Sept. 14, 2011 -- MRSA is also sometimes resistant to antibiotics found in over-the-counter ointments like Neosporin and Polysporin, a study shows.
The study is published in Emerging Infectious Diseases. It suggests that these ointments may be one of the factors behind the spread of an especially severe MRSA strain, called USA300, around the world.
It also means that antibiotic ointments probably wouldn't treat or prevent MRSA skin infection, though experts say they've never been recommended for that purpose.
A spokeswoman for the drug company that makes Neosporin and Polysporin says the study doesn't prove a link between the ointments and MRSA resistance to antibiotics.
MRSA stands for methicillin-resistant Staphylococcus aureus. The majority of MRSA infections are picked up in hospitals. But they are becoming more common in the community. MRSA bacteria are spread through skin-to-skin contact and often strike people who are prone to cuts and scrapes like children and athletes.
The infections may first be mistaken for a pimple. But they can quickly worsen into deep pus-filled sores. Such infections, if not promptly treated, can spread to the blood, lungs, and other organs and may become life-threatening.
The study tested 259 samples of MRSA bacteria that caused human infections treated at two hospitals in Japan.
Nineteen of the samples were USA300, a strain that is frequently found the U.S. but is rare in Japan. Scientists are worried about USA300 because it has features that make it especially dangerous.
In addition to being resistant to a host of antibiotics, for example, it makes a toxin that's responsible for its "flesh-eating" ability. It can also block the body's ability make to infection-fighting white blood cells. And USA300 appears to be replacing other, less severe MRSA strains as a cause of serious infections.
A study presented in June at the European Congress of Clinical Microbiology and Infectious Diseases in Milan, Italy, for example, found that while hospitalizations for MRSA infections in the U.S. increased only moderately between 2004 and 2008, hospitalizations related to USA300 strains more than tripled during that same period. Researchers grew all the MRSA samples on gel food in petri dishes alongside paper disks that were saturated with the antibiotics bacitracin and neomycin.
Bacitracin is among the active ingredients in Neosporin and Polysporin ointments and generic versions of those products. Neomycin is an active ingredient in Neosporin ointment.
Nearly half of the USA300 samples grew unhampered by the antibiotics bacitracin and neomycin, indicating that they were resistant to those drugs. Another USA300 sample was resistant to bacitracin, but susceptible to neomycin.
In contrast, none of the 240 samples of other MRSA strains found in Japan were resistant to bacitracin, though more than half of the other strains demonstrated at least partial resistance to neomycin.
"I think this indicates that spreading of USA300 may be related with problems in North America, specifically," says study researcher Yoshitsugu Iinuma, MD, PhD, professor in the department of infectious diseases at Kanazawa Medical University in Ishikawa, Japan, in an email. Iinuma notes that antibiotic ointment is rarely used in countries outside North America.