A new superbug, which is fully resistant to one of the most powerful antibiotics in the medical arsenal, has been identified in a 40-year-old hospital patient, increasing fears that science will have trouble keeping ahead of the spread of potentially harmful bacteria.

Staphylococcus aureus is a common bacterium carried in the nose and on the skin of 10% of the population, but it can cause serious blood poisoning if it enters a wound.

It was once treated easily with penicillin, but it has now mutated to the point where it is totally resistant to it. The next defence on the antibiotic ladder has been methicillin, but strains of the bug that are resistant to methicillin have become increasingly common, especially in hospitals. They are known as MRSA (methicillin-resistant Staphylococcus aureus) - and dubbed the superbug.

MRSA is treated with the antibiotic vancomycin, which has been regarded as the last defence. There have been a few cases of partial resistance to vancomycin, but now the first case of a staphylococcus bug that is totally resistant to the antibiotic has been detected in the US.

"This is very disturbing," said David Livermore, director of the public health laboratory service's antimicrobial resistance monitoring and reference laboratory in London.