The present invention provides methods for treating or preventing diseases and disorders caused by iron-dependent pathogenic microorganisms, such as bacteria, fungi, and parasites, by applying a gallium compound to an affected area. In particular, the present invention provides methods for treating or preventing dental caries, vaginal infections, skin infections, and so forth. Gallium compounds can be formulated as toothpaste, mouthwash, cream, ointment, gel, solution, eye drops, suppository, and the like. Furthermore, the invention provides methods for controlling microbial growth on environmental surfaces, including those of toothbrush, denture, dental retainer, contact lens, catheter, food stuff, and so forth. In addition, the present invention provides animal feeds which contain gallium compounds that promote the animal growth and prevent the animals from infections as well as protect consumers from post processing infections.