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A select few. Just five out of hundreds of S. aureus strains are responsible for 70% of drug resistance cases, a study finds.

When Antibiotics Fail, a Few Strains Are at Fault

A handful of devilish strains may cause most of the world's drug resistant Staphylococcus aureus infections. Researchers have found that just five strains are overwhelmingly the culprits in more than 3000 samples of resistant S. aureus collected from patients around the world; the small number suggests that relatively few strains can easily develop resistance to antibiotics, allowing scientists to focus on these few and determine what makes them so virulent.

Before the widespread introduction of antibiotics in the early 1940s, S. aureus killed most people who contracted it, overwhelmingly hospital patients with weakened immune systems. Though antibiotics once tamed the bugs easily, strains have evolved resistance, the toughest to treat being methicillin-resistant S. aureus (MRSA). Scientists have remained divided over how easily the hundreds of different S. aureus strains acquire resistance, and whether slight genetic differences allow some strains to evolve drug resistance more easily.

To find out, microbiologist Alexander Tomasz of Rockefeller University in New York City teamed up with molecular geneticists Herminia de Lancastre and Duarte Oliviera, from the Universidade Nova de Lisboa, in Lisbon, Portugal. The scientists collected some 3000 samples of drug-resistant S. aureus from hospital patients in Europe, Latin America, the United States, and Japan. By comparing differences in the gene that confers antibiotic resistance, as well as other pieces of DNA, the team determined that 70% of their samples included just five bacterial strains. A closer look at these five revealed that they are descendents of two strains, one of which was responsible for the first reported case of MRSA, in Britain in 1961. The work suggests that only a few S. aureus strains have what it takes to become drug resistant. Still, the qualities that make it easier for certain bacteria to pick up the resistance gene, and other genes that enhance that one's performance, are unclear, the researchers report in the 1 March issue of Lancet Infectious Diseases.

"It's a very elegant and detailed piece of investigative work," says Frank Lowy, a microbiologist at Columbia University College of Physicians and Surgeons in New York City. Studying these two S. aureus families might reveal what genetic factors, from virulence to reproduction, make these particular bugs successful at evading antibiotics.

Related sites
CDC information on MRSA
Rockefeller Laboratory of Microbiology