Effect of Selective Enrichment Storage Temperature and Duration Time on the Detection of Salmonella in Food

Abstract

Background: For pathogen detection in food, there are occasions where samples cannot be processed immediately after selective enrichment or need to be reexamined days or weeks later for confirmation or retest.

Objective: This study aimed to investigate the effect of different prolonged period of storage of selective enrichments of food at 4 ± 2°C and room temperature (20-22°C) on the detection and isolation of Salmonella.

Method: This study included two experiments involving 34 types of foods to compare the effect of 4 ± 2°C and room temperature storage on the detection of Salmonella in 204 selective enrichments (Rappaport-Vassiliadis [RV] and Tetrathionate [TT] broths) during a 42-day storage (Experiment I); and to monitor the survival of Salmonella in 300 selective enrichments (RV and TT) with different pre-enrichment broths (Lactose broth [LB] or Buffered peptone water broth [BPW]), stored at 4°C for 60 days (Experiment II). All the samples were subjected to Salmonella analysis following the FDA BAM method.

Results: During multiple samplings, the positive detection rate for Salmonella remained consistent through Day 42 after selective enrichment, irrespective of Salmonella serotype, storage temperature, pre-enrichment broth, or selective enrichment broth in both Experiment I and II. However, on Day 60 sampling in Experiment II, seven previously positive results turned to negatives. These data indicated that storage of RV and TT enrichments at 4 ± 2°C or room temperature for up to 42 days after selective enrichment did not compromise the detection of Salmonella in the tested food categories, regardless of Salmonella serotypes and the broths used for pre-enrichment and selective enrichment.

Conclusions: At least for the food types studied in this experiment, the recovery of Salmonella from selective enrichments could be postponed for a limited period of time (e.g., <42 days) if needed without adversely affecting the test results. However, the delayed analysis of TT and RV enrichments does pose a risk of reduced detection sensitivity, as evidenced by the seven negative results on Day 60 compared to previous positives. We do not recommend or endorse delaying the analysis of TT and RV enrichments.

Highlights: In the food matrixes investigated in this experiment, the plating and isolation of Salmonella from selective TT and RV enrichments stored at 4 ± 2°C or room temperature could be deferred for a period (up to 42 days) without any negative effect on the test results, if necessary.

Figure 1.

Flow chart for experimental design. LB, Lactose broth; UP, Universal pre-enrichment broth; BPW, Buffered peptone water broth; RV, Rappaport-Vassiliadis broth; TT, Tetrathionate broth; XLD, Xylose lysine desoxycholate agar; HE, Hektoen enteric agar; BS, Bismuth sulfite agar; TSI, Triple sugar iron agar (slant); LIA, Lysine iron agar (slant).

Figure 2.

Results of Salmonella detection in Experiment I. Number of positive tests among 34 RV or 34 TT enrichments at different sampling times during 42 days of storage at 4 ± 2°C and room temperature. RM, room temperature; RV, Rappaport-Vassiliadis broth; TT, Tetrathionate broth.

Figure 3.

Results of Salmonella detection in Experiment II. A, Results of Trial# 1, number of positive tests among 16 RV or 16 TT enrichments at different sampling times during 42 and 60 days of storage at 4 ± 2°C. B, Results of Trial# 2, number of positive tests among 52 RV or 52 TT enrichments at different sampling times during 42 and 60 days of storage at 4 ± 2°C. LB, Lactose broth; BPW, Buffered peptone water broth; RV, Rappaport-Vassiliadis broth; TT, Tetrathionate broth.