Researchers think they now know why a particularly virulent form of E. coli that swept through northern Germany last May was so hard to trace: The germs responsible eluded detection by going into a self-induced deep sleep.
Two new studies show that when stressed, E. coli can turn off most signs of life. That’s a problem for food-safety officials because their germ-screening techniques rely on germs reproducing to establish the presence of live bacteria.
Microbiologists look for life by attempting to culture — or grow — bacteria in nutrients. But near-comatose germs don’t reproduce, explains microbiologist James Oliver of the University of North Carolina at Charlotte, who did not take part in the new work. Such bacteria instead “become viable but nonculturable,” he says, “what we call VBNC.”
Although dozens of different bacteria can enter this state in the lab, he notes, data have been sparse about the extent to which living germs go dormant in the environment or on foods. And that, he says, points to the importance of the two new studies.
“Whenever you have a foodborne outbreak and can’t trace it, it’s likely because you had bacteria that turned VBNC,” Oliver says.
In one of the new studies, microbiologists at the Robert Koch Institute, a federal research center in Wernigerode, Germany, tested E. coli O104:H4 isolated from patients who fell ill in this year’s massive food-poisoning outbreak. O104:H4 germs don’t ordinarily produce deadly toxins or bloody diarrhea. This strain, which probably traveled and spread on tainted sprouts, did both, killing 54 people and sickening more than 3,700 others.
In the lab, the scientists stressed the germs by exposing them to copper. Within a few days, many of the bacteria entered the dormant state and remained that way unless the researchers removed copper from the germs’ growth medium. Antje Flieger and her colleagues describe their findings in the December Environmental Microbiology.
Once resuscitated, the germs still had all of the features needed to be infective. For example, they had retained the genes to produce their lethal toxin and to make the sticky hairlike features that foster gut attachment. “That’s why we speculate they should be active and allow infection,” Flieger says.
To simulate what might happen in farm fields, microbiologists at Agriculture and Agri-Food Canada in Summerland, British Columbia, inoculated lettuce with either of two strains of E. coli O157:H7, a germ linked to deaths from eating tainted hamburger, lettuce and other types of produce.
Withholding the nutrients these bacteria would ordinarily acquire while passing through the gut caused the germs to enter hibernation. Within a few days, observes Susan Bach of Agriculture and Agri-Food Canada, more than half of the germs were still alive but could not be cultured. “We showed they remained active metabolically, but at a very low level.” Moreover, even in their dormancy, the cells were a source of toxin.
Depending on the conditions, some bacteria can hibernate for very long periods, Oliver says, pointing to one study in which bacterial cells remained dormant for over six years before reawakening.
One limitation of the new E. coli studies: Neither proved that once resuscitated, the germs would still induce disease. But that’s certainly the expectation, Oliver says. His team has shown that, the Vibrio bacteria that frequently taint shellfish is still infectious — and lethal — after resuscitation from a chill-induced hibernation.
Such data “show that VBNC cells may be really important in food safety,” Bach says, especially for types like O157:H7 where very few cells are needed to induce potentially life-threatening disease.