Rifaximin and melaleuca armillaris essential oil combinations against Staphylococcus aureus.

BULDAIN, DANIEL; BUCHAMER, ANDREA; MARCHETTI, MARÍA LAURA; ALIVERTI, FLORENCIA; MESTORINO, NORA

Roma

Conferencia; Ninth International Conference on Antimicrobial Agents In Veterinary Medicine (AAVM); 2018

Rifaximin (RIF) is an antibiotic with high activity against Staphylococcus spp. However, it isnecessary a prudent use to avoid resistance selection. Another antimicrobial resource choice isrepresented by the essential oils (EO). There is evidence that Melaleuca armillaris Sm. EO hasantimicrobial activity. Our objective was to evaluate the existence of a possible synergistic effectbetween the EO and RIF at different pH conditions. EO was obtained by steam distillation of leavesand herbaceous branches. Susceptible S. aureus strains wild type (n=3) isolated from Holstein cowsand S. aureus ATCC 29213 were used. The antimicrobial activity of RIF, EO and RIF/EO combinationwas evaluated by microdilution in broth using the checkerboard method. The concentration rangewas 256-0.007 μg/mL for RIF and 50-0.1 μL/mL for EO. The existence or not of synergism betweenboth was determined by the FIC index (Fractional Inhibitory Concentration). Time kill assays wereused to evaluate the antibacterial activity index E, defined as the difference between the Log10values of the number of viable bacteria (CFU/mL) at initial time (nt-0) and at the end of the test (nt-24) according to: E= nt-24h - nt-0 (E= 0 bacteriostatic effect, E= -3 bactericidal effect, E= -4 virtualeradication effect). The concentrations evaluated corresponded to 1MIC (mixture with lower FIC),0.5MIC, 2MIC, 4MIC and 8MIC maintaining the proportion of both compounds (dilution orconcentration depending on the case). RIF had high potency (0.032 μg/mL) against S. aureusindependently of the pH conditions. The MIC of EO was 25 μL/mL for ATCC 29213 and 12.5 mL/mL forwild types at pH 7.4. Both values decreased two-fold at pH 5.Combining RIF with EO, we found asynergic effect where the antibiotic was potentiated, particularly at pH 5 (MIC decreased 8-fold with1.6 μL/mL of EO). A mix of 0.004 μg/mL of RIF and 12.5 μL/mL of EO allows reaching a virtualeradication effect (E= -4.0) against wild types strains at pH 7.4. Something similar occurs forreference strain. The acidification of media improves the EO/RIF activity. We can conclude thatrifaximin is a potent antibiotic against S. aureus and does not require the use of adjuvants. However,these results are important findings for the treatment of staphylococcal infections of difficultresolution, because with the emergence of antimicrobial resistance of S. aureus to conventionalantibiotics the treatment options for infections have become limited.