An antimicrobial composition includes an aqueous and/or ethanolic solution of iodine and hydrogen peroxide, which is acidified. The acidifying agent may be aspirin, hydrochloric acid, or other acidic agent tolerable to the body. The composition may also include a witch hazel extract, or alternately hamamelitannin, gallic acid, or other components thereof that effect quorum signaling inhibition that blocks and erodes microbe-induced biofilm on cellular and device surfaces. Acetohydroxamic acid may be combined with the composition as a counter to microbe-induced alkalinization, particularly in urine. Moreover, a metal, halogen, and combinations of these, such as a silver colloid, may be added as a secondary microbicidal agent. The antimicrobial composition may be applied topically to or instilled through an indwelling urinary catheter or applied as a device surface-coating on other medical devices.