Treatment of localized abscesses induced by methicillin-resistant Staphylococcus aureus (MRSA) using MRgFUS: First in vivo results

Background. In the present work we study the therapeutic effect of focused ultrasound on localized abscess induced by methicillin-resistant Staphylococcus aureus (MRSA). MRSA is a major nosocomial pathogen in health-care facilities. The people, particularly those who are immunocompromised are prone to develop infectious sites that often are non-responsive to regular treatments. Because of its capability to induce a rise of temperature at a very precise location, the use of focused ultrasound represents a considerable opportunity to propose a new therapy for MRSA-related infections. Methods. A 50μL subcutaneous injection of MRSA strain USA 400 bacteria at a concentration of 7×10³/μL was made on the left thigh of BALB/c mice and an abscess of 6±2 mm-length formed after 48hrs. A transducer operating at 3 MHz with a focal length of 50mm and diameter of 32mm was used to treat the abscess. The focal point was positioned 2mm under the skin at the abscess center. Forty-eight hours after injection 4 ultrasound exposures of 9s-each were applied to each abscess under Magnetic Resonance-guidance. Each exposure was followed by a 1 min pause. Real-time estimation of change of temperature was done using a communication toolbox (matMRI) developed in our laboratory. Three experimental groups of 6 animals each were tested: moderate temperature (MT), high temperature (HT) and control. MT and HT groups reached, respectively, 55°C and 65°C at end of exposure. Effectiveness of the treatment was assessed by culturing bacteria of the treated abscess 1 and 4 days after treatment. Spleen samples were cultured to test for septicemia. Results. Macroscopic evaluation of treated abscess indicated a diminution of external size of abscess 1d after treatment. Treatment did not cause open wounds. Bacteria counting 1 day after treatment was 0.7±1.1 × 10⁵, 0.5±0.7 × 10⁵ and 1.1±2.3 × 10⁵ CFU/μl for MT, HT and control groups, respectively; for the 4-day end point, the count was 0.6±0.6 × 10⁴, 0.09±0.2×10⁴ and 1.0±1.3 × 10⁴. Absence of bacteria in spleen samples indicated that the therapy did not trigger septicemia. Conclusions. Focused ultrasound induces a therapeutic effect in abscesses induced by MRSA. This effect is observed as a reduction of the number bacteria in the abscess. These initial results indicate that focused ultrasound is a viable option for the treatment of MRSA-related infections.