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A new, untreatable strain of the sexually transmitted disease gonorrhea has been discovered in Japan, according to an international team of infectious disease experts. The strain, named H041, is resistant to all known forms of antibiotics.

The finding was presented Monday with extensive laboratory evidence at a conference in Quebec City, Canada — and it comes just three days after the Centers for Disease Control and Prevention (CDC) warned that U.S. gonorrhea samples had being showing new signs of drug resistance as well. Although drugs have remained effective in almost all U.S. cases, the CDC said that analysis of bacteria samples taken from 2000 to 2010 showed that the gonorrhea bug was becoming less and less susceptible to the frontline drugs, cephalosporins, as the years went by.

"This is both an alarming and a predictable discovery," Dr. Magnus Unemo said in a statement about H041. Unemo, based at the Swedish Reference Laboratory for Pathogenic Neisseria, worked with Japanese colleagues to characterize the new H041 multidrug-resistant gonorrhea strain.

Multidrug resistance is "predictable," in Unemo's words, because most gonorrhea strains worldwide are already resistant to at least one major class of antibiotics. Bacteria become resistant to antibiotics through evolution. Some naturally occurring genetic variation exists among Neisseria gonorrhoeae, the bacterial organism that causes gonorrhea, and that means that any one given bacterium may, by chance, be slightly more susceptible to antibiotics than another. When a colony of bacteria first comes in contact with antibiotics, therefore, the antibiotics will kill off the most susceptible bacteria at higher rates. This leaves behind a disproportionately robust batch of surviving bacteria, and when the survivors reproduce, they pass on their more-robust-than-average genes to their offspring. With repeated exposure to antibiotics, and over many generations of bacteria, eventually all the bacteria that are spreading are drug-resistant.

In the U.S., gonorrhea strains resistant to penicillin and tetracycline have been circulating since the 1970s and became widespread by the early 1980s, according to the CDC. Since then, most Neisseria gonorrhoeae have also become resistant to fluoroquinolines, and today the CDC recommends treating gonorrhea with both a cephalosporin and either azithromycin or doxycycline (two relatively commonly administered antibiotics).

But the recent CDC report from last week suggests that some U.S. samples of Neisseria gonorrhoeae are no longer responding quite as well as they used to against cephalosporins either — the bacteria have become less susceptible — and now the new strain in Japan is reportedly resistant to everything.

That brings us back to what researcher Magnus Unemo called "alarming." Gonorrhea is by no means the most deadly of sexually transmitted infections. But it is among the most common — with 700,000 new cases in the U.S. each year, and an estimated 340 million new cases each year globally.

The disease often causes painful intercourse (among women), tender and swollen testicles (among men), and pain when urinating (among infected people of both sexes). Left untreated, gonorrhea can lead to internal scarring of parts of the reproductive tract and, in women, pelvic inflammatory disease or lifelong infertility.

"The potential emergence of gonococcal cephalosporin resistance is of particular concern because the U.S. gonorrhea control strategy relies upon effective antibiotic therapy," the CDC announced Friday. "No other well-studied and effective antibiotic treatment options or combinations currently are available [once the bacteria are resistant to cephalosporins]."

As for H041, the strain found in Japan: "While it is still too early to assess if this new strain has become widespread, the history of newly emergent resistance in the bacterium suggests that it may spread rapidly unless new drugs and effective treatment programs are developed," Unemo said in his statement.

Indeed, when researchers grew the drug-resistant bacterium in culture with other strains of gonorrhea, the new strain was able to pass its resistance quickly, increasing the other strains' resistance to cephalosporins some 500-fold. That suggests that H041 could spread resistance swiftly in the real world.

• Laura Blue