[In vitro studies on the combined effects of arbekacin and fosfomycin on methicillin-resistant Staphylococcus aureus]

We investigated the bactericidal activity and post-antibiotic effect (PAE) of arbekacin (ABK) and fosfomycin (FOM) on clinical isolates of methicillin-resistant Staphylococcus aureus (MRSA) strain 1936, and the morphological changes of the bacterium. Both antibiotics were used in combination at concentrations lower than their respective MICs. The following results were obtained. 1. When the bacterium was simultaneously exposed to ABK and FOM, growth inhibition and a PAE longer than that effected by ABK alone were observed. 2. When exposed to ABK after treatment with and removal of FOM, bactericidal activity was stronger than that obtained when exposed to ABK alone. However, no increase in bactericidal activity was obtained when exposed to FOM after pretreatment with ABK. 3. Transmission electron microscopy revealed that, when the bacterium was exposed to ABK after treatment with and removal of FOM, large bacterial cells with cross walls at irregular sites and lysed cells showing destruction of the cell wall at the site of bacterial segmentation caused by ABK and FOM. These results demonstrated that ABK in combination with FOM shows stronger bactericidal activity against MRSA in vitro than ABK alone.