ATLANTA - The sexually transmitted disease gonorrhea is now among the "superbugs" resistant to common antibiotics, leading U.S. health officials to recommend wider use of a different class of drugs to avert a public health crisis.

The resistant form accounts for more than one in every four gonorrhea cases among heterosexual men in Philadelphia and nearly that many in San Francisco, according to a survey that led to Thursday's recommendation by the Centers for Disease Control and Prevention.

Gonorrhea, which is believed to infect more than 700,000 people in the United States each year, can leave both men and women infertile and puts people at higher risk of getting the AIDS virus.

Since the early 1990s, a class of drugs known as fluoroquinolones has provided a relatively easy cure. These antibiotics, taken as tablets, include the drug Cipro.

But a growing number of gonorrhea cases is resistant to those drugs, and officials at the CDC for the first time are urging doctors to stop using fluoroquinolones and switch to cephalosporins, a different class of antibiotics, to treat everyone.

Those drugs _ which include the generic ceftriaxone or brand name Rocephin, made by Swiss drug maker Roche Holding AG _ must be given as a shot and aren't as readily stocked as Cipro on most doctor's shelves.

"Gonorrhea has now joined the list of other superbugs for which treatment options have become dangerously few," said Dr. Henry Masur, president of the Infectious Disease Society of America. "To make a bad problem even worse, we're also seeing a decline in the development of new antibiotics to treat these infections."

The CDC made the new recommendation after discovering that nearly 7 percent of gonorrhea cases among heterosexual men in a survey of 26 U.S. cities last year were drug-resistant. In 2001, only about 0.6 percent of gonorrhea cases among heterosexual men were drug-resistant.

"That leaves us with a single class of highly effective antibiotics," said Dr. John Douglas Jr., director of the CDC's division of STD prevention. Other experts called the situation perilous.

"We are running out of options to treat this disease," added Douglas, who said there are "no new drugs for gonorrhea in the drug development pipeline."

Previously, CDC recommended against fluoroquinolones to treat drug-resistant gonorrhea among men who have sex with men and in certain states, including California and Hawaii where most of these cases were turning up.

Described by Douglas as a "very wily" disease, gonorrhea has worked its way through decades of other treatment regimens, from sulfa drugs used in the 1930s and 1940s, to penicillin, which was used from the 1940s until the mid-1980s.

Gonorrhea, spread through sexual contact, is the second most commonly reported infectious disease in the United States, trailing only chlamydia, which the CDC says affects more than 2.1 million people yearly in the U.S.

The highest rates of infection are among sexually active teens, young adults and African-Americans. Because many people don't have obvious symptoms, they can unknowingly spread it to others. And having it makes people more susceptible to HIV. Gonorrhea's spread is preventable through consistent and proper use of condoms, experts said.

In women, the infection can cause pelvic inflammatory disease. In men, it can cause epididymitis, a painful condition of the testicles that can lead to infertility if untreated, the CDC said.

In the survey of gonorrhea cases among heterosexual men in 26 cities last year, Philadelphia had the highest percentage of drug-resistant cases with almost 27 percent, a dramatic increase from only 1.2 percent in 2004.

San Francisco's drug-resistant cases more than doubled between 2004 and 2006, from 10.3 percent to 22.5 percent. During the same period, Miami's cases spiked from 2.1 percent to 15.3 percent and Atlanta's climbed from 1 percent to 5.7 percent.