Methicillin-resistant Staphylococcus aureus (MRSA) is unaffected by most antibiotics - only vancomycin is powerful enough to destroy it.

However, infection usually only occurs in hospitals before and its appearance among native American communities has alarmed doctors.

There have been several instances of MRSA evolving into a bacterium resistant to vancomycin, a development some specialists have said marks the end of the era of antibiotics.

Half affected

Teams from the Indian Health Service in New Mexico, the Centers for Disease Control - which monitors antibiotic-resistant bacteria - and the Minnesota health department found the bug in Minnesota among native Americans living in the countryside.

Bacteria develop resistance after long-term exposure to antibiotics

They checked 112 patients with Staphylococcus aureus infections and found that 62 of them had the methicillin-resistant form.

Only 16 of them had been in a hospital or care centre, or had any of the other risk factors for an antibiotic-resistant infection, leading the researchers to conclude the bacteria were at large in the community.

There have been previous accounts of infection occurring in the community -in Chicago and Canada - but these have been isolated incidents.

Barbara Murray, a specialist in infectious diseases at the University of Texas, said MRSA was no longer a problem isolated in hospitals.

"This reports it in rural American Indian communities - so it is out there in the real world," she said.

Danger

MRSA usually lives harmlessly in humans, either on their skin or in their nostrils.

However, if it gets into the blood stream it can cause blood poisoning or pneumonia.

Bacteria are more likely to develop resistance to antibiotics in hospitals because they contain patients who take antibiotics for long periods, sometimes months.

In this time, some bacteria are bound to survive, and - because these are genetically predisposed to resist the antibiotic - they multiply and evolve into a resistant form.

Dr Barry Cookson, of the UK's Public Health Laboratory Service, said there was always an increased risk of transmission in close-knit communities.

"Coughs and sneezes spread diseases - get enough people together and it will get passed around," he said.

This was why hospitals and nursing homes were natural breeding grounds for resistant strains of bacteria.

Community problems

But another well-recorded problem was that of the indigenous people in Western Australia, many of whom were infected with MRSA.

Because their communities were so closely knit, the bug had found a strong breeding ground and in some cases was transferring from the community into hospitals.

Dr Cookson said the Minnesota study showed that hospitals in the area would have to be careful.

"The hospitals need to make sure they are well prepared for this - they need to screen people as they come into hospitals - as they do now when people are transferred from nursing homes."