Compositions and methods, including novel homogeneous microparticulate suspensions, are described for treating natural surfaces that contain bacterial biofilm, including unexpected synergy or enhancing effects between bismuth-thiol (BT) compounds and certain antibiotics, to provide formulations including antiseptic formulations. Previously unpredicted antibacterial properties and anti-biofilm properties of disclosed BT compounds and BT compound-plus-antibiotic combinations are also described, including preferential efficacies of certain such compositions for treating certain gram-positive bacterial infections, and distinct preferential efficacies of certain such compositions for treating certain gram-negative bacterial infections.