Researchers Trace Origins of Superbugs

A new study examining the increasing growth of antibiotic-resistant Staphylococcus aureus has found that these microbes are all members of a few large families that have managed to spread around the world. The study was published in the March 1, 2002, issue of Lancet Infectious Diseases.

S aureus is the most versatile of human pathogens, readily surviving and proliferating in today's hostile, antibiotic-laden environment. Today, at least half of S aureus infections in U.S. hospitals are caused by strains resistant to many antibiotics, including penicillin, tetracycline, erythromycin and methicillin. Medical practitioners use the antibiotic vancomycin as a last resort therapy when fighting these resistant strains.

Scientists at the Laboratory of Microbiology at Rockefeller University (New York NY, USA) and from the Laboratory of Molecular Genetics at the New University of Lisbon (Portugal) conducted a major study of methicillin-resistant S aureus (MRSA), which became the prime cause of nosocomial infections worldwide six to seven years ago.

The study involved collecting a large number of MRSA strains, or clones, from diseased patients in 160 hospitals located in southern and eastern Europe, five Latin American countries, the United States and Japan. DNA fingerprinting techniques were used to identify and categorize the different clones. The findings were surprising. In more than two thirds of the 3,067 MRSA isolates, the researchers identified the fingerprints of as few as two drug-resistant clones of MRSA. "We kept seeing the same two bugs again and again,” says Duarte Oliveira, first author of the study.

One of the clones was a direct descendent of the very first MRSA strain detected in the United Kingdom in 1961. Members of this family have since spread in a pandemic fashion to cause disease in such faraway places as Argentina, Brazil, the Czech Republic, Hungary, Portugal and Spain. The second clone was identified as the cause of MRSA disease among 60 % of the 258 drug-resistant S aureus isolates collected at 29 health- care facilities in the United States in 1998. The same clone was found in abundance in a hospital in Tokyo, Japan, and represents almost half of all MRSA recovered from 12 New York City hospital patients infected with the bacteria in 1996.

"The secrets of the spectacular success of these S aureus lineages may be hidden in their unique genetic background and may ultimately lead to new strategies to help fight these dangerous microbes,” says Alexander Tomasz, Ph.D., head of the Laboratory of Microbiology at The Rockefeller University and second author of the paper.




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