Antibiotic-Resistant Bacterium Causing Bladder Infections
A newly identified, antibiotic-resistant strain of a common bacterium is contributing to an increase in relatively hard-to-treat bladder infections in women in at least three U.S. cities, according to a study published Thursday.
Genetic analysis and other laboratory tests pinpointed the strain of Escherichia coli bacteria as the culprit in a substantial percentage of drug-resistant urinary tract infections among female university students in Berkeley, Minneapolis and Ann Arbor, Mich.
The microbes implicated in the Michigan and Minnesota infections were almost identical to the California ones, which suggests the recent arrival or emergence of a new drug-resistant strain that has spread rapidly to different parts of the country.
“I think calling it an epidemic is reasonable,” said Glenn Morris, professor and chairman of the department of epidemiology and preventive medicine at the University of Maryland Medical School. “You probably do have strains that are coming from a common source and are responsible for an increase above expected numbers” of drug-resistant infections.
About 11% of women suffer at least one bladder infection annually, and more than half of all women experience at least one in their lifetime. It is thought that bacteria from the large intestine gain access to the bladder, perhaps during sexual activity or because of predisposing factors. The infections make urination frequent and often painful, but most cases can be easily treated with antibiotics. E. coli is a common cause.
Not much is known about how new strains of bacteria that infect the urinary tract arise and spread. E. coli and many other bacteria are normally present in the large intestine, and diarrhea-causing strains are sometimes spread through contaminated food. There is no evidence that the strain causing the bladder infections in California, Michigan and Minnesota came from food, but researchers said that was a possibility.
There is also no evidence that the new strain of E. coli is causing any increase in kidney infections or other serious complications. The bladder infections of women in the study, although resistant to some drugs, remained susceptible to other antibiotics.
Researchers at UC Berkeley undertook the study because they were concerned about a recent rise in bladder infections among women treated at the student health clinic that were resistant to trimethoprim-sulfamethoxazole, a popular two-drug combination.
Amee Manges, a graduate student who led the study, said she and her colleagues were surprised to find virtually the same microbe causing bladder infections in three states. “We were really expecting to see maybe some small clusters of similar organisms, but we were not expecting to see ... that all these unrelated women would have urinary tract infections due to the identical strain,” she said.
The California researchers found the same strain of E. coli in a number of stool samples obtained from healthy men and women seen at the clinic. However, Manges said, they were not able to determine how and when the strain first arrived in the population.
“It could be that this organism has been around forever and has now acquired resistance to several antibiotics,” she said. Alternatively, she added, it could have been introduced, “maybe not too long ago,” perhaps through contaminated food. The research team analyzed bacteria in urine samples but did not interview the infected women or read their medical records, so they could not determine whether they had any common exposures or risk factors.
“I would guess that [the strain] is fairly recent in origin,” said Walter Stamm, a professor of medicine at the University of Washington who wrote a commentary accompanying the study in Thursday’s issue of the New England Journal of Medicine.
Drug-resistant intestinal bacteria from farm animals, which are often given antibiotics in feed, have sometimes been transmitted to farmers, food handlers or other people, Stamm said. “Those pathways have been documented, but not for this strain or for strains causing urinary tract infections,” he said.
In the study, researchers studied 255 samples of E. coli isolated from the urine of women with bladder infections seen at the Berkeley clinic. Fifty-five (22%) were resistant to trimethoprim-sulfamethoxazole. They compared them with 18 similarly resistant samples of E. coli from women treated in Minneapolis and 29 from women in Ann Arbor. Researchers analyzed the genetic “fingerprints” of each sample using two techniques, tested each for antibiotic susceptibility and performed serotyping, a method of grouping bacteria according to proteins on their surfaces.
The tests showed that more than half of the resistant infections at the Berkeley clinic were caused by a single strain. A nearly identical strain was implicated in 38% of the resistant infections in Michigan and 39% in Minnesota. The strain showed a pattern of resistance to multiple antibiotics and had genetic characteristics called “virulence factors” that increased its ability to infect the urinary tract.
In his commentary, Stamm noted that widespread use of trimethoprim-sulfamethoxazole has probably contributed to the recent increase in resistance to the drug. He said risk factors for resistant bladder infections include recent use of antibiotics, recent travel or attending a day care center.
