Methicillin sodium salt is primarily used in microbiological laboratories for antibiotic susceptibility testing. It helps detect methicillin-resistant strains of bacteria, especially Staphylococcus aureus (MRSA), by inhibiting the growth of susceptible organisms. Due to its resistance to degradation by certain bacterial enzymes, it serves as a key tool in identifying beta-lactamase-producing strains. Its use in clinical treatment has been discontinued due to adverse effects and the development of more effective antibiotics, but it remains important in research and diagnostic settings for studying antibiotic resistance mechanisms.