High-level summary

Alt: Antibiotic spectrum basics chart by organism groups.

A compact reference of common antibiotics grouped by their usual activity against Gram-positive bacteria (Enterococcus, Staph, Strep), Gram-negative bacteria (enterics, Pseudomonas, ESBLs) and anaerobes, plus key agents for resistant organisms.

Quick-reference table (verbal)

• Narrow anti-staphylococcal penicillins (nafcillin, dicloxacillin): target MSSA and many strep species; not active vs MRSA.
• Aminopenicillins (amoxicillin, ampicillin): good for strep, some enterococci, and susceptible enterics; prone to beta-lactamase degradation.
• Beta-lactam/beta-lactamase inhibitors (amox/clav, amp/sulbactam): extend aminopenicillin coverage to many beta-lactamase producers and anaerobes.
• Anti-pseudomonal penicillins (piperacillin/tazobactam): broad gram-negative including Pseudomonas plus anaerobes.
• Carbapenems (imipenem, meropenem, doripenem): very broad (Gram+, Gram-, anaerobes) and active vs ESBL producers; ertapenem: similar but does NOT cover Pseudomonas.
• Cephalosporins: 1st gen (cefazolin/cephalexin) — MSSA, strep, some enterics; 2nd gen (cefuroxime) — improved gram-neg; 3rd gen (ceftriaxone) — broad community gram-neg and strep; ceftazidime — anti-Pseudomonas; 4th gen (cefepime) — broad gram-neg incl Pseudomonas.
• Monobactam (aztreonam): aerobic Gram-negative including Pseudomonas; useful in severe penicillin allergy.
• Glycopeptides & related: vancomycin — MRSA, most Gram+ (not VRE); daptomycin & linezolid — options for MRSA and VRE (daptomycin: not for pulmonary infections).
• TMP-SMX: good oral option for many enterics and community MRSA strains.
• Tetracyclines (doxycycline): covers atypicals, some MRSA and some Gram-negatives.
• Clindamycin: Gram+ and anaerobes (particularly above diaphragm), useful for toxin suppression in strep/staph.
• Aminoglycosides (gentamicin): potent Gram-negative activity and synergy for enterococcal endocarditis; notable nephro/ototoxicity.
• Fluoroquinolones: ciprofloxacin/levofloxacin — strong Gram-negative (cipro best for Pseudomonas); levo covers some Gram+/atypicals; moxifloxacin — better anaerobe/Gram+ but not Pseudomonas.
• Metronidazole: anaerobes (below diaphragm) and protozoa.
• Nitrofurantoin: oral agent for uncomplicated cystitis (E. coli); not for pyelonephritis.
• Macrolides (azithromycin): atypicals and some respiratory pathogens.

Important resistance-related points

• MRSA: treat with vancomycin, daptomycin (not for pneumonia), or linezolid.
• VRE: linezolid or daptomycin are first-line options.
• ESBL-producing Enterobacterales: often resistant to penicillins and cephalosporins; carbapenems (imipenem/meropenem/doripenem) are preferred.
• Pseudomonas: requires anti-pseudomonal agents (piperacillin-tazobactam, ceftazidime, cefepime, aztreonam, carbapenems except ertapenem, ciprofloxacin, aminoglycosides).

Drug-specific clinical pearls

• Ertapenem: broad-spectrum but no Pseudomonas coverage; useful for community ESBL or polymicrobial infections excluding Pseudomonas.
• Daptomycin: inactivated by lung surfactant — avoid for pneumonia.
• Aztreonam: safe in severe penicillin allergy; covers Gram-negatives only.
• Nitrofurantoin: oral, concentrates in urine — for cystitis only, not systemic infections.
• Metronidazole: excellent for anaerobes and C. difficile (treatment choice historically), but oral vancomycin or fidaxomicin now preferred for severe C. difficile.
• Aminoglycosides: monitor levels; nephro- and ototoxicity risks.

How to choose empiric therapy (practical steps)

1. Identify likely pathogens by site of infection and host factors (community vs healthcare-associated, recent antibiotics).
2. Assess need for Pseudomonas / ESBL / MRSA coverage.
3. Check allergies — if severe beta-lactam allergy, consider aztreonam (Gram-neg), vancomycin/linezolid (Gram+).
4. Start broad empiric therapy for severe infections; narrow (de-escalate) once culture/susceptibilities return.
5. Consider drug properties: oral bioavailability, tissue penetration (e.g., lung, CSF), renal/hepatic dosing, toxicity.

Common clinical examples (concise)

• Uncomplicated cystitis: nitrofurantoin or TMP-SMX (based on local resistance).
• Skin & soft tissue (non-purulent): beta-lactam active vs streptococci (e.g., cefazolin); purulent MRSA — TMP-SMX or doxycycline.
• Community-acquired pneumonia: ceftriaxone + macrolide or respiratory fluoroquinolone (site/severity dependent).
• Intra-abdominal/aspiration: agents with anaerobic coverage (piperacillin-tazobactam, ertapenem, or ceftriaxone + metronidazole).
• Sepsis/critically ill: broad Gram-negative + Gram-positive coverage including anti-pseudomonal and anti-MRSA agents until cultures guide therapy.

Rapid checklist for exams/wards

• If suspect MRSA → add vancomycin/linezolid/daptomycin.
• If suspect Pseudomonas → choose anti-pseudomonal beta-lactam (cefepime, ceftazidime, piperacillin-tazobactam) ± aminoglycoside or ciprofloxacin.
• If suspect ESBL → use carbapenem (imipenem/meropenem/doripenem); ertapenem if Pseudomonas unlikely.
• For anaerobes below the diaphragm → metronidazole or beta-lactam/beta-lactamase inhibitor or carbapenem.

Final reminders

• Always tailor antibiotics to culture results and local antibiogram data.
• Weigh efficacy vs toxicity, site penetration, and route (IV vs oral) when switching therapy.
• De-escalate promptly to limit resistance and adverse effects.