broad-spectrum antibiotic

**Definition:** Antibiotics with activity against many bacterial classes (gram-positive, gram-negative, often anaerobes and/or atypicals), in contrast to narrow-spectrum agents (e.g., penicillin G, vancomycin, which mostly cover gram-positives) that target limited groups.

**Examples (high-yield):** - Penicillins: ampicillin, amoxicillin (extended-spectrum); piperacillin-tazobactam (adds Pseudomonas and anaerobes) - Cephalosporins: 3rd/4th generation (ceftriaxone, cefepime) - Carbapenems: imipenem, meropenem, ertapenem (very broad; imipenem is given with cilastatin; seizure risk) - Fluoroquinolones: levofloxacin, ciprofloxacin, moxifloxacin - Tetracyclines: doxycycline, tigecycline - Macrolides (e.g., azithromycin) have moderately broad coverage, including atypicals - Chloramphenicol

**Clinical use:** - Empiric therapy in sepsis, febrile neutropenia, hospital-acquired pneumonia, intra-abdominal infections, and other polymicrobial infections, before culture and sensitivity results are available - Best practice is to de-escalate to a narrow-spectrum agent once the organism and susceptibilities are known (antibiotic stewardship)

**Adverse consequences:** - Disruption of normal flora, which leads to: - *Clostridioides difficile* infection or pseudomembranous colitis (clindamycin, fluoroquinolones, cephalosporins, and ampicillin are classic culprits) - Superinfection or overgrowth, such as *Candida* (oral thrush, vaginal candidiasis) - Selection for resistant organisms (MRSA, VRE, ESBL-producing Enterobacterales, carbapenem-resistant organisms) - Drug-specific toxicities (e.g., tendon rupture and QT prolongation with fluoroquinolones; tooth discoloration and photosensitivity with tetracyclines; gray baby syndrome and aplastic anemia with chloramphenicol)

**Board pearls:** - Anaerobic coverage is lacking in aminoglycosides (they need oxygen for uptake), so they are not truly broad-spectrum for anaerobes. - Vancomycin is a gram-positive agent (narrow), but it is often paired with a broad gram-negative agent in empiric regimens. - "Broad" does not mean "covers everything." Examples: ceftriaxone does not cover Enterococcus, MRSA, or Pseudomonas; ertapenem lacks Pseudomonas and Enterococcus coverage. - Always match the spectrum to the likely source and local resistance patterns.