WASHINGTON: The US Food and Drug Administration has approved a new antibiotic to treat an intestinal infection that afflicts more than 700,000 patients each year in the United States and sometimes can prove fatal.

Optimer Pharmaceuticals Inc.’s Dificid tablets were approved Friday to treat Clostridium difficile, an infection that usually affects older patients and can cause symptoms ranging from diarrhea to potentially life-threatening inflammation of the colon. It is most common in hospitals and nursing homes where the bacteria spores can be found in bed linens, bathrooms and medical equipment.

Clostridium difficile recently surpassed a type of staphylococcus bug as the most common hospital-acquired infection, according to researchers at Duke University Medical Center.

Optimer’s twice-a-day tablet is the first new drug approved for the infection in nearly 25 years, according to the company.

In recent decades, some varieties of germs have grown immune to popular antibiotics like penicillin, creating anxiety about the dwindling number of new antibiotics available. The new drug, known generically as fidaxomicin, is only the third antibiotic cleared by the FDA since 2006, according to Optimer.

‘‘When we started out, this was a relatively small segment of the market, and it was not clear how a drug could be better than vancomycin,’’ said Optimer Chief Executive Pedro Lichtinger, referring to the currently prescribed antibiotic. ‘‘But our scientists were able to see the growth of this market and the weakness of the current treatment.’’

In company trials, Dificid worked as well as vancomycin in treating the infection. The drug proved superior at preventing re-infection three weeks after treatment when compared with the older drug.