IDSA coverage

USA 300’ MRSA clone becoming dominant strain

Infections are susceptible to clindamycin

The latest evidence of the continuing emergence of a single predominant strain of community-acquired methicillin-resistant Staphylococcus aureus (CA-MRSA) comes from Atlanta, where hospital-based investigators found the so-called "USA 300 clone" is causing 90% of incoming skin and soft tissue infections.

The 300 clone, which gets its name from its identifying markers using pulsed-field molecular epidemiology, was reported last year by the Centers for Disease Control and Prevention (CDC). Investigators found the strain typically is resistant to beta-lactam drugs and erythromycin but susceptible to clindamycin.1

The 300 clone appears to be outstripping USA 400 clone, which caused four fatal infections in children that signaled the emergence of CA-MRSA in the late 1990s. That finding was strongly reflected in research presented recently in Boston at the annual meeting of the Infectious Disease Society of America (IDSA).

"This [USA 300] is widely circulating in the U.S.," says Mark King, MD, MS, an epidemiologist at the Emory University School of Medicine in Atlanta. King and colleagues conducted laboratory surveillance for S. aureus (SA) isolated from skin and soft tissue infections within 48 hours of admission or at an outpatient visit.

"We were trying to [target] people who truly had community-onset infections," he tells Hospital Infection Control. "We looked at all of all our consecutive skin and soft tissue infections that were caused by Staph aureus over 3½ months that fit the definition of community onset."

MRSA accounted for 279 of 389 identified infections, while the remainder were due to susceptible staph strains. Among all staph isolates, 243 (62%) were considered CA-MRSA Looking only at MRSA, they found that 243 (87%) of 279 total infections were due to CA-MRSA. When restricted only to isolates in which pulsed-field analysis, an overwhelming 159 (90%) of 176 isolates were the USA 300 strain.

"We compared the types we got with the CDC standard types and showed that about 72% of our community-onset staphylococcal skin and soft tissue infections are caused by MRSA," King says. "Ninety percent were USA 300, and only 1% were associated with USA 400."

Asked if he saw any cases of MRSA associated necrotizing fasciitis, which was also reported at IDSA, King says, "We have seen a few cases you can probably define as that. That wasn’t the goal of our study, and we didn’t look carefully at that. But we have seen invasive infections, there is no doubt about it."

In the report given at IDSA, 14 cases of necrotizing fasciitis caused by CA-MRSA occurred in Los Angeles. All patients survived, but three needed skin grafts. The infections were so serious that 10 of the patients had to be admitted to intensive care units. Part of the problem appeared to be initial empiric therapy, as MRSA still remains an unsuspected pathogen coming in from the community. All patients were eventually cleared of infection with vancomycin and/or clindamycin.

Lead investigator Loren Miller, MD, MPH, at the LA Biomedical Research Institute called the fasciitis cases "about as serious an infectious disease emergency as you can get."

In other MRSA news at the meeting, investigators reported that infection with the resistant bacterium can severely complicate influenza. They looked at 17 cases from nine states — 15 (88%) of which were due to MRSA — the investigators found that otherwise healthy people can develop fatal pneumonia when coinfected with both the bacterium and flu virus.


1. McDougal LK, Steward CD, Killgore GE, et al. Pulsed-field gel electrophoresis typing of oxacillin-resistant Staphylococcus aureus isolates from the United States: Establishing a national database. J Clinic Micro 2003; 41:5,113-5,120.