A new type of gonorrhea that can't be treated with the antibiotic tetracycline has begun to appear in several East Coast cities, government medical researchers reported last week, and health officials expect the bacteria to continue to evolve in response to new drugs.

Gonorrhea bacteria "have an enormous capacity to mutate," says Dr. John Zenilman of the Centers for Disease Control in Atlanta. "Basically, they're smarter than we are."

The CDC reports 12 cases of the new strain in Atlanta and Philadelphia. Additional unconfirmed cases of the sexually transmitted infection have occurred in Boston, Alabama and Prince George's County, Maryland, Zenilman says.

Expanded studies in the Atlanta area showed that about 4 percent of all cases were resistant to tetracycline, but it is not known whether the prevalence of the new bacteria is the same nationally.

In the past 40 years, the penicillin dose necessary to treat gonorrhea has increased almost 100 times as the bacteria have mutated. But in most cases, the inexpensive antibiotic is still an effective cure.

Tetracycline was originally used for gonorrhea in patients who were allergic to penicillin. More recently, the CDC has advised doctors to use either drug they prefer.

But in light of the new strain, the CDC plans to advise doctors next month to use both drugs on all patients not allergic to penicillin, Zenilman says.

What researchers now fear is a strain of gonorrhea resistant to both drugs, which would force use of more expensive antibiotics such as spectinomycin -- now usually held in reserve for only the most difficult cases.

"As these drugs are used more and more," Zenilman says, "we expect that there will be some resistant [cases]. We can't predict when that will occur."