A hospital strain of antibiotic-resistant staph that causes bloodstream infection is five times more deadly than other staph strains, U.S. researchers have found.

The strain, called USA600, seems to be partly immune to an antibiotic use to treat the infection, researchers from Henry Ford Hospital in Detroit said at an annual meeting of the Infectious Diseases Society of America in Philadelphia.

"While many MRSA strains are associated with poor outcomes, the USA600 strain has shown to be more lethal and cause high mortality rates," said Carol Moore, a research investigator in Henry Ford's Division of Infectious Diseases and lead author of the study.

"In light of the potential for the spread of this virulent and resistant strain and its associated mortality, it is essential that more effort be directed to better understanding this strain to develop measures for managing it," she added in a release.

MRSA, or methicillin-resistant Staphylococcus aureus, causes infections in the skin and bloodstream. It can also infect surgical wounds and cause pneumonia.

MRSA mostly affects people in hospitals, but there have also been cases outside health-care settings.

The study found that half of patients infected with the strain died within 30 days compared to 11 per cent of patients infected with other MRSA strains.

The average 30-day mortality rate for MRSA bloodstream infections ranges from 10 per cent to 30 per cent, the researchers said.

The USA600 strain appears to be more immune than other strains to the antibiotic vancomycin, which is often given to treat other MRSA strains.

People infected with the USA600 strain tended to be older. The average age of patients with the USA600 strain was 64 compared with 52 for other MRSA strains.

It's not clear whether age, other diseases or spread of infection contributed to the higher mortality rate among those infected with USA600.

The meeting ended on Sunday.