Outcomes of using wet pooling to detect STEC and Salmonella

Authors: Bosilevac, Joseph - Mick

Submitted to: Meeting Abstract
Publication Type: Abstract Only
Publication Acceptance Date: 2/18/2017
Publication Date: 2/28/2017
Citation: Bosilevac, J.M. 2017. Outcomes of using wet pooling to detect STEC and Salmonella. Meeting Abstract. [Abstract]. Beef Industry Safety Summit, February 28-March 2, 2017, Houston, Texas. Available:http://bifsco.org/search.aspx?qal=2017 abstracts.

Interpretive Summary:

Technical Abstract: Objective: The objective of this work was to examine the reliability of wet pooling sample broths. Experimental Design & Analysis: Fresh sample enrichment broths (n=737) were used to prepare 148 wet pools of 5 broths each. The initial broths and the pools were screened for STEC and Salmonella. The broths were held at room temperature until the screening result of the pool was known (3-4 hours), then the implicated broths of a positive pool were individually re-screened to identify those responsible for the positive pool. Results were analyzed to identify 1) the percentage of pools that accurately reflected the expected result based on the individual samples used; 2) the percentage of broths that could be confirmed to contain a positive sample after the samples had been held 3-4 hours; and 3) whether the samples contributing to a positive pool were consistently identified. In cases of incongruence between samples and pools, the levels of background bacteria (APC and EBC) in the pre-enrichment starting materials was examined to determine if its overgrowth correlated to the unexpected results. Key Results: Based on individual sample prevalence, 19 of 147 pools and 49 of 134 pools were predicted to be positive for STEC and Salmonella respectively. Of the 19 predicted STEC pools, 13 (68%) were positive and 6 were negative. All of the pools predicted to be STEC negative, were found negative. Follow-up analysis of the STEC positive pools showed that 92% could be confirmed, while two-thirds of the negative predicted-positive pools were still shown to have samples test positive for STEC. Of the 49 predicted Salmonella pools, 36 (74%) were positive and 13 were negative. All, except for one, of the pools predicted to be Salmonella negative were found negative. Follow-up analysis of the Salmonella positive pools showed that 94% could be confirmed, while a third of the negative predicted-positive pools were found to have samples that still tested positive for Salmonella. Follow-up analysis of the one predicted-negative pool that tested positive could not confirm a positive sample was present. There was no relationship between the APC or EBC of a sample and the screening results. How can this information be applied in the industry?: Wet pooling is commonly used to save material and equipment costs associated with screening tests. Pooling is essentially diluting the sample and low concentration target cells can be missed when diluted in the pool (as likely seen here). If pooling is used, validation is warranted to ensure all positive samples are detected. The sample responsible for the positive pool can be identified over 90% of the time, but if greater accuracy is needed then initial samples should be prepared before pooling then tested if a pool is positive rather than going back into samples after the fact.