CHICAGO - Doctors wrote 12 million antibiotic prescriptions in a single year for colds, bronchitis and other respiratory infections against which the drugs are almost always useless a study found.

Such indiscriminate use of antibiotics has contributed to the emergence of drug-resistant bacteria in the United States, the researchers said.

More than 90 percent of upper respiratory infections, including bronchitis and colds, are caused by a virus and are therefore impervious to antibiotics, researchers stated in the Journal of the American Medical Association."

In 1992, doctors prescribed antibiotics to two-thirrls of broil; chitis sufferers who visited them' iIt their offices andhaIf of common-cold sufferers and patients with other upper respiratory in fections, the researchers found.

That amounted to 12 million prescriptions, or one in every five antibiotic prescriptions written for adults that year, the researchers said.