Gram positive cocci antibiotics
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Antibiotics for Gram-Positive Cocci
Gram-positive cocci include Staphylococcus, Streptococcus, and Enterococcus species. Antibiotic selection depends heavily on the organism, susceptibility profile, and site of infection.
1. Streptococcus Species
| Organism | First Choice | Alternatives |
|---|---|---|
| S. pneumoniae | Penicillin G | Ceftriaxone, antipneumococcal quinolone (levofloxacin/moxifloxacin), doxycycline, macrolide, linezolid |
| S. pyogenes (Group A) | Penicillin G/V or clindamycin | Erythromycin, amoxicillin, 1st-gen cephalosporin |
| S. agalactiae (Group B) | Penicillin (± aminoglycoside) | Vancomycin |
| Viridans streptococci | Penicillin | 1st/3rd-gen cephalosporin, vancomycin |
Notes: Macrolide resistance in Group A Strep is ~5% in the US. Azithromycin is not recommended empirically for pneumococcal infections due to resistance concerns. High-dose amoxicillin can overcome penicillin resistance in S. pneumoniae.
2. Staphylococcus aureus
| Susceptibility | First Choice | Alternatives |
|---|---|---|
| β-lactamase negative (rare) | Penicillin G | 1st-gen cephalosporin, vancomycin |
| β-lactamase positive (MSSA) | Nafcillin, oxacillin, dicloxacillin | 1st-gen cephalosporin, vancomycin |
| MRSA | Vancomycin | TMP-SMX, doxycycline (>8 yrs), clindamycin (up to 40% resistant), linezolid, daptomycin, tigecycline, minocycline |
Notes on MRSA:
- Vancomycin is preferred for bacteremia, endocarditis, and meningitis (but penetrates poorly into lungs, bones/joints)
- TMP-SMX generally >90% susceptible
- Doxycycline generally >90% susceptible (use in patients >8 years)
- Linezolid if other anti-MRSA agents cannot be used
- Delafloxacin (300 mg IV q12h or 450 mg PO q12h): active against some MRSA; FDA-approved for skin/soft tissue infections and CAP
3. Enterococcus Species
| Susceptibility Profile | First Choice | Alternatives |
|---|---|---|
| Ampicillin/penicillin susceptible | Penicillin or ampicillin | Ampicillin + aminoglycoside or ceftriaxone (endocarditis), vancomycin |
| Ampicillin-resistant, vancomycin susceptible | Vancomycin | Linezolid, daptomycin |
| VRE (ampicillin- and vancomycin-resistant) | Linezolid or daptomycin | Tigecycline, quinupristin/dalfopristin (E. faecium only) |
4. Coagulase-Negative Staphylococci (CoNS)
Often associated with prosthetic devices and catheters. Commonly methicillin-resistant — treat like MRSA with vancomycin.
Antibiotic Classes with Gram-Positive Cocci Activity
| Class | Agents | Coverage |
|---|---|---|
| Penicillins | PCN G/V, ampicillin, nafcillin, oxacillin | Strep, MSSA, Enterococcus (susceptible strains) |
| Cephalosporins (1st gen) | Cephalexin, cefazolin | MSSA, streptococci |
| Glycopeptides | Vancomycin, telavancin | MRSA, MRSE, most strep and enterococci |
| Lipopeptides | Daptomycin | MRSA, VRE (NOT for pulmonary infections — inactivated by surfactant) |
| Oxazolidinones | Linezolid | MRSA, VRE, streptococci |
| Streptogramins | Quinupristin/dalfopristin | E. faecium (including VRE), MRSA |
| Macrolides/Lincosamides | Clindamycin, azithromycin, erythromycin | Streptococci, MSSA (bacteriostatic) |
| TMP-SMX | Trimethoprim-sulfamethoxazole | MRSA (skin/soft tissue), S. pneumoniae |
| Tetracyclines | Doxycycline, minocycline, tigecycline | MRSA (skin/soft tissue), streptococci |
| Fluoroquinolones | Levofloxacin, moxifloxacin, delafloxacin | Streptococci, some MRSA (delafloxacin) |
Key Clinical Pearls
- Always check local antibiogram — resistance rates vary by institution and geography
- MRSA bacteremia/endocarditis → vancomycin is the standard; daptomycin is an alternative but check for tolerance mutations if the patient has prior vancomycin exposure
- Daptomycin is inactivated by pulmonary surfactant → never use for pneumonia; use linezolid or vancomycin instead
- Clindamycin and MRSA: check for inducible resistance via D-zone test before relying on it
- VRE is increasing in both adults and pediatrics — empiric daptomycin or linezolid is warranted
- Transition from IV to oral therapy whenever possible (linezolid and TMP-SMX have excellent oral bioavailability)
— Katzung's Basic and Clinical Pharmacology 16e; Washington Manual of Medical Therapeutics; Harriet Lane Handbook 23e
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