Antibiofilm activity of thymol against Salmonella Infantis on polystyrene and stainless steel surfaces.

Salmonella enterica serovar Infantis (S. Infantis) is one of many Salmonella serovars that have gained attention in the past two decades as a leading cause of foodborne illness globally. This serovar is well known for forming biofilms that are resistant to standard cleaning and disinfection protocols used in processing plants. Thymol, a plant-derived monoterpene phenol, has demonstrated antimicrobial activity against many foodborne pathogens, including Salmonella. This study investigated the efficacy of thymol in inhibiting S. Infantis biofilm formation and inactivating established biofilms on common food industry surfaces. A biofilm inhibition assay was performed by allowing S. Infantis to grow and form biofilms at 37°C and 20°C on polystyrene and stainless steel surfaces in the presence or absence of sub-inhibitory concentrations of thymol. The inactivation study examined the effects of thymol at various concentrations (0.125 %, 0.25 %, or 0.5 %) on 48 h old biofilms developed on both surfaces at 37°C and 20°C for 1, 5, or 10 min. The surviving S. Infantis in the biofilms were enumerated by serial dilution and plating. Biofilms were inactivated within 10 min to below detection limits (∼6-7 log reduction) on both surfaces by 0.5 % thymol, whereas a 0.125 % thymol dose reduced counts significantly (3-4 logs) after 1 min on polystyrene and 5 min on stainless steel. Also, treatment with sub-inhibitory concentration of thymol downregulated the expression of S. Infantis genes (P < 0.05) associated with biofilm formation (csgA, csgB, csgD, adrA, crl) and quorum sensing (luxS, sdiA). LC-MS/MS-based proteomic analysis showed that thymol downregulated the expression of proteins associated with bacterial chemotaxis and motility (CheY and ProQ), and upregulated proteins involved in functions such as survival, cell wall maintenance, cellular repair, and adaptation to environmental stresses (P < 0.05). The results suggest that thymol could be used as a natural disinfectant to control S. Infantis biofilms on plastic and steel surfaces commonly found in poultry and food processing facilities.