Antimicrobial activity of free and liposome-encapsulated thymol and carvacrol against Salmonella and Staphylococcus aureus adhered to stainless steel

Abstract

Antimicrobial activity of thymol, carvacrol and thymol/carvacrol liposomes (TCL) was evaluated against two bacterial pools, each one consisting of four strains of Staphylococcus aureus or Salmonella enterica. TCL were prepared using thin-film hydration, showing 270.20nm average diameter (polydispersity index of 0.33) and zeta potential of +39.99mV. Minimum inhibitory concentration (MIC) of thymol, carvacrol and TCL against S. aureus pool was 0.662mg/ml, while MIC for Salmonella pool was 0.331mg/ml for thymol and carvacrol, and for TCL was 0.662mg/ml. Bacterial pools (8.0logCFU/ml), allowed in contact on stainless steel AISI 304 coupons in UHT skim milk for 15min, resulted in adhered populations of 5.6-6.1logCFU/cm². Adhered S. aureus (±6.1logCFU/cm²) were inhibited after 1-min and 10-min treatments using thymol or carvacrol at MIC and 2.0 MIC. Reductions of 1.47-1.76logCFU/cm² and 1.87-2.04logCFU/cm² were obtained using 0.5 MIC of thymol and carvacrol, respectively. A 10-min contact with free (MIC and 2.0 MIC) and encapsulated (MIC) antimicrobials inhibited attached Salmonella (±6.0logCFU/cm²); however, after 1-min of contact, 2.0 MIC of thymol and carvacrol were not able to inactivate adhered Salmonella MIC of TCL inactivated S. aureus and Salmonella after 10min; however, after 1-min contact, adhered S. aureus and Salmonella populations were decreased in 1.62logCFU/cm² and 2.01logCFU/cm², respectively. Considering antimicrobial concentrations and contact times, thymol, carvacrol, and TCL could be employed in food-contact surfaces to prevent biofilm formation at early stages of bacterial attachment. Further investigations should be performed considering long-term antibacterial effects of TCL.

Keywords: Carvacrol; Liposomes; Salmonella; Staphylococcus aureus; Thymol.