narrow-spectrum antibiotic

**Definition:** Narrow-spectrum antibiotics inhibit or kill a restricted group of bacteria. This contrasts with broad-spectrum agents (e.g., carbapenems, piperacillin-tazobactam, fluoroquinolones, 3rd/4th-generation cephalosporins, tetracyclines), which cover many gram-positive, gram-negative, and often anaerobic organisms.

**Why spectrum differs (pathophysiology):** - Spectrum depends on drug target, ability to penetrate the bacterial cell envelope, and susceptibility to bacterial resistance mechanisms. - Gram-negative bacteria have an outer membrane with porins. Large or hydrophobic drugs (vancomycin, penicillin G, macrolides) cannot cross it well, so they are mostly gram-positive-only. - Aminoglycosides require oxygen-dependent transport into the cell, so they are ineffective against anaerobes. They are mainly active against aerobic gram-negative rods. - Some drugs target pathways present only in specific organisms (e.g., isoniazid blocks mycolic acid synthesis, so it acts only on mycobacteria).

**High-yield examples:** - *Mainly gram-positive:* - Penicillin G/V: Strep, Treponema pallidum, Neisseria meningitidis, and anaerobes above the diaphragm. - Penicillinase-resistant penicillins (nafcillin, oxacillin, dicloxacillin): MSSA. - Vancomycin: MRSA, enterococci, and oral C. difficile. - 1st-generation cephalosporins (cefazolin): gram-positive cocci plus *Proteus, E. coli, Klebsiella* (the PEcK organisms). They are often called narrow, but their activity is somewhat broader. - Linezolid, daptomycin. - *Mainly gram-negative:* - Aminoglycosides (gentamicin, tobramycin). - Aztreonam: aerobic gram-negative rods only, with no gram-positive or anaerobic coverage. Safe in most penicillin-allergic patients. - Polymyxins. - *Organism-specific:* - Isoniazid, rifampin, and ethambutol for mycobacteria. - Metronidazole for anaerobes and protozoa. - Fidaxomicin for C. difficile. - Nitrofurantoin for uncomplicated UTI.

**Clinical significance:** - *Advantages:* - Less disruption of normal flora, so lower risk of C. difficile colitis and superinfection (e.g., candidiasis). - Less selection pressure for resistance (e.g., ESBL, VRE, MRSA). - Often lower toxicity and cost. - Targeted therapy is a core principle of antimicrobial stewardship. - *Disadvantages and when to use broad agents instead:* - They require a known or highly probable pathogen (from Gram stain, culture, or syndrome). - They are not appropriate for empiric therapy in sepsis, neutropenic fever, or polymicrobial infections (intra-abdominal infection, necrotizing fasciitis). - *Best practice:* Start broad empirically in critically ill patients, then de-escalate to a narrow-spectrum agent once culture and sensitivity results are available.

**Board pearls:** - Narrow spectrum does not mean weak. It means a limited range of organisms. - Vancomycin, aztreonam, and aminoglycosides are classic examples of limited coverage tested on Step 1. Aminoglycosides and aztreonam are gram-negative only. Vancomycin is gram-positive only, and its IV form does not cover gram-negatives. - Broad-spectrum use is the major risk factor for C. difficile infection (clindamycin, fluoroquinolones, cephalosporins, and ampicillin are classic). - Spectrum is distinct from bactericidal versus bacteriostatic activity, which describes the mode of action.