Implementation of molecular screening for a more efficient Shiga toxin-producing Escherichia coli testing workflow

Culture-independent diagnostic tests (CIDT) have emerged as a preferred diagnostic approach for the rapid detection of etiologic agents of gastroenteritis. Although the relative ease and speed of CIDT support their use as a primary diagnostic test, there is still a need to isolate the organism from CIDT-positive specimens for disease surveillance. The objective of this study was to assess the performance of the testing algorithm used at the California Department of Public Health Foodborne & Waterborne Diseases Section (CDPH-FWDS) for Shiga toxin-producing Escherichia coli (STEC) recovery from CIDT-positive human stool specimens in transport medium, with particular focus on molecular screening of enrichment broths for a Shiga toxin gene (stx). When STEC was not isolated by culture of the submitted stool specimen, CDPH-FWDS subcultures from an enrichment broth started from the stool specimen. Enrichment broth can improve STEC recovery by allowing target bacteria to multiple while limiting the growth of normal fecal flora. The enrichment broths were screened for stx1 and stx2 using a lab developed real-time PCR assay. STEC was isolated from only 3% of the enrichment broths that were negative for stx1 and stx2 by molecular screening. This work demonstrates the utility of molecular screening of enrichment broths prior to culturing for bacterial isolation. Discontinuation of culture from stx molecular screen negative enrichment broths will result in decreased hands-on laboratory testing time and cost savings with minimal impact on STEC surveillance efforts.