Overcoming Multidrug Resistance in Salmonella spp. Isolates Obtained From the Swine Food Chain by Using Essential Oils: An in vitro Study

Abstract

Antimicrobial resistance (AMR) is a global concern, and new approaches are needed to circumvent animal and food-borne resistant pathogens. Among the new strategies, the combination of antibiotics with natural compounds such as essential oils (EOs) could be an alternative to challenge bacterial resistance. The present study evaluates the phenotypic and genotypic antibiotic resistance of 36 Salmonella enterica (16 S. Typhimurium, 3 monophasic variant S. Typhimurium, 8 S. Enteritidis, 6 S. Rissen, 1 S. Typhi, and 2 S. Derby) strains, isolated from the swine production chain. The isolates displayed phenotypic resistance to gentamicin, amikacin, tobramycin, and tetracycline, while the resistance genes most commonly detected were parC, catA, nfsB, nfsA, blaTEM, tetA, and tetB. Then 31/36 Salmonella isolates were chosen to evaluate resistance to tetracycline and Thymus vulgaris, Eugenia caryophyllata, and Corydothymus capitatus EOs by determining minimum inhibitory concentrations (MICs). Finally, the synergistic effect between tetracycline and each EOs was evaluated by the checkerboard method, calculating the fractional inhibitory concentration (FIC) index. Among the EOs, C. capitatus displayed the best bioactivity in terms of MICs, with the lowest values (0.31 and 0.625 μl/ml). On the contrary, the strains showed the ability to grow in the presence of the maximum concentration of tetracycline employed (256 μg/ml). While not displaying a real synergism according to the FIC index, the combination of tetracycline compounds and the three EOs resulted in a significant reduction in the MIC values to tetracycline (4 μg/ml), suggesting a restoration of the susceptibility to the antibiotic in Salmonella spp.

Prevalence, Antibiotic-Resistance, and Replicon-Typing of Salmonella Strains among Serovars Mainly Isolated from Food Chain in Marche Region, Italy

“…The fact that only 49% of strains resistant to tetracycline were positive for at least one tet gene is not surprising because this resistance could be due to different genes or other mechanisms such as mutations within the ribosomal binding site, activity of efflux pumps or enzymatic inactivation of tetracycline drugs [ 42 ]. In agreement with other authors, the genes most frequently detected in the present work were tet A and tet B belonging to Group-I and associated with an efflux pump mechanism [ 43 , 44 , 45 , 46 ]. On the contrary, all strains of our collection were negative for tet C and tet D. This result is not uncommon and could be due to the low ability of those genes to confer resistance to tetracycline [ 46 ], leading to their infrequent detection.…”

Unraveling the Antimicrobial Effectiveness of Coridothymus capitatus Hydrolate against Listeria monocytogenes in Environmental Conditions Encountered in Foods: An In Vitro Study

“…Carvacrol was the principal component (98.9%) revealed in the applied hydrolate ( Table 1 ). This molecule is characterized by the presence of a hydroxyl group and a system of delocalized electrons [ 7 , 27 ]. These chemical structures allow carvacrol to act as a proton exchanger, which is able to reduce the transmembrane gradient.…”

“…These chemical structures allow carvacrol to act as a proton exchanger, which is able to reduce the transmembrane gradient. The consequence is the collapse of the proton motive force and the depletion of the ATP pool, which can lead to cell death [ 7 , 27 ]. In our case, exposure for 1 h at 37 °C at CHY concentrations between 250 and 400 µL mL −1 was not sufficient to inactivate the cells, but it probably disturbed the transmembrane gradient and hampered the energy production, thus leaving the cells with lower amounts of ATP, which required instead a response to stressing conditions.…”

“…EOs, especially those rich in monoterpenes, demonstrate high antimicrobial efficacy and antioxidant features [ 5 ]. Among these, Coridothymus capitatus EO exerts strong antimicrobial effectiveness due to a high concentration of carvacrol [ 6 , 7 ]. Unfortunately, some difficulties in the use of EOs have been reported, mainly due to their low water solubility, high volatility, strong odor and flavor [ 8 ].…”

“…Hydrolates (also called hydrosols) can represent a “mild” and effective alternative to EOs. Hydrolates are basically composed by the condensing water in the EO distillation process, in which small amounts of volatile oil components, usually in concentrations lower than 1 g/L, are dispersed [ 7 ]. Their hydrophilic nature, together with their gentle sensory profile, makes HYs much more versatile than EOs for some applications [ 5 , 9 ].…”