Scientists have made a major breakthrough in the war against superbugs, which attack hospital patients and are increasingly resistant to treatment, by unravelling the genetic codes of two of the most feared antibiotic resistant organisms.

Scientists have made a major breakthrough in the war against superbugs, which attack hospital patients and are increasingly resistant to treatment, by cracking the genetic codes of two of the most antibiotic-resistant organisms.

Researchers in Japan have worked out the chemical sequences making up the DNA of two drug resistant strains of the infectious bug Staphylococcus aureus. One, methicillin resistant Staphylococcus aureus (MRSA) has become a major problem in hospitals where it infects people who have undergone operations.

The other, VRSA, is a variant strain resistant to the "last resort" antibiotic vancomycin. Scientists hope the genome sequences will unlock secrets about the bugs which may lead to new ways of combating them.

Keiichi Hiamatsu and colleagues from the Department of Bacteriology at Juntendo University, Tokyo, described the work today in the Lancet medical journal. Among the findings was the discovery of 70 new genes affecting virulence which could be potential targets for new drugs.

The research also showed that the genes of S. aureus have been partly acquired from a wide variety of living things, ranging from different bacteria to human beings. This characteristic, combined with the repeated duplication of harmful genes, give the bug much of its immense infective power.

A third discovery was that critical antibiotic-resistance genes exist as mobile lengths of DNA called "plasmids". They have the worrying ability to spread between strains of S. aureus as well as across species boundaries.

Fourthly, five new classes of genes were discovered, labelled "pathogenicity islands," that produce potentially lethal toxins. Among other conditions, they are responsible for toxic shock syndrome, which can be triggered by wearing tampons.

Over the last 10 years, MRSA has become a major cause of post-operative hospital infection worldwide. Between 1990 and 2000, the proportion of reported S. aureus infections in England and Wales that were resistant to methicillin rose from 1.7 per cent to 42.1 per cent, according to figures from the Public Health Laboratory Service.

The total number of recorded S. aureus infected blood samples in that time soared from 4,890 a year to 11,162. MRSA can kill when it triggers blood poisoning, and it may also contribute to deaths from other causes. The number of people who die because of MRSA is unknown, but hospital infections cause 5,000 deaths each year in England alone.

For a time, vancomycin was the only antibiotic effective against MRSA. But in 1997, a vancomycin-resistant version of the bug emerged and now poses a serious threat.

Two experts commenting on the research in the Lancet, Dr Dlawer Ala'Aldeen (correct) and Dr Hajo Grundmann, from the University Hospital of Nottingham, said the discovery of 70 new virulence genes could be significant.

They said: "If proven important, these, and those found previously, may become possible targets for the design of novel therapeutic agents as well as vaccines." However, they said developing vaccines against S. aureus would be "extremely difficult".