The present invention relates to methods for preventing or treating infectious diseases caused by extracellular microorganisms, such as bacteria and fungi, by systemically administering to a patient a compound containing gallium. The extracellular microorganisms targeted by the present methods include methicillin-resistant Staphylococcus aureus (MRSA), vancomycin-resistant Enteracoccus faecalis (VRE), E. coli O157:H7, fluoroquinolorte-resistant Salmonella typhi, and the like. Furthermore, in the present methods, gallium compounds can be co-administered with one or more conventional antimicrobial agents to treat infectious diseases with reduced risks of creating multi-drug resistant pathogens. The methods of the present invention is also applicable to those microorganisms, such as ulcer-causing Helicobacter pylori, complete eradication of which so far has been difficult to achieve.
