Methicillin sodium is a narrow spectrum β-lactam antibiotic related to Penicillin. β-lactams interfere with PBP (penicillin binding protein) activity involved in the final phase of peptidoglycan synthesis. PBP's are enzymes which catalyze a pentaglycine crosslink between alanine and lysine residues providing additional strength to the cell wall. Without a pentaglycine crosslink, the integrity of the cell wall is severely compromised and ultimately leads to cell lysis and death. Resistance to β-lactams is commonly due to cells containing plasmid encoded β-lactamases. Methicillin is mostly resistant to β-lactamases. Methicillin targets primarily the cell wall of Gram positive organisms especially Staphylococcus aureus. Water (Karl Fisher): ≤10%