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This is the story of a good bug gone bad.

The bug, a bacterium named Branhamella catarrhalis (commonly called B.cat), has been living in people's noses, mouths and throats for generations. It was like the good kid on the block. Nobody paid much attention until he got into bad company, flexed his muscles - and there went the neighborhood."A wolf in sheep's clothing," is how the Hearing Journal labeled B.cat's role in otitis media, the common ear infection that, if not successfully treated, can threaten a child's hearing.

The article quoted a 1986 report that B.cat had jumped from 7 percent to 24 percent of otitis media cases seen at the University of Texas Health Science Center, Dallas. That made the illness harder to treat.

"Five short years ago, B.cat was not recognized as a cause of infection," said Dr. Gary Doern, director of clinical microbiology at the University of Massachusetts Medical Center, Worcester.

"We have known for 30 years that the organism lives in the upper respiratory tract of healthy people. It's part of the normal bacterial flora of our mouths, throats and noses that, for whatever reason, usually don't cause disease.

"Now we recognize B.cat as the third most common cause of middle ear infections and a very, very common cause of lung infections, bronchitis and pneumonia."

What happened?

Nobody is altogether sure, said Doern.

Recent research has shown that B.cat bacteria increasingly produce an enzyme, called beta lactamase, that makes antibiotics ineffective.

"We are investigating the possible role of beta lactamase as a factor that allows the bug to cause disease," Doern said.

One question scientists are asking: Has the common - some say unnecessarily frequent - use of antibiotics in recent years contributed to the change in B.cat from a benign bacterium to a drug-resistant troublemaker? Answers to those questions will take time.

Meanwhile, Doern said, too few doctors consider B.cat when they encounter stubborn ear infections in children or the bronchitis and pneumonia that can send older people to the hospital.

He cited a 15-state telephone survey, done for Beecham Laboratories, that showed 84 percent of doctors questioned were unaware of B.cat's emergence as a dangerous bacterium.

However, doctors in their first 10 years of community practice were more likely to know about it. What makes that survey significant is the doctors' choice of drug.

"Penicillin, ampicillin and amoxicillin are commonly used to treat infections of the middle-ear and many lung infections, yet they won't work against B.cat," Doern said. There is, however, a combination of amoxicillin and clavulanate sodium, called Augmentin, that works, he said. It's made by Beecham.

Dr. Kenneth Ratzan, chief of infectious disease at Mount Sinai Medical Center in Miami Beach, agrees with Doern that many community based physicians might not think of B.cat when treating stubborn bronchitis, sinusitis or inner ear problems.

"It's not commonly recognized or even heard of," he said. "It's not terribly important in terms of frequency . . . (but) it should be considered in a situation whereby a patient has the symptoms of a bacterial infection and is not improving on antibiotics."

Then Augmentin might be used, he said, because its secondary drug, clavulanate potassium, combats the enzyme that makes B.cat resistant to antibiotics. However, the physician may choose another drug, especially if a patient is sensitive to penicillin, Ratzan said.

Dr. Jose Wenger, a Miami pediatrician, said he doesn't like the diarrhea that Augmentin causes in some children.

All of those decisions, however, are usually out of the hands of the patient or parents.

"Parents need to be aware of their children's state of health and when a child develops symptoms that are suggestive of a middle ear infection that child should be seen by a physician," Doern said.

"In adults, particularly the elderly, in whom bronchitis and pneumonia are the greatest problem, when these conditions arise, we need to seek health care."

As for B.cat, he said: "There's nothing unique about the diseases produced by this bug that would allow the normal, non-medical person to know much."

Even doctors don't recognize it without doing tests. But when treatment isn't going well, it might be worth discussing with the doctor.