Valutazione dell’attività antimicrobica di oli essenziali, composti naturali identici e estratti da piante officinali su batteri di interesse veterinario

Abstract

According to WHO, antibiotic resistance is one of the biggest threats to global health, food security, and development today. Antimicrobial resistance has become a critical problem for infection treatment both in human and veterinary medicine, both in food-producing and affection animals. Finding alternatives to traditional antibiotics molecules is critical to fighting antimicrobial resistance identified by international authorities such as EFSA, WHO, and OIE. This study aimed to test the antimicrobial activity against four bacterial strains (Escherichia coli - EC, Salmonella Typhimurium - ST, Staphylococcus aureus - SA, and Methicillin-Resistant Staphylococcus aureus - MRSA) of compounds suggested as an alternative to antibiotics as Essential Oils - EOs - (Cinnamon, Lavender, Tea tree, Mint, Oregano, Rosemary, Clove bud, Thyme), Nature Identical Compounds - NICs -(Carvacrol, Cinnamaldehyde, Menthol, Terpineol, Thymol) and Plants Extracts – PEs- (Marshmallow, Chamomile, Mallow). EOs and NICs were also tested in combination with Tween 20 and Tween 80 (only for EOs) as emulsifiers. The results showed that for all the tested EOs average Minimal Inhibitory Concentration (MIC) value is lower than 4% and lower than 2% for the most. Overall, MIC was lower in the presence of Tween 20 for all the EOs and tested bacteria. The most susceptible bacterium to EOs was EC; contrariwise, the most resistant was ST. The lowest MIC average among all the tested EOs was found for Oregano oil, followed by Thyme oil, Tea tree oil, and Rosemary oil. The highest MIC average was found for Clove oil and Cinnamon oil, followed by Lavender oil and Mint oil. For what concerns PEs, no antimicrobial activity against the four tested strains was detected. Checkerboard assays between EOs and Tween 20 showed a prevalence of indifference (37,5%) and additivity (34,4%) among all tested EOs and bacteria, compared to antagonism (15,6%) and synergy (12,5%). Interesting to consider is that EOs MICs generally increased after about one year, showing a time-dependent decrease in antimicrobial activity. Among all the tested NIC, the highest MIC was found for Menthol and Terpineol. The lowest MIC was found for Cinnamaldehyde against SA. For what concerns checkerboard assays between NICs and Tween 20, generally the combinations showed a prevalence of indifference, no synergy, and additivity for Terpineol with Tween 20 against EC, ST, and SA and Menthol with Tween 20 for EC. In conclusion, we can hypothesize that the administration of these essential oils, identical natural compounds, and their respective active ingredients may benefit gut microbiota, acting as inhibiting agents against the most common pathogenic bacteria of zootechnical animals. Their combination with Tween 20 could be helpful to lower the doses administered in husbandry.