Provided herein are methods and compositions for treatment of a microbial infection or a microbial biofilm in a subject or on a surface by administering to the subject or surface determined to have or suspected of having a microbial infection/film an antimicrobial agent in combination with a silver-containing compound (e.g., a silver salt). In some embodiments, a silver-containing compound can increase activity of the antimicrobial agent. In other embodiments, addition of a silver-containing compound to an antimicrobial agent can expand the antimicrobial spectrum of the antimicrobial agent such that the antimicrobial agent originally indicated for treatment of one microbial strain (e.g., Gram-positive microbes) becomes effective for treating additional microbial strains (e.g., Gram-negative microbes). Other aspects relating to methods and compositions for delivering an agent to a microbe by increasing the membrane permeability of the microbe are also provided herein.