The natural antimicrobial carvacrol and thymol disrupt desiccation resistance in Salmonella enterica serovar Tennessee: Advancing safety of low-moisture foods

Research poster
Ahmed Abdelhamid
Post Doctoral
Ahmed Yousef
Department of Food Science and Technology

Introduction: Salmonella enterica can survive effectively in low-moisture (LM) foods. Consequently, large multi-state salmonellosis outbreaks have been attributed to consumption of foods such as peanut butter, almonds and others. S. enterica employs desiccation resistance mechanisms to survive LM conditions by increasing concentration of intracellular ions (e.g. K+) and compatible solutes (e.g. trehalose). 
Purpose: To overcome desiccation resistance mechanisms in S. enterica serovar Tennessee, a strain isolated from peanut butter, using two membrane-active food ingredients, namely carvacrol and thymol. 

Methods:  Ten food ingredients were screened for potassium leakage activity before carvacrol and thymol were selected. Leakage of potassium ions and concentration of intracellular trehalose in Salmonella Tennessee, exposed to 0-800 µg/mL carvacrol or thymol, was measured using fluorescence of a potassium-sensitive probe, and trehalase assay, respectively.  Luminescence ATP detection assay (Luciferin/Luciferase) estimated total ATP content in treated and untreated Salmonella Tennessee. Transcriptional analysis of desiccation-related genes was conducted using RT-qPCR.

Results: Carvacrol and thymol at ≥ 100 µg/mL caused leakage of cellular k+, in a concentration-dependent manner (P < 0.05), from cells of Salmonella Tennessee compared to cells of untreated control. Carvacrol or thymol at MIC sensitized Salmonella Tennessee (1.5 log CFU reduction) to desiccation stress. Carvacrol and thymol at 100-400 and 50-200 µg/mL, respectively, inhibited trehalose biosynthesis, and decreased ATP content (12-59%) in Salmonella Tennessee, compared to the untreated control. Transcriptional analysis revealed that carvacrol at 100-200 µg/mL down-regulated (i.e., > 2-fold decrease) some of the desiccation-related genes.

Significance: Carvacrol and thymol would be valuable food ingredients to combat desiccation-resistant Salmonella and enhance the safety of low-moisture foods.