Developing a Universal Enrichment Broth for the Foodborne Bacterial Pathogen Salmonella

Introduction: For successful prevention of foodborne illness, rapid and reliable methods are needed for detection. Depending on the food matrix, varying pre-enrichment broths are used by FDA field labs as outlined in Bacteriological Analytical Manual for the detection of Salmonella.  In addition to the preparation of multiple pre-enrichment broths being labor intensive and costly, the use of multiple enrichment broths is a major roadblock when trying to develop pathogen detection methods in multiple food matrices.  

Purpose: The purpose of this study is to evaluate and modify current enrichment broths to propagate the foodborne bacterial pathogen, SalmonellaTyphimurium.

Methods: Salmonella Typhimurium (4.65 ± 0.89 log CFU/ml) was inoculated into candidate enrichment broths, incubated at 37°C and samples were taken periodically over 30 h.  Candidate broths included the currently used Lactose broth (LB), Tryptone Soy Broth (TSB), Universal Pre-Enrichment Broth (UPB), and Modified Buffered Peptone Water (mBPW) as well as Yersinia pestis Enrichment Broth (YpE). Two modified broths, mBPW with 0.25% glucose and YpE broth with a buffer system (buffered YpE), were also examined. Growth was characterized by CFU/ml from TSA plates, optical density from absorbance readings at 600nm (OD₆₀₀), and Ct values from TaqMan qPCR. 

Results: After 24 h in LB, Salmonella Typhimurium had the lowest growth (7.93 ± 0.17 log CFU/ml, 30.98 ± 1.54 Ct, P < 0.05).   Salmonella Typhimurium growth was greatest after 24 h in YpE broth (OD 1.27 ± 0.02) which was significantly greater (P < 0.05) than buffered YpE broth (OD₆₀₀ 1.10 ± 0.01).  Salmonella Typhimurium in mBPW with 0.25% glucose had significantly greater growth (P < 0.05) than in mBPW without glucose with OD₆₀₀readings of 0.57 ± 0.04 and 0.19 ± 0.10, respectively.

Significance: Growth characteristics of Salmonella Typhimurium varied significantly in the different enrichment broths examined.  The identification of the vital constituents for Salmonella growth in the enrichment broths will ultimately be used for the identification of a universal enrichment broth for the detection of Salmonella Typhimurium in multiple food matrices.