Mapping MRSA's Family Tree

Present-day methicillin-resistant Staphylococcus aureus (MRSA) might have been worse if it had descended directly from a 1950s version of the bug, according to a study co-authored by Barry N. Kreiswirth, PhD, a professor at the Public Health Research Institute of UMDNJ-New Jersey Medical School, and published in the Proceedings of the National Academy of Sciences.

In the early 1950s, a penicillin-resistant version of S. aureus known as phage-type 80/81 emerged in hospitals and quickly spread, causing a decade-long pandemic. The pandemic subsided in the mid-1960s, a few years after the introduction of a new type of antibiotic called methicillin.

By now, however, many bacteria found in hospitals and elsewhere have become resistant to methicillin, and scientists have suspected that these deadly modern MRSA strains descended directly from the 1950s strains. However the bugs DNA tells a different story. Kreiswirth, Frank DeLeo, PhD, of the National Institute of Allergy and Infectious Diseases and colleagues compared the DNA in a total of 284 old and new S. aureus strains and found that the newer strains are not directly descended from the older ones but rather that all evolved from a common ancestor.

Present-day MRSA strains had DNA mutations that rendered them less deadly but more adept at infecting the nasal passages, mutations that were not present in the 1950s strains. These genetic changes make MRSA less deadly in humans but better able to survive and spread, especially in hospital settings where the population has a higher susceptibility to infection than the general population. Had modern MRSA descended directly from phage type 80/81, the authors believe death rates among those infected by MRSA might have been even higher than have been experienced.