A new kind of genetic sleuthing suggests hospital outbreaks of drug-resistant staph bacteria don't always spread from one patient to another, but that numerous people - patients, visitors or staff - bring in the deadly germ.
The finding came as British scientists used in-depth gene scanning to track how a dangerous strain of this bacteria, called MRSA, has spread around the world.
The result, published Thursday in the journal Science: Practical evidence about why some hospital defenses fall short, and a possible new tool to help figure out better ones.
"To the lay public, MRSA just means one big, bad, ugly organism," said Dr. Buddy Creech, a Vanderbilt University infectious disease specialist who wasn't involved in the research but called it an important step - because it uncovers multiple subtypes that standard testing can't.
"It proves to us that not all MRSA are created equally," he said.
MRSA stands for methicillin-resistant Staphylococcus aureus, a form of the incredibly common staph family of germs. About one in every three people carries staph aureus in their noses with no symptoms but still can infect others; about 1 million people in the U.S. carry the MRSA type. The germs usually cause skin infections but can be deadly if they penetrate the bloodstream or organs, and MRSA is blamed for about 18,000 U.S. deaths a year.
Older tests hunt for a handful of bacterial mutations to classify MRSA into different strains.
Scientists at Britain's Wellcome Trust Sanger Institute went a big step further: They created genetic blueprints of 63 samples of MRSA collected around the world between 1982 and 2003, and tracked even minor DNA variation to trace how one common strain - called ST239 - spread, from continent to continent and within one hospital in Thailand.
The resulting family tree suggests this MRSA strain originated in Europe in the 1960s, just as antibiotics were being widely used. It spread to South America and became dominant in parts of Asia.
Perhaps most eye-opening: Of 20 ST239 patients during seven months in that Thai hospital, only five shared the same exact infection. The germs' genetic diversity showed the rest were coincidentally brought into the hospital.
Hospital-acquired MRSA has long been thought more dangerous than MRSA caught in ordinary community settings, perhaps because the already hospitalized are more vulnerable.
This study shows "there's a blurring in distinction" between the two, said study co-author Dr. Sharon Peacock of the University of Cambridge. Hospital efforts such as hand-washing and room-cleaning only help so much if MRSA spreaders keep walking in the front door, she said.
Britain last year began requiring that some patients be tested for MRSA upon hospital admission, she said, and some U.S. hospitals do that, too. But Vanderbilt's Creech cautioned that a simple yes-no MRSA swab doesn't give the detail necessary to tell which subtype could cause trouble.
This more sophisticated testing takes a few weeks and about $300 per sample, making it suitable mostly for researchers tracing outbreak hot spots. But Creech said similar study is under way to better understand MRSA's genetics in hopes of developing a vaccine.