Emergence of Heat-resistant Salmonella and Escherichia coli Isolated from Food Products

Introduction: Foodborne illnesses of microbial origin continue to be a major public health threat. Epidemiology of foodborne diseases is rapidly evolving as newly recognized pathogens increase in prevalence or become coupled with new food products. Of interest, Salmonella and Escherichia coli which are associated with the consumption of raw or undercooked meat along with other food vehicles. There is a growing concern with these bacteria as they have developed resistance to various controlling methods and conditions including thermal treatment.

Purpose: The purpose of this study was to determine the D- and z-values of newly isolated Salmonella and E. coli bacterial strains obtained from local foods (chicken, tahini, and raw meat).

Methods: The isolated Salmonella and E. coli bacterial strains were heat treated at 55, 60, 65, 70 and 75°C. The colony forming unit (CFU) was recorded for each temperature every minute for 10 minutes. Survival curves were plotted and D-values as well as z-values were calculated and compared using two-sample t-test.

Results: All bacterial strains tested were totally resistant to heating at 55°C. E. coli strains isolated from two sources of raw meat recorded D₆₀ of 7.32 and 28.80; D₆₅ of 0.65 and 1.02; D₇₀ of 0.50 and 0.11; and D₇₅ of 0.38 and 0.11 minutes; respectively. The Salmonella strain isolated from tahini had D₆₀, D₆₅, D₇₀ and D₇₅ of 9.45, 0.49, 0.19 and 0.19 minutes; respectively. The other Salmonella strains isolated from poultry samples had D₆₀ of 3.64 and 3.00; D₆₅ of 0.47 and 0.69; D₇₀ of 0.19; and 0.54 and D₇₅of 0.13 and 0.46 minutes; respectively.

Significance: This study highlights emerging concerns regarding the safety of certain food products in the Lebanese market such as tahini and poultry. These results suggest further revision of associated processing conditions to ensure compliance with food safety standards and avoid outbreaks.