Science News - January 14, 2012

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year’s massive food poisoning outbreak. O104:H4 germs don’t ordinarily produce deadly toxins or bloody diarrhea. But this strain, which probably traveled and spread on tainted sprouts, did both, killing more than 50 people and sickening more than 3,700.

The scientists stressed the germs by exposing them to copper. Within a few days, many of the bacteria went dormant and stayed that way until the researchers removed copper from the germs’ growth medium. Once resuscitated, the germs still had all of the features needed to be infective, Antje Flieger and her colleagues report in the December Environmental Microbiology.

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, says study coauthor Susan Bach, 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,” she says. Moreover, even in dormancy the cells were a source of toxin, the researchers report in the December Applied and Environmental Microbiology.

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 Vibrio bacteria that frequently taint shellfish are still infectious — and lethal — after resuscitation from a chill-induced hibernation.

Some of these E. coli cells
found on lettuce are dead
(red), but most are in a
dormant state (green) that
makes them hard to detect.

Microbes sleep to evade detection
Stressed bacteria can dodge screening tests by laying low

By Janet Raloff

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 detect live bacteria.

Scientists have watched dozens of kinds of bacteria enter a dormant state in the lab but don’t know to what extent living germs do so in the environment or on foods, says microbiologist James Oliver of the University of North Carolina at Charlotte. And that, he says, points to the importance of the two new studies.

In one study, microbiologists at the Robert Koch Institute in Wernigerode, Germany, tested E. coli O104:H4 isolated from patients who fell ill in last

DNA to flutter by

Scientists have deciphered the complete genetic instruction
book of monarch butter;ies. It is the ;rst butter;y genome com-
pleted and the ;rst of a long-distance migrating insect. Steven
Reppert of the University of Massachusetts Medical School
in Worcester and his colleagues found genes that may help
the insects sense the sun’s position and navigate to forests
in Mexico where they spend the winter. Reporting in the Nov.
23 Cell, the team also notes that, when in migration mode,
monarchs make more of certain small genetic molecules, called
microRNAs, that are involved in building muscle, regulating tem-
perature sensitivity and storing fat. The 273 million DNA units
that make up the monarch genome also include a complete
set of genes for producing juvenile hormone, which summer
butter;ies use to kick-start reproduction. Migrating males use
different strategies than females do to turn off the hormone,
the team discovered. — Tina Hesman Saey
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