By Julie Steenhuysen

CHICAGO, Sept 18 (Reuters) - Adding a new antibiotic to the standard mix of drugs used to treat tuberculosis could shave at least two months off the current grueling six-month regimen, U.S. researchers said on Tuesday.

By substituting the Bayer <BAYG.DE> antibiotic moxifloxacin for an older drug, researchers said they saw a 17 percent increase in effectiveness.

"Our finding shows that moxifloxacin is potent against tuberculosis," Dr. Richard Chaisson of Johns Hopkins University in Baltimore, said in an interview. "It shows very dramatically that people get better faster."

Chaisson, who presented his findings at the Interscience Conference on Antimicrobial Agents and Chemotherapy meeting in Chicago, said adding this antibiotic could cut treatment time by about two months.

"If we simplify treatment, it will be easier for people to take the drugs," he said.

People often do not take their full regimen of TB drugs, which has in turn spawned drug resistance, making TB more dangerous and more difficult to treat.

Shortening treatment time could help people stick to the prescribed therapy better and reduce the development of resistant strains, Chaisson said.

About 1.6 million people died from TB in 2005, according to the World Health Organization.

Chaisson's eight-week study, which was funded by the U.S. Food and Drug Administration, tested 170 men and women with TB in Brazil. They took the traditional mixture of drugs, which includes the antibiotic ethambutol, or a mix replacing ethambutol with moxifloxacin.

After two months, cultured sputum samples from moxifloxacin patients were far less likely to grow TB bacteria.

Sold under the brand name Avelox, moxifloxacin is approved for respiratory infections including pneumonia. It is marketed in the United States by Schering-Plough <SGP.N>.

TB researchers are already preparing to study this finding in larger clinical trials, with support from the Global Alliance for TB Drug Development.

"We really do hope to shorten therapy to four months," Melvin Spigelman of the alliance told reporters, adding that it will take a number of large trials to get there.

Meanwhile researchers at Johns Hopkins are working on even shorter treatment regimens.

A study led by Dr. Jacques Grosset showed the antibiotic rifapentine achieved cures in eight weeks or less in TB infected mice. Chaisson said he plans to study that combination in humans and he thinks it could yield even better results.

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