Antibiotic spectrum refers to the range of bacterial species susceptible to a given antimicrobial agent, and optimal selection balances the need for effective coverage against the causative pathogen with the principle of minimizing collateral ecological damage. Narrow-spectrum antibiotics target a limited range of organisms: penicillin G kills primarily gram-positive cocci (Streptococcus pyogenes, Streptococcus pneumoniae) by inhibiting transpeptidase (PBP) during cell wall synthesis; isoniazid targets only Mycobacterium tuberculosis by inhibiting mycolic acid synthesis. Broad-spectrum antibiotics cover diverse gram-positive, gram-negative, and sometimes anaerobic organisms: carbapenems (meropenem, imipenem) have the broadest beta-lactam coverage, binding multiple PBPs across species; fluoroquinolones (levofloxacin, moxifloxacin) inhibit DNA gyrase and topoisomerase IV in both gram-positive and gram-negative bacteria. The antimicrobial stewardship principle of 'start broad, narrow early' guides clinical practice: in critically ill patients with unknown pathogens (sepsis, neutropenic fever), empiric broad-spectrum coverage is initiated immediately to avoid delays in effective therapy (each hour of delay in sepsis increases mortality by 7-8%). Once culture and sensitivity results return at 48-72 hours, therapy is de-escalated to the narrowest effective agent — this is the most important stewardship intervention. Unnecessary...