Small change

The CDC researchers identified the EspW mutation by comparing the genetic sequences of 729 isolates of the new E. coli strain—dubbed REPEXH01—to genetic sequences of 2,027 other E. coli O157:H7 isolates. Of the 729 REPEXH01 strains, all but two had a single nucleotide deletion in EspW (the remaining two had ambiguous sequences), while the deletion was present in less than 4 percent of the non-REPEXH01 E. coli strains. The finding suggests the tiny change could be a genetic signature of the strain, and its persistence in a key disease protein may offer the strain an advantage.

For now, it’s unclear what that advantage might be. The deletion of a single DNA base (an adenine) shifts the frame of the three-sequence protein code for the rest of EspW. This could result in a shorter protein. It could also cause the molecular machinery that translates the genetic code to slip, leading to proteins of various lengths. In any case, the deletion is likely to result in a less fully functional EspW protein.

The CDC researchers suggest this could help E. coli when it’s on lettuce and other produce. For example, EspW might spur an immune response from an infected plant that causes stomata—pores on the surfaces of leaves—to close, blocking the bacteria’s ability to invade. Thus, cutting back EspW may help E. coli sneak in—an adaptation in the ongoing arms race between the bacteria and its host. Another possibility is that EspW could function like HopW1, leading to more severe infection in plant tissues, which could lower the chances that those infected leaves are harvested and make it to grocery stores and atop burgers. Thus, cutting back on EspW could help E. coli move to its human victims.

Ultimately, additional research will be needed to understand what’s going on. As the CDC researchers conclude: “the role of the single base pair mutation in this strain’s colonization and survival on leafy vegetables could yield valuable insights.”