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

•Planktonic and attached bacteria showed different susceptibilities to thymol and carvacrol.•Thymol/carvacrol liposome showed a slower release of encapsulated compound.•TCL inhibited attached S. aureus and Salmonella pool after 10 min contact.•Free carvacrol and thymol reduced attached S. aureus and Salmonella pool after 1 min.•The antimicrobials reduced the adhered bacteria even in the presence of milk residues.

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 270.20 nm average diameter (polydispersity index of 0.33) and zeta potential of + 39.99 mV. Minimum inhibitory concentration (MIC) of thymol, carvacrol and TCL against S. aureus pool was 0.662 mg/ml, while MIC for Salmonella pool was 0.331 mg/ml for thymol and carvacrol, and for TCL was 0.662 mg/ml. Bacterial pools (8.0 log CFU/ml), allowed in contact on stainless steel AISI 304 coupons in UHT skim milk for 15 min, resulted in adhered populations of 5.6-6.1 log CFU/cm2. Adhered S. aureus (± 6.1 log CFU/cm2) were inhibited after 1-min and 10-min treatments using thymol or carvacrol at MIC and 2.0 MIC. Reductions of 1.47-1.76 log CFU/cm2 and 1.87-2.04 log CFU/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 (± 6.0 log CFU/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 10 min; however, after 1-min contact, adhered S. aureus and Salmonella populations were decreased in 1.62 log CFU/cm2 and 2.01 log CFU/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.