Emergence of MRSA bacteria that are increasingly resistant to vancomycin

Jun 17 2004

Researchers had previously thought that such bacteria were emerging from only one sort of the MRSA superbug, but the new study has shown that they have evolved in all major types.

The news has worried scientists because vancomycin has been the last antibiotic that could be relied on to kill the MRSA superbug that is responsible for many infections in hospitals and in the community.

This finding, published in the journal Emerging Infectious Diseases, suggests that bacteria which are fully-resistant to vancomycin may emerge as a major public health problem more quickly than previously thought.

The study, from the Universities of Bath, Bristol and Southmead Hospital in Bristol, examined MRSA from eight countries - the UK, the USA, France, Japan, Sweden, Poland, Norway and China. In these countries they found bacteria with increased resistance to vancomycin that had evolved from all five major types of MRSA.

The type of bacteria with increased resistance is called VISA, or Vancomycin-intermediate Staphylococcus aureus, which is a strain of bacteria that needs higher levels of vancomycin to kill it. Scientists believe that it may be an intermediate stage to the development of bacteria that are fully resistant to vancomycin.

Dr Mark Enright, of the University of Bath, who is the lead author of the recent paper, said he believed that resistance to vancomycin would become more common as more of the antibiotic was used to treat increasing numbers of people with MRSA infections.

No one drug would then be able to tackle the bacteria, though combinations of antibiotics might still be effective, he said.

"The results of our study show that the problem is much more serious than was previously thought," said Dr Enright, who is a senior research fellow in the Department of Biology and Biochemistry at the University of Bath.

"Whereas we had previously thought that the intermediate strain had developed from one lineage of MRSA known as the New York/Japanese clone, we have now found that all major MRSA have evolved resistance.

"It's only a matter of time, perhaps just years, before bacteria that cannot be killed by vancomycin develop in some areas. There have already been three cases of this in the USA but we believe these will become more common.

"With vancomycin being ineffective, doctors will then have to use combinations of antibiotics to try and tackle infections. But this can be a matter of trial and error before the right combination is found, and for very ill patients, any delay can be serious.

"There are some new antibiotics which hold out some hope, but we cannot be sure how effective they will be."

Dr Enright said that VISA strains had not been found more often in the past because current hospital technology did not ordinarily detect it. He stressed that fully-resistant bacteria had not been found in the UK by his study.

The scientists studied 101 samples of MRSA collected from the eight countries and identified nine distinct strains of the VISA.

Staphylococcus aureus are commonly-occurring bacteria found in the nose or on the skin of healthy people. These bacteria, often referred to simply as "staph," can occasionally cause a skin infection - such as pimples and boils - and most can be treated without antibiotics.

However, staph bacteria can also cause serious infections, such as surgical wound infections and pneumonia. These infections are normally treated with penicillin-related antibiotics but, over the past 50 years, staph bacteria have become resistant to various types of antibiotic. These resistant bacteria are called methicillin-resistant Staphylococcus aureus, or MRSA, after the type of antibiotics they are resistant to.

Vancomycin is an antibiotic that has usually to be injected because the body absorbs it poorly. As it is toxic, it has to be used very carefully and over long period of time, which allows the development of resistance.

The first staph bacteria with reduced vancomycin susceptibility were isolated in 1997 and similar isolates have since been discovered in several countries. These VISA isolates are relatively rare compared to the MRSA, which is increasingly common in UK hospitals - only 21 have been recorded to date. To date, three occurrences of fully-resistant (VRSA) bacteria have been identified.

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