正确

错误

The fraction of time it is in use

The maximum power that can be produced

The average power that is produced

The ratio of average power to maximum power

正确

错误

Y

N

NG

Three billion years of evolution have turned S.aureus into a pretty mean bacterium. Although it is found on human skin, its preferred habitat is up the nose. When it gets inside the body, it can manifest itself as anything from harmless, pimples to life-threatening diseases, such as endocarditis (inflammation of the heart tissue) and septicaemia. The over use of antibiotics in the past fifty years means that S.aureus is now resistant to treatment. In America alone, every year 2 million people acquire bacterial infections while in hospital and 90,000 of them die as a result, according to the Centres for Disease Control and Prion in Atlanta. The methicillin-resistant strain of S.aureus, MRSA, is of particular concern. Infections are a growing global problem.

First spotted in 1961, MRSA is now endemic in many hospitals. In many Asian countries 70-80% of the strains isolated from diseased tissue have the MRSA form. of S.aureus. In America, the figure is around 40%. In Britain, the percentage of death certificates mentioning it as a factor contributing to death has shown a staggering rise since 1993. The disease is also hyper- endemic in Italy, Turkey and Argentina. In a study published earlier this year, among 500 otherwise healthy children attending a hospital outpatient clinic in Nashville, 9.2% had MRSA up their noses. The same study three years earlier had put this figure at 0.8%.

According to the Lancet, countries that have more or less ignored MRSA, such as China, South Korea and Japan, have some of the highest rates of incidence. Meanwhile, those with a low prevalence of MRSA, such as Finland, Denmark and the Netherlands, have high levels of surveillance and strictly enforced contact precautions. The bug's spread can be greatly reduced by scrupulous hygiene. Hospitals in the Netherlands isolate patients with MRSA and screen everyone who comes into contact with them. Once a patient has become iii with MRSA, there are only a few expensive antibiotics left that can treat it. Strains resistant to these are already emerging. It is a war between man and a bacterium, and the outcome is by no means certain.

Many people believe that the main stumbling block is a lack of new antibiotics. Fewer and fewer antibiotics are being discovered. Richard Wise, who chairs the committee on anti-microbial resistance for the Department of Health in Britain, is one of many who are concerned. Of those few new drugs that have emerged, he says, most are not sufficiently novel to combat resistance to old drugs. Antibiotics are not big earners for the pharmaceutical companies. Drugs for chronic conditions are far more profitable because they keep working and remain saleable, unlike antibiotics. In December this year, the British Department of Health will bring industry and academia together to try to address some of these problems in Europe. It won't be easy. Most drug companies have cut back on their efforts. An American study last year found that out of 506 drugs in development, only five were new antibiotics.

Others think that vaccines might be the answer. As antibiotics attack bacteria directly, this leads to an evolutionary pressure on the bacteria to evade this. Vaccines stimulate the body to mount its own, far more deft, defences. According to Alison Holmes, director of infection control