MIAMI BEACH – Antimicrobial resistance to Staphylococcus aureus varies by patient age, agent, and geographic region, but overall resistance patterns have remained steady over 5 years, according to a Florida-based study of nearly 300,000 isolates.
The analysis of 289,584 patient isolates collected in the Sunshine State from 2006 through 2010, revealed S. aureus resistance to oxacillin is running steady at approximately 50% over 5 years. Oxacillin is important because laboratories typically use resistance to this agent as a measure of overall methicillin-resistant S. aureus (MRSA).
With the exception of penicillin, resistance also remains stable for most antibiotics commonly deployed to combat S. aureus. Resistance to penicillin increased slightly from 91% in 2006 to 95% in 2010, said Dr. Richard S. Hopkins, acting state epidemiologist based in Tallahassee.
Quest Laboratories in Tampa provides the health department with a quarterly report of every S. aureus isolate result. Most of the data come from outpatient testing, which "gives you an idea of what is going on in the community," he said at the Florida Antimicrobial Stewardship Symposium sponsored by the University of Miami.
Resistant strains run highest among young children ages 1 year to 4 years old, Resistance to oxacillin exceeds 60% in that age group compared with approximately 50% for infants younger than 1 year and patients older than 4 years. These toddlers also tend to have S. aureus with the highest resistance rates to amoxicillin-clavulanic acid, cefazolin, and erythromycin.
Regarding geographic patterns of resistance, the panhandle of Florida consistently features a higher proportion of resistant S. aureus isolates. In contrast, the proportion tends to be lowest in south Florida. "We don’t know why the pattern is this way," said Dr. Hopkins, who studied geographical distribution along with his colleague, Catherine Lesko, an epidemiologist at the Florida Department of Health.
Although a few isolated S. aureus cases that demonstrate intermediate resistance to vancomycin are reported each year, the state health department has yet to see a strain completely resistant to vancomycin. "When we do, we will need to respond very strongly from a public health and clinical perspective," he said.
The intermediate resistant infections are almost always in hospitalized patients who have been treated with vancomycin, he said. "We think it’s evolution of vancomycin resistance right there in the patient under your care, rather than it is being spread from person to person in the hospital."
Dr. Hopkins is employed by the Florida Department of Health Bureau of Epidemiology.
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