Antibiotic Spectrum Calculator
Identify common pathogens by infection site and select appropriate empiric antibiotic therapy. View antibiotic coverage spectrum including Gram-positive, Gram-negative, anaerobic, and atypical organisms.
Infection Details
Patient Risk Factors
Antibiotic Coverage Spectrum
| Antibiotic | Gram (+) | Gram (-) | Anaerobes | Atypicals | Notes |
|---|---|---|---|---|---|
| Penicillin G | +++ | - | + | - | Streptococci, some anaerobes |
| Amoxicillin | +++ | + | - | - | Strep, some E. coli |
| Amoxicillin-Clavulanate | +++ | ++ | ++ | - | Broad spectrum, β-lactamase coverage |
| Nafcillin/Oxacillin | +++ | - | - | - | MSSA (not MRSA) |
| Piperacillin-Tazobactam | ++ | +++ | +++ | - | Pseudomonas coverage |
| Cefazolin (1st gen) | +++ | + | - | - | Surgical prophylaxis, MSSA |
| Ceftriaxone (3rd gen) | ++ | +++ | - | - | Broad Gram-negative, CNS penetration |
| Cefepime (4th gen) | ++ | +++ | - | - | Pseudomonas, nosocomial infections |
| Meropenem | +++ | +++ | +++ | - | Broadest spectrum, reserve for resistant organisms |
| Aztreonam | - | +++ | - | - | Gram-negative only, safe in PCN allergy |
| Vancomycin | +++ | - | + | - | MRSA, resistant Gram-positives |
| Daptomycin | +++ | - | - | - | MRSA, VRE (not for pneumonia) |
| Linezolid | +++ | - | - | - | MRSA, VRE |
| Ciprofloxacin | + | +++ | - | ++ | Pseudomonas, atypicals |
| Levofloxacin | ++ | +++ | - | +++ | Respiratory fluoroquinolone |
| Azithromycin | ++ | + | - | +++ | Atypicals, pertussis |
| Doxycycline | ++ | + | + | +++ | Atypicals, tick-borne |
| Clindamycin | +++ | - | +++ | - | Anaerobes, CA-MRSA |
| Metronidazole | - | - | +++ | - | Anaerobes only, C. diff |
| Trimethoprim-Sulfa | ++ | ++ | - | - | UTI, CA-MRSA, PCP |
Antibiotic Stewardship Principles
Key Concepts
Empiric Therapy
- • Based on most likely pathogens
- • Consider local resistance patterns
- • Broader coverage initially if severe
- • Start within 1 hour for sepsis
De-escalation
- • Narrow based on culture results
- • Use narrowest effective spectrum
- • Reduces resistance development
- • Reassess at 48-72 hours
Organism Categories
- •Gram-Positive: Staphylococci, Streptococci, Enterococci (thick cell wall, purple stain)
- •Gram-Negative: E. coli, Klebsiella, Pseudomonas (thin cell wall, pink stain)
- •Anaerobes: Bacteroides, Clostridium, Peptostreptococcus (grow without oxygen)
- •Atypicals: Mycoplasma, Chlamydia, Legionella (no cell wall, require special antibiotics)
Special Situations
- •Pseudomonas Coverage: Needed for severe HAP/VAP, neutropenic fever, severe burns. Use piperacillin-tazobactam, cefepime, or meropenem
- •MRSA Coverage: Add vancomycin, daptomycin, or linezolid for suspected MRSA (IV drug use, recent hospitalization, severe cellulitis)
- •Anaerobic Coverage: Needed for aspiration pneumonia, intra-abdominal infections, diabetic foot infections. Use metronidazole or β-lactam/β-lactamase inhibitor
Frequently Asked Questions
What does "broad spectrum" mean?
Broad-spectrum antibiotics are effective against a wide range of bacteria, including both Gram-positive and Gram-negative organisms. Examples include piperacillin-tazobactam and carbapenems (meropenem). While useful for empiric therapy in severe infections, they should be narrowed once the causative organism is identified to prevent resistance.
When should I cover for Pseudomonas?
Pseudomonas coverage is indicated for hospital-acquired pneumonia, ventilator-associated pneumonia, neutropenic fever, severe burns, cystic fibrosis exacerbations, and in patients with recent healthcare exposure or prior Pseudomonas infections. Use antipseudomonal β-lactams (piperacillin- tazobactam, cefepime), fluoroquinolones (ciprofloxacin), or carbapenems.
What is antibiotic de-escalation?
De-escalation is the practice of switching from broad-spectrum to narrow-spectrum antibiotics once culture results identify the causative organism and its sensitivities. This reduces antibiotic resistance, decreases side effects, and is more cost-effective. For example, switching from piperacillin-tazobactam to ceftriaxone once E. coli is identified.
Why do atypical bacteria need different antibiotics?
Atypical bacteria (Mycoplasma, Chlamydia, Legionella) lack a peptidoglycan cell wall, making them resistant to β-lactam antibiotics (penicillins, cephalosporins). They require antibiotics that target intracellular processes, such as macrolides (azithromycin), fluoroquinolones (levofloxacin), or tetracyclines (doxycycline).
What is MRSA and when should I cover for it?
MRSA (Methicillin-Resistant Staphylococcus aureus) is resistant to all β-lactam antibiotics. Cover for MRSA in severe skin infections (especially if purulent), IV drug users, recent hospitalization, healthcare-associated infections, and severe pneumonia. Use vancomycin, daptomycin (not for pneumonia), linezolid, or ceftaroline.
How do I choose between different antibiotics in the same class?
Consider spectrum of activity, pharmacokinetics, dosing frequency, side effect profile, cost, and local resistance patterns. For example, among cephalosporins: 1st generation (cefazolin) for MSSA and simple skin infections, 3rd generation (ceftriaxone) for Gram-negatives and meningitis, and 4th generation (cefepime) for Pseudomonas and nosocomial infections.
What are β-lactamase inhibitors?
β-lactamase inhibitors (clavulanate, tazobactam, sulbactam) are combined with β-lactam antibiotics to overcome bacterial resistance mechanisms. Bacteria produce β-lactamase enzymes that destroy β-lactam antibiotics. Inhibitors protect the antibiotic, extending coverage to resistant organisms like ESBL-producing bacteria. Examples: amoxicillin-clavulanate, piperacillin-tazobactam.
When should I use carbapenems?
Carbapenems (meropenem, imipenem, ertapenem) have the broadest spectrum of all β-lactams and should be reserved for severe infections with multidrug-resistant organisms, ESBL-producing bacteria, or as empiric therapy in critically ill patients with suspected resistant pathogens. Overuse contributes to carbapenem resistance, so use judiciously and de-escalate when possible.