German E. coli outbreak caused by previously unknown strain

Abstract

The bacterium responsible for the current outbreak of enterohaemorrhagic Escherichia coli (EHEC) infections in Germany is a strain that has never before been isolated in humans. The discovery, announced today by the food safety office of the World Health Organization (WHO) in Geneva, Switzerland, means that the infection could prove unusually difficult to bring under control. Scientists in Germany are feverishly analysing the genome sequence of the bacterium, and have found clues as to how this strain — which has so far infected more than 1,500 people and killed 18 — is making so many people ill. The bacteria are relatively unusual in that they produce extended-spectrum β-lactamases — enzymes that render the bacteria resistant to many different antibiotics. Patients with E. coli infections are not typically treated with antibiotics anyway, because the bacteria are thought to respond to the medication by increasing production of the Shiga toxin, which can lead to the life-threatening complication haemolytic–uremic syndrome. But antibiotic resistance might have helped the bacteria to survive and persist in the environment. "EHEC outbreaks usually only last around two weeks, but this outbreak has been going on since 1 May or earlier," says Angelika Fruth, a microbiologist based in Wernigerode who works for the Robert Koch Institute, the federal agency responsible for disease control. The number of new cases is still rising, suggesting that whatever their source, the bacteria are still infecting people. That source remains a mystery. A case–control study of female patients and healthy women, conducted by epidemiologists from the Robert Koch Institute, revealed that the ill women were more likely than the controls to have eaten tomatoes, cucumbers and salad vegetables prior to contracting the disease, but exactly which vegetables are responsible, if any, is unclear. Early reports that the outbreak originated in cucumbers imported from Spain were later shown to be incorrect — the cucumbers contained the bacterial toxins, but not the bacteria responsible for the ourbreak themselves. Fresh vegetables are still the prime suspect, but Flemming Scheutz, head of the WHO Collaborative Centre for Reference and Research on Escherichia and Klebsiella in Copenhagen, suggests that the bacteria might not have originated in the food chain at all. "This strain has never been found in any animal, so it is possible that it could have come from straight from the environment into humans". Lothar Wieler, a veterinary microbiologist at the Free University of Berlin, cautiously agrees with this theory. In addition to the antibiotic-resistance genes, the bacteria contain a gene for resistance to the mineral tellurite (tellurium dioxide). Tellurium oxides were used as antimicrobial agents against diseases such as leprosy and tuberculosis before the development of antibiotics. Some strains of bacteria may have evolved resistance to tellurium during its historical medical use, or after its use in the mining and electronics industries increased its presence in the environment. According to Wieler, the strain's resistance characteristics could point towards an environmental source, such as water or soil.