MPC was 2 × MIC. Once isolated and identified as Salmonella Typhimurium, the MIC against OEO of all strains obtained in the induction test indicated a possible reduction in susceptibility. However, the result obtained for both strains coincided with MIC of the original strains, rejecting a priori such a reduced susceptibility of Salmonella Typhimurium to OEO.
Keywords: Salmonella Typhimurium; enrofloxacin; oregano essential oil; resistant mutants.