Staphylococcus aureus bacterium is a major cause of sometimes fatal hospital infections.

Strains of the bug have already proved resistant to all other antibiotics, but now scientists think it is on the brink of developing full resistance to their last defence - vancomycin.

Vancomycin was the last antibiotic that could kill all strains completely.

Doctors have warned that the appearance of such a strain could mean the era of antibiotics is at an end.

Hospital infections

The New England Journal of Medicine reports three cases of vancomycin-resistant S. aureus, one of them fatal.

In two of the cases, the vancomycin-resistant strain had not developed resistance to all other antibiotics and doctors were able to defeat it with a cocktail of drugs.

But in the third case, a combination of antibiotics failed and the 79-year-old patient died.

An isolated case of vancomycin-resistant S. aureus was reported in Japan in 1997. The increasing incidence has worrying implications for medicine.

Dr Alexander Tomasz is professor of microbiology at New York's Rockefeller University.

He said: "This is the tip of the iceberg. At the moment such strains are very, very rare.

"But they are not as susceptible as they used to be. They have moved up the threshold of resistance. It is a warning."

Beating resistance

Tests suggest that similar bacteria have accumulated the raw material needed to develop a full-blown resistance to the antibiotic.

One small mutation may be enough to complete the process.

Dr Theresa Smith of the US Centers for Disease Control and Prevention reported two of the cases in the journal.

Her report said the appearance of such bacteria "threatens to return us to the era before the development of antibiotics".

Dr Krzysztof Sieradzki of Rockefeller University reported on the fatal case.

War on bacteria

All four reported cases appeared in patients who had received repeated and prolonged treatment with vancomycin, Dr Smith and colleagues said.

They advised hospitals to lookout for vancomycin-resistant strains and be prepared to tackle it aggressively using a combination of antibiotics.

Dr Francis Waldvogel of University Hospital in Geneva commented on the reports in an editorial. He said they were unsurprising.

"The adaptive potential of the microbial world is such that for each new antibiotic that is introduced, several escape mechanisms are soon devised," he said.