Single MRSA Bacteria Strain Causes Severe Infections

Researchers have discovered that the most dangerous form of staph infection now occurring in the United States originates from a single strain of bacteria, not multiple strains as previously believed. These findings were published in the Proceedings of the National Academy of Sciences (PNAS).

Community-associated methicillin-resistant Staphylococcus aureus (CA-MRSA), an emerging public health concern, causes soft-tissue infections such as boils as well as life-threatening conditions that are difficult to treat. It is estimated that almost 100,000 cases of MRSA occurred in the United States in 2005 and almost 20 percent resulted in death.

Our study confirms that a single strain, called USA300, of CA-MRSA is responsible for many of the devastating infections which have spread rapidly across the U.S. in recent years, says James Musser, MD, PhD, co-author of the PNAS paper and co-director of The Methodist Hospital Research Institute in Houston. Musser is also the director of the Center for Molecular and Translational Human Infectious Disease Research Center at TMHRI.

The study, led by Frank R. DeLeo, PhD, at the National Institute of Allergy and Infectious Diseases Rocky Mountain Laboratories in Hamilton, Mont., found that the USA300 group of CA-MRSA strains collectively called the epidemic strain comprises nearly identical clones that have emerged from a single bacterial strain.

It is the first time scientists have used large-scale comparative genome sequencing to reveal the origins of epidemic CA-MRSA.

This discovery may lead to better tools to allow physicians to identify, diagnose, and treat this disease that has spread with extraordinary speed across the United States during the past five years.

To understand how CA-MRSA evolves in complexity and spreads geographically, researchers sequenced the genomes of 10 patient samples of the USA300 bacterium recovered from individuals treated for MRSA at different U.S. locations between 2002 and 2005. They then compared these genomes to each other and to a baseline USA300 strain used in earlier studies. Eight of the 10 USA300 patient samples were found to have nearly indistinguishable genomes, indicating they originated from a common strain. The remaining two bacteria were related to the other eight, but more distantly.

Of the eight nearly indistinguishable USA300 patient samples, two caused far fewer deaths in laboratory mice than the others, highlighting an emerging view that tiny genetic changes among evolving strains can profoundly affect disease severity and the potential for drug resistance to develop.

Other study collaborators included Fred C. Tenover, PhD, of the Centers for Disease Control and Prevention in Atlanta, and Barry N. Kreiswirth, PhD, of the International Center for Public Health (ICPH) in Newark, N.J. 

Source: Methodist Hospital, Houston