CA-MRSA strains found to be increasingly deceptive

October 2005

A study of how the immune system reacts to antibiotic-resistant strains of community-associated Staphylococcus aureus has shown for the first time that these strains are more deadly and better at evading human immune defenses than S. aureus strains that originate in hospitals and other health care settings.

Infections from community-associated methicillin-resistant S. aureus (CA-MRSA) are difficult to treat and are increasing at an alarming rate, according to a National Institute of Allergy and Infectious Diseases (NIAID) release.

Scientists at NIAID’s Rocky Mountain Laboratories (RML) in Hamilton, Mont., examined the ability of MRSA strains to cause disease in mice and avoid destruction by human neutrophils, which make up about 60% of all white blood cells and are the first line of defense against bacteria. Scientists know that community-associated strains differ from hospital strains, but they do not know why community strains cause more serious infection in otherwise healthy people.

A study published in The Journal of Immunology identified specific S. aureus genes that potentially control the bacterium’s escape from neutrophils. Among thousands of S. aureus genes analyzed in the five different strains used in the study, the scientists identified a large group of genes whose role in helping spread infection is unknown. Frank DeLeo, PhD, the investigator who led the study, and colleagues plan to determine if some of the unknown genes help promote disease. If the researchers can learn how genes control the ability of S. aureus to evade and destroy neutrophils, their work could lead to new medical treatments.

S. aureus strains acquired in health care settings can be challenging to resolve because of antibiotic resistance, which limits the choices for treatment. The situation, however, can become more serious with newer community-associated strains, DeLeo said. “We do not know why cases of community-[associated] MRSA infections are increasing, let alone how they flourish,” he said in a release.

Treatment challenge

Physicians can easily treat mild S. aureus infections such as impetigo. But, S. aureus can be more severe and difficult to treat, affecting multiple vital organs and leading to producing toxins poisoning the blood and infection overwhelming the heart that can have systemic manifestations that lead to a higher mortality and complications.

“The reason that some mild infections become severe or fatal is not well understood,” said Jovanka Voyich, PhD, of RML. “These results suggest that community-[associated] MRSA causes disease in healthy people, in part, because it has [an] enhanced ability to circumvent killing by neutrophils.”

For more information:

• Voyich J, Braughton KR, Sturdevant DE, et al. Insights into mechanisms used by Staphylococcus aureus to avoid destruction by human neutrophils. J Immunol. 2005;175:3907-3919.