Modelling the growth and death of Staphylococcus aureus against Melaleuca armillaris essential oil at different pH conditions

BULDAIN, DANIEL; GORTARI CASTILLO, LIHUEL; MARCHETTI, MARÍA LAURA; JULCA LOZANO, KAREN; BANDONI, ARNALDO; MESTORINO, NORA

Conferencia; Tenth International Conference on Antimicrobial Agents in Veterinary Medicine (AAVM); 2020

Bacterial resistance is a critical problem for both human and animal health and it is overriding finding alternatives to antibiotics. Essential oils are plant extracts with recognized antimicrobial activity, and the interest of study and application of these in veterinary medicine is being increase particularly for organic dairy producers. Mastitis is an infectious disease of the udder usually treated by antibiotics. We evaluated the essential oil of Melaleuca armillaris (EO) obtained in Buenos Aires (Argentina) against S. aureus isolated from dairy cows. The EO was characterized physicochemically and the antimicrobial activity was analysed by establishing the Minimum Inhibitory and Bactericidal Concentrations (MIC and MBC respectively), and the mathematical modelling of growth and death data after expose seven S. aureus strains to different concentrations of EO. Experiments were carried out at pH 7.4, 6.5 and 5.0. The main component of the EO was 1.8cineol as was observed in different places of the world. Respect to the antimicrobial activity, the MIC was between 25-12.5 L/mL at pH 7.4 and decreased slightly when the media was acidified from pH 7.4 to 6.5 and 5.0. The relation MBC/MIC was between 1-4, so the EO can be consider bactericidal. Applying a sigmoid model (similar to maximum response model of Hill) to data obtained in the time kill assay we observed that acidifying the media, the time and EO concentrations necessaries to reach the 50% of the maximum bacterial drop were lower. The analysis of bacterial growth in absence of EO (control) and subinhibitory concentrations (0.5 MIC) of this by the Gompertz model showed that the growth rate and the maximum population density were lower and the latency period was longer because of the presence of EO. The antimicrobial activity analysis in terms of pH is important for pathologies as mastitis, where the milk pH is higher than normal milk. Moreover S. aureus can internalize in phagolysosomes of host cells were pH is near to 5 and alternatives antimicrobials must be effective at this level. The M. armillaris EO had an interesting antimicrobial activity against S. aureus which is improved by acidity conditions.