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CDC: MRSA USA300 Strain Resistant to Topical Antibiotics

Most MRSA isolates susceptible to bacitracin, but USA300 isolates resistant to bacitracin, neomycin

FRIDAY, Sept. 16 (HealthDay News) -- Most methicillin-resistant Staphylococcus aureus (MRSA) isolates are susceptible to bacitracin, but MRSA USA300 isolates show resistance to bacitracin and neomycin, according to a study published online Sept. 14 in the U.S. Centers for Disease Control and Prevention's Emerging Infectious Diseases.

Masahiro Suzuki, Ph.D., from the Aichi Prefectural Institute of Public Health in Nagoya, Japan, and colleagues investigated the susceptibility of MRSA isolates, including the USA300 clone, to the antimicrobial drugs in over-the-counter (OTC) triple-antibiotic ointments (TAO). A total of 259 MRSA isolates, including nine USA300 isolates from two strains, were collected and tested for susceptibility to bacitracin and neomycin using the Kirby-Bauer disk diffusion method. The agar dilution method was used to determine the maximum inhibitory concentration (MIC) of bacitracin, neomycin, and polymyxin B (PL-B) for USA300 strains.

The investigators found that most USA300 isolates were resistant to both bacitracin and neomycin, whereas nearly all MRSA isolates that were a type other than USA300 were susceptible to bacitracin. Intermediate resistance to neomycin was observed in 55 percent of the non-USA300 MRSA isolates, while 4.5 percent were resistant to neomycin. The MICs of USA300 isolates with resistance to both bacitracin and neomycin for bacitracin, neomycin, and PL-B were 400 units/mL, 128 µg/mL, and 400 units/mL, respectively. The TAOs had approximately 10 to 30 times higher concentrations of neomycin and PL-B than the MICs of both the antibiotics.

"The emergence of MRSA USA300 depends partly on the virulence of MRSA USA300, but it may be influenced by usage of OTC drugs," the authors write.

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