On November 20, the Centers for Disease Control and Prevention (CDC), in conjunction with the Food and Drug Administration (FDA) and several state agencies, announced that it is currently investigating an outbreak of Shiga toxin-producing E. coli (STEC) O157:H7 infections that has so far impacted at least 17 people across 8 states. The first reported illnesses date back to September 24, 2019. Investigators are looking into a branded chicken Caesar salad as a potential source, after the Maryland Department of Health (MDH) identified E. coli O157 in an unopened package. However, MDH is still conducting a whole genome sequencing (WGS) analysis to determine if it is closely related genetically to the E. coli identified in this outbreak.

As previously reported on this blog, in January 2019, FDA’s Food Safety and Inspection Service (FSIS) transitioned to using only WGS for Shiga toxin-producing E. coli (STEC) in an effort to update its analytical methods to the state of the art. The method of WGS determines the order of all of the DNA building blocks (nucleotides) in an organism’s entire genome in a single laboratory process, and a comparison of the DNA sequence of an isolated bacterial pathogen to the sequences from other samples in a DNA database can pinpoint the source of a foodborne disease outbreak.

The recent outbreak follows a similar outbreak of E. coli 0157:H7 from 2018 that was ultimately traced to romaine lettuce. The 2018 outbreak included 62 cases from 16 states and the District of Columbia, and prompted FDA to issue recommendations for leafy greens growing operations as well as a partnership between FDA and leafy greens stakeholders in Arizona to enhance food safety. Subsequent research from the U.S. Department of Agriculture’s Agricultural Marketing Service found that pest flies were a potential vector in the spread of the E. coli O157:H7 and contamination of leafy greens.