In a dangerous boomerang effect apparently caused by antibiotics, E. coli is on the rise among premature babies and has overtaken strep as the most common infection in such infants, a disturbing new study suggests.

The shift is worrisome because E. coli bacteria can be more deadly than streptococcus germs.

The rate of group B streptococcus blood infections in newborn preemies fell by nearly three-quarters during the 1990s, probably because more women in labor now get antibiotics to keep from passing the bacteria on to their babies during delivery, the researchers said.

During that same period, the rate of E. coli infections doubled, apparently because ampicillin, the antibiotic commonly used the wipe out strep, gave E. coli room to flourish, according to the researchers.

The study was funded by the National Institutes of Health and was published in Thursday's New England Journal of Medicine. It was led by Dr. Barbara J. Stoll, professor of pediatrics at Emory University School of Medicine.

Group B strep and E. coli are among the bacteria that live harmlessly in many people's intestinal tract. If they spread into a woman's vagina during pregnancy, though, they can overwhelm the newborn's weak immune system, sometimes causing mental retardation, hearing or vision loss, or death.

Over the last decade, giving women preventive antibiotics intravenously during labor has cut the once-predominant group B strep infections by 70 percent in all newborns, but it still kills about 80 each year.

NIH scientists and doctors at about a dozen hospitals and medical schools compared 5,447 very low-birth-weight infants born from 1998 through 2000 with 7,606 similar births from 1991 through 1993. Very low-birth-weight is defined as 3 1/2 pounds or less.

Over the study, the more-dangerous "gram-negative" type of bacteria, predominantly E. coli, overtook group B streptococcus and other related bacteria as the most common type of infection in the premies. Gram-negative bacteria have innate resistance to antibiotics and can acquire further resistance mechanisms from other germs.

E. coli infections rose from 3 per 1,000 births to 7 per 1,000 births, while group B strep infections dropped from 5.9 per 1,000 births to 1.7 per 1,000 births. The overall rate of sepsis dropped from 19.3 per 1,000 births to 15.4 per 1,000 births.

The research appears along with a related study from the Centers for Disease Control and Prevention.

Since 1996, the CDC, the American Academy of Pediatrics and the American College of Obstetricians and Gynecologists have backed two strategies as equally effective for deciding which mothers should get antibiotics during labor to prevent group B strep transmission.

One involves testing mothers for presence of streptococcus in the vagina near the end of the pregnancy; the other recommends antibiotics for all mothers who have risk factors linked to transmission of the bacteria: a fever above 100 degrees, preterm delivery or rupture of the protective amniotic membrane 18 hours or more before delivery.

In the first study comparing the approaches, researchers found that testing for streptococcus cut the risk of a baby being infected 54 percent more than the risk-factor approach.

"While the risk-factor approach is definitely catching some women, it's missing a good proportion of women," said Stefanie J. Schrag, a CDC epidemiologist.

As a result, the CDC in August will release new guidelines recommending screening all women for presence of group B strep, she said.

The researchers studied records on 5,144 babies born at about 170 hospitals around the country.

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