Enhanced Enumeration of Heat-injured Salmonella Typhimurium in Synthetic and Food Models by Combining Thin Agar Layer Method and Sodium Pyruvate Supplement

Introduction: Salmonella is a leading cause of foodborne illness throughout the world.  Previous research indicates thin agar layer (TAL) method can effectively recover pathogenic bacteria sublethally injured under common food processing conditions. Moreover, supplementation of peroxide decomposer chemicals to media improve the recovery of injured bacterial cells.  

Purpose: This study aimed to determine the combined effect of these two approaches for recovering heat-injured Salmonella Typhimurium from synthetic and food models. 

Methods: The TAL procedure was developed by overlaying one or two layers of non-selective plate count agar (PCA) onto pre-poured and solidified Salmonella selective medium, xylose lysine deoxycholate (XLD) agar. The recovery effect of these agar media with various peroxide decomposers in PCA was evaluated with healthy and heat-injured Salmonella Typhimurium cells inoculated in peptone water, canned chicken broth, and fresh lettuce washing solution.   

Results: Supplementation of PCA with sodium pyruvate (SP) significantly enhanced the recovery of heat-injured Salmonella Typhimurium in peptone water; the count increased by 0.76 and 2.67 logs compared with PCA and XLD, respectively. Adding SP or FeSO₄ into XLD directly did not show any significant effect on the recovery. Heat-injured Salmonella Typhimurium in peptone water was counted 2.51 and 3.62 log CFU/ml on one and two layers of SP-supplemented PCA overlaid onto XLD, respectively.  The count was only 0.20 and 0.85 log CFU/ml on one and two layers of PCA overlaid onto XLD, respectively.  Supplementation of two layers of PCA in TAL with SP increased the count of heat-injured Salmonella Typhimurium in chicken broth and lettuce washing solution by 1.48 and 2.78 logs, respectively.  This method did not influence the selectiveness of XLD in TAL on background flora in lettuce washing solution. 

Significance: The combination of TAL method and SP supplement could potentially be used for rapid and accurate enumeration of injured pathogenic bacteria in food samples.