WASHINGTON - Cases of a drug-resistant bacterial infection known as MRSA have risen by 90 percent since 1999, and they are increasingly being acquired outside hospitals, researchers reported on Tuesday.

They found two new strains of methicillin-resistant Staphylococcus aureus - MRSA for short - were circulating in patients and they are different from the strains normally seen in hospitals.

Ramanan Laxminarayan of Princeton University in New Jersey and colleagues studied data on lab tests from a national network of 300 microbiology laboratories in the United States for their study.

"We found during 1999-2006 that the percentage of S. aureus infections resistant to methicillin increased more than 90 percent, or 10 percent a year, in outpatients admitted to U.S. hospitals," they wrote in a report published in the journal Emerging Infectious Diseases.

"This increase was caused almost entirely by community-acquired MRSA strains, which increased more than 33 percent annually."

MRSA is now entrenched in U.S. hospitals. It was also known to be circulating in the community but it was not clear whether patients were carrying the infections out of hospitals, or the other way around.

Laxminarayan's team found that many more people were being diagnosed with the community-acquired strains, and these strains were not replacing the known hospital strains. Instead, they are just adding to the overall number of MRSA cases.

"Our findings have implications for local and national policies aimed at containing and preventing MRSA," they wrote.

For one thing, new, fast tests are needed so patients can be diagnosed and treated quickly. It is possible to treat MRSA but doctors need to know straight away so they start patients on the correct antibiotics.

"Lastly, infection control policies should take into account the role that outpatients likely play in the spread of MRSA and promote interventions that could prevent spread of MRSA from outpatient areas to inpatient areas," they added.

MRSA is one of the most common causes of hosptial-acquired infections. It can also now be picked up in schools, at fitness centers and elsewhere.

Symptoms range from abscesses to bloodborne infections that can kill quickly.

The researchers estimate that 20,000 people in the United States die each year from MRSA, and treating MRSA can range from $4,150 to more than $48,500 per case.