Widespread 'superbug' is expert at acquiring drug-resistance

August 08, 2001

The findings may lead to new strategies for combating the infectious microbe

One of the most widely disseminated strains of an antibiotic-resistant bacterium responsible for hundreds of infections in European hospitals can be traced back to the 1950s, according to researchers at The Rockefeller University. Using the molecular tool called DNA fingerprinting, they have shown that this persistent lineage of Staphylococcus aureus is an expert at acquiring resistance to antibiotics.

"The capacity of this bacterium to acquire resistance traits against antibiotics is amazing," says Alexander Tomasz, Ph.D., co-author of the paper and head of the Laboratory of Microbiology at Rockefeller.

The findings, reported in the August 14 issue of The Proceedings of the National Academy of Sciences, provide new insight into why this particular staphylococci lineage is so successful and ultimately may lead to the discovery of new antibiotics.

According to a recent report by the World Health Organization, "Drug-resistant infections in rich and developing nations alike are threatening to make once treatable diseases incurable." This chilling announcement fits most accurately Staphylococcus aureus, the number one cause of potentially life-threatening hospital-borne infections in the United States and all over the world.

Although many of the spectacular early successes of antibiotics were observed in this microbe, clinical records in hospitals today tell a very different story: an alarmingly large proportion of staphylococci have become resistant to virtually all antibiotics, precluding their use for therapy and earning the name "superbug" for this microbe in the popular press.

Today, almost half of all staphylococcal infections in U.S. hospitals are caused by methicillin-resistant (MRSA) strains. These strains have acquired a piece of DNA, called the mecA gene, which not only confers resistance to methicillin but to an entire class of antibiotics, called b-lactam antibiotics, which includes penicillin.

"Of all the numerous drug-resistance mechanisms acquired by staphylococci the most devastatingly effective one was the acquisition of mecA since it provides the bacteria with a wide-spectrum resistance against the largest and most effective group of antimicrobial agents," says Hermínia de Lencastre, Ph.D., senior research associate at Rockefeller and head of the Laboratory of Molecular Genetics at the Instituto de Tecnologia Química e Biológica, Universidade Nova de Lisboa, Portugal, who led the current research effort.

In order to better understand the origin of MRSA, Rockefeller researchers in the Laboratory of Microbiology combined forces with scientists from the Laboratory of Molecular Genetics at the Instituto de Tecnologia Química e Biológica and the State Serum Institute in Denmark. The aim of the group was to identify the nature of the first staphylococcal strain to receive the mecA gene.

The very first MRSA was detected in 1961 in a British hospital within one year of the introduction of the methicillin class of antibiotics into clinical practice.

Shortly afterwards, in 1963, MRSA appeared among staphylococci causing blood stream infections in Danish hospitals. Danish colleagues had saved all blood isolates of staphylococci since 1957, including both drug-susceptible and drug-resistant strains.

Using a combination of various molecular techniques, the researchers were able to obtain a characteristic fingerprint of the first MRSA isolates from the United Kingdom and Denmark.

Interestingly, these early isolates from the two countries proved to be identical. Equipped with this information the scientists then began a search among methicillin-susceptible strains collected in the same era in order to find bacteria with the same molecular portrait as that of the first MRSA strains.

To the researchers' surprise, they found that the staphylococci that must have served as the first recipients of the mecA gene were not only abundant among the clinical isolates of this era, but these bacteria already carried drug-resistance traits to as many as four antibiotics: penicillin, streptomycin, tetracycline and?often?to erythromycin as well.

Each one of these four antibiotics was introduced into therapy between 1943 and 1960?prior to the year when the methicillin type of antibiotics were first put into clinical practice. "It was like recognizing that a convict already had a long criminal history," says Tomasz.

"The second surprise was even more startling," says Inês Crisóstomo, first author of the paper and a visiting student at Rockefeller from the Instituto de Tecnologia Química e Biológica. "Using the same molecular techniques we produced DNA fingerprints of MRSA currently most abundant in hospitals. It turned out that an MRSA clone responsible for hundreds of infections in European hospitals was a direct descendant of the first MRSA identified in England and Denmark."

Strains belonging to this so-called "Iberian" clone, first identified in a hospital outbreak in Barcelona, Spain, in 1986, were resistant to even more antibiotics than their ancestors from the 1960s.

In addition to penicillin, streptomycin, tetracycline, erythromycin and methicillin, these strains also were resistant to just about every one of the generic antibiotics developed during the last 50 years, with the exception of vancomycin?an antibiotic now used as last resort therapy against MRSA infections.

The Iberian clone has spread throughout hospitals in Spain, Portugal, France, Belgium, Italy, Scotland, Germany and Poland, and also caused a recent outbreak in New York.

"DNA fingerprints have identified a very large number of different staphylococcal strains, each carrying a variety of drug-resistance traits. But only a very few of these lineages have successfully spread around the world like the Iberian clone," says de Lencastre.

"The stability of this clone over a period of close to a half century documents the remarkable fitness of this staphylococcal lineage."

"The most challenging question to answer next will be what factors, what genetic determinants are responsible for the evolutionary success of such epidemic strains," says Tomasz. "Understanding what makes these microbes different, will help us learn how to block their spread."
The research was funded by a grant from the U.S. Public Health Service to Tomasz and by a grant from the Fundação para a Ciência e Tecnologia (FCT), Ministry of Science and Technology, Lisbon, Portugal, to de Lencastre.

This paper is available online at The Proceedings of the National Academy of Sciences Web site: http://www.pnas.org/cgi/content/full/161272898v1.

Tomasz is the Plutarch Papamarkou Professor of Microbiology and Infectious Diseases at The Rockefeller University. Other authors include Henrik Westh, M.D., Ph.D., from the Hvidovre Hospital and the Statens Serum Institute in Copenhagen, Denmark; Duarte C. Oliveira, a visiting student from the Instituto de Tecnologia Química e Biológica; and Marilyn Chung, B.S., from Rockefeller.

John D. Rockefeller founded Rockefeller University in 1901 as The Rockefeller Institute for Medical Research. Rockefeller scientists have made significant achievements, including the discovery that DNA is the carrier of genetic information. The University has ties to 21 Nobel laureates, six of whom are on campus. Rockefeller University scientists have received the award in two consecutive years: neurobiologist Paul Greengard, Ph.D., in 2000 and cell biologist Günter Blobel, M.D., Ph.D., in 1999, both in physiology or medicine. At present, 33 faculty are elected members of the U.S. National Academy of Sciences, including the president, Arnold J. Levine, Ph.D. Celebrating its centennial anniversary in 2001, Rockefeller?the nation?s first biomedical research center?continues to lead the field in both scientific inquiry and the development of tomorrow?s scientists.

Rockefeller University

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