Kai Kupferschmidt, who initially reported it was E. coli O104 that has killed three and sickened over 400 in Germany, writes for Science Insider today that an initial sorta-case control study hasn’t provided any clues as to the source of the outbreak.

But Kupferschmidt does provide excellent background on E. coli O104, and notes that German authorities reported earlier today that the number of hemolytic-uremic syndrome (HUS) cases had reached 140; there are normally 60 HUS cases in Germany in a year.

Most EHEC infections are caused by a notorious serotype called O157:H7; researchers refer to that serotype and four others frequently found in Europe as the "gang of five." But the German reference laboratory for EHEC in Wernigerode has so far identified the serotype of EHEC in stool samples from five patients as O104.

Scientists have been baffled not only by the outbreak’s size and rapid spread in northern Germany but also by the fact that it affects mostly adults and females, an odd pattern because EHEC usually sickens children.

That makes the outbreak highly unusual, says Helge Karch, head of the National Consulting Laboratory on Hemolytic Uremic Syndrome in Münster. Among Karch’s E. coli isolates from 588 HUS patients collected over the past 20 years, only two are O104. In another unusual twist, Karch has found the strain to be eae-negative. The gene eae codes for the protein intimin, which the bacteria uses to attach to the intestinal wall. Most pathogenic EHEC serogroups are eae-positive.

It is still unclear whether the serotype might explain the strange pattern of infections. E. coli O104 first emerged as a pathogen in a small outbreak in Helena, Montana, in early 1994. Four people developed abdominal cramps and bloody diarrhea. Experts at the U.S. Centers for Disease Control and Prevention (CDC) in Atlanta identified a serotype called O104:H21 as the culprit. A CDC investigation later found up to 18 patients; most of them were women and the median age was 36 years.