Study Shows Potential of Thymol for Controlling Salmonella Biofilms on Food-Contact Surfaces
Need to Know
- Although the antimicrobial and antibiofilm activity of thymol, a plant-derived antimicrobial compound, is well-established, there is limited research on the effects of thymol on Salmonella Infantis biofilms
- USDA-ARS and University of Arkansas, Fayetteville researchers investigated thymol’s ability to inhibit and inactivate S. Infantis biofilms on polystyrene and stainless steel surfaces, as well as its effects on genes and proteins involved in biofilm formation
- Thymol showed inhibitory effects against S. Infantis at sub-inhibitory concentrations of 0.015 percent and 0.03 percent, with reductions up to 3.5 log CFU/mL depending on concentration and sampling time
- On both surface types, treatment with 0.5 percent thymol reduced S. Infantis in mature biofilms to below detectable levels after ten minutes or less at both 20 °C and 37 °C; even the lowest concentration tested against established biofilms showed significant reductions
- The researchers concluded that thymol could have potential as a natural disinfectant for controlling S. Infantis biofilms on plastic and stainless steel surfaces in poultry and food processing facilities.
A new study led by researchers at the U.S. Department of Agriculture’s Agricultural Research Service (USDA-ARS) and the University of Arkansas, Fayetteville has demonstrated the potential of thymol, a plant-derived antimicrobial compound, to inhibit and inactivate Salmonella Infantis biofilms on polystyrene and stainless steel surfaces commonly found in poultry and food processing environments.
Published in Poultry Science, the study investigated thymol’s efficacy against S. Infantis biofilms, as well as its effects on genes and proteins involved in biofilm formation. The poultry-associated pathogen is known to persist in processing environments partly because of its ability to form biofilms that are resistant to cleaning and disinfection. Although the antimicrobial and antibiofilm activity of thymol is well-established, there is limited research on the effects of thymol on S. Infantis biofilms.
Thymol Inhibited and Inactivated S. Infantis Biofilms
Researchers formed biofilms comprising S. Infantis isolated from ground chicken on polystyrene plates and stainless steel coupons. Biofilm formation was evaluated at 20 °C and 37 °C. The researchers found greater biofilm formation at 20 °C than at 37 °C, although the effect of temperature was significant only on polystyrene.
To investigate biofilm inhibition, the researchers exposed S. Infantis to sub-inhibitory thymol concentrations of 0.015 percent and 0.03 percent. At 20 °C, 0.03 percent thymol reduced S. Infantis counts by approximately 3–3.5 log colony forming units per milliliter (CFU/mL) on polystyrene. Reductions of approximately 0.5–3.5 log CFU/mL were observed on stainless steel with the two thymol concentrations, with results dependent on concentration and sampling time.
The researchers also tested thymol concentrations of 0.125, 0.25, and 0.5 percent against mature 48-hour S. Infantis biofilms. Increasing thymol concentration and exposure time generally increased biofilm inactivation.
On polystyrene, treatment with 0.5 percent thymol reduced S. Infantis to below detectable levels after five or ten minutes at both temperatures. On stainless steel, 0.5 percent thymol reduced S. Infantis to below detection limits after ten minutes at both temperatures. Overall, the highest thymol treatment achieved approximately 5–7 log CFU/mL reductions in mature biofilms.
Even the lowest concentration tested against established biofilms showed significant activity. Treatment with 0.125 percent thymol achieved approximately 4–5 log CFU/mL reductions after one minute on polystyrene, and approximately 2–3.5 log CFU/mL reductions after five minutes on stainless steel.
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Fluorescence microscopy also showed increasing bacterial cell membrane damage as thymol concentrations increased.
Thymol Affected Biofilm-Related Gene and Protein Expression
The researchers also investigated how thymol affected the expression of genes important to S. Infantis biofilm formation. Treatment with 0.015 percent thymol significantly downregulated genes associated with extracellular polymeric substance (the structural and protective matrix encompassing a biofilm) by approximately twofold. Certain quorum-sensing genes (which help facilitate communication among bacteria within a biofilm) were also significantly downregulated.
Proteomic analysis identified 745 proteins in S. Infantis. Thymol-treated biofilms showed downregulation of proteins involved in bacterial chemotaxis and motility. Proteins involved in bacterial survival, cell wall maintenance, cellular repair, efflux pump transport, and adaptation to environmental stresses were upregulated, which the researchers associated with bacterial responses to thymol-induced stress.
Based on the findings, the researchers concluded that thymol could have potential as a natural disinfectant for controlling S. Infantis biofilms on plastic and stainless steel surfaces in poultry and food processing facilities.
However, the study did not evaluate thymol against S. Infantis on chicken carcasses or other food products. The researchers called for additional studies investigating these applications, as well as thymol's effects on mixed biofilms of Salmonella serovars prevalent in poultry environments.
