What are the specific antibiotics for gram-positive, gram-negative, and anaerobic bacteria?

Specific Antibiotics by Bacterial Type and Associated Diseases

Gram-Positive Coverage

Methicillin-Susceptible Staphylococcus aureus (MSSA)

For serious MSSA infections, penicillinase-resistant penicillins (flucloxacillin, dicloxacillin) or first-generation cephalosporins (cefazolin, cephalexin) are the antibiotics of choice. 1

• Oral options: Dicloxacillin, cephalexin, clindamycin 2
• IV options: Nafcillin, oxacillin, cefazolin 2
• Alternative for penicillin allergy: Clindamycin (300 mg PO three times daily or IV) 2, 3

Common diseases: Skin and soft tissue infections (impetigo, cellulitis, abscesses), endocarditis, osteomyelitis 2

Methicillin-Resistant Staphylococcus aureus (MRSA)

For MRSA infections, vancomycin (15 mg/kg IV q8-12h) remains first-line for serious infections, though linezolid and daptomycin show superior clinical cure rates in some studies. 2

• IV options: Vancomycin, linezolid (600 mg IV q12h), daptomycin (10 mg/kg IV once daily), ceftaroline, dalbavancin, tedizolid 2
• Oral options: Linezolid (600 mg PO q12h), trimethoprim-sulfamethoxazole (160-800 mg twice daily), doxycycline (100 mg twice daily), tedizolid 2
• Duration: 7-14 days based on clinical response 2

Common diseases: Complicated skin and soft tissue infections, hospital-acquired pneumonia, bacteremia, endocarditis 2

Streptococcal Infections

Penicillin (benzylpenicillin or phenoxymethylpenicillin) remains the drug of choice for streptococcal infections. 2

• For Streptococcus pyogenes/agalactiae: Penicillin plus clindamycin for necrotizing infections 2
• For Streptococcus pneumoniae: Penicillin-susceptible strains respond to penicillin or amoxicillin 2
• Alternative: Clindamycin, cephalexin, erythromycin 2

Common diseases: Pharyngitis, cellulitis, necrotizing fasciitis, pneumonia, meningitis 2

Enterococcal Infections

Ampicillin is the preferred agent for Enterococcus faecalis; vancomycin for ampicillin-resistant strains. 2

• E. faecalis: Ampicillin, piperacillin-tazobactam, vancomycin 2
• Vancomycin-resistant E. faecium: Linezolid (600 mg IV/PO q12h) 2

Common diseases: Healthcare-associated intra-abdominal infections, urinary tract infections, endocarditis 2

Gram-Negative Coverage

Enterobacteriaceae (E. coli, Klebsiella, Proteus)

For community-acquired infections, narrow-spectrum agents like ampicillin-sulbactam or cefazolin plus metronidazole are preferred; for healthcare-associated infections, use carbapenems or piperacillin-tazobactam. 2

• Mild-moderate community infections: Ampicillin-sulbactam (1.5-3.0 g IV q6h), cefazolin plus metronidazole, ertapenem (1 g IV daily) 2
• Severe/healthcare-associated: Meropenem (1 g IV q8h), imipenem (1 g IV q6-8h), piperacillin-tazobactam (4.5 g IV q6h) 2, 4

Common diseases: Urinary tract infections, intra-abdominal infections, bacteremia 2

Pseudomonas aeruginosa

Antipseudomonal beta-lactams combined with an aminoglycoside or fluoroquinolone provide optimal coverage for serious Pseudomonas infections. 2

• Beta-lactam options: Piperacillin-tazobactam (4.5 g IV q6h), cefepime (2 g IV q8h), ceftazidime (2 g IV q8h), meropenem (1 g IV q8h) 2, 5
• Non-beta-lactam options: Ciprofloxacin (400 mg IV q8h), aminoglycosides (amikacin 15-20 mg/kg IV q24h, gentamicin 5-7 mg/kg IV q24h) 2
• For multidrug-resistant strains: Colistin or polymyxin B 2

Common diseases: Ventilator-associated pneumonia, healthcare-associated infections, diabetic foot infections 2

Haemophilus influenzae

Amoxicillin-clavulanate or second/third-generation cephalosporins are effective against H. influenzae. 2, 5

• Options: Amoxicillin-clavulanate (500/875 mg PO twice daily), ceftriaxone (1 g IV q12h), cefuroxime 2

Common diseases: Respiratory tract infections, otitis media, sinusitis 5

Anaerobic Coverage

Bacteroides fragilis and Other Anaerobes

Metronidazole is the gold standard for anaerobic coverage, particularly for Bacteroides species. 2

• Metronidazole: 250-500 mg PO four times daily or 500 mg IV 2
• Alternatives: Clindamycin (300 mg PO three times daily), piperacillin-tazobactam, carbapenems (ertapenem, meropenem, imipenem) 2, 4
• Beta-lactam/beta-lactamase inhibitor combinations: Ampicillin-sulbactam, piperacillin-tazobactam 2, 4

Common diseases: Intra-abdominal infections, aspiration pneumonia, necrotizing soft tissue infections, brain abscesses 2

Clostridium perfringens

Penicillin plus clindamycin is the regimen of choice for clostridial necrotizing infections. 2

• Combination: Penicillin (500 mg IV q4-6h) plus clindamycin (600-900 mg IV q8h) 2

Common diseases: Gas gangrene, necrotizing fasciitis 2

Mixed Infections Requiring Broad Coverage

Animal/Human Bites

Amoxicillin-clavulanate provides optimal coverage for polymicrobial bite infections involving aerobes and anaerobes. 2

• Oral: Amoxicillin-clavulanate (500/875 mg twice daily) 2
• IV: Ampicillin-sulbactam (1.5-3.0 g q6h), piperacillin-tazobactam (3.37 g q6-8h) 2

Pathogens: Pasteurella multocida (animal), Eikenella corrodens (human), Staphylococcus, Streptococcus, anaerobes 2

Intra-Abdominal Infections

For community-acquired infections, ertapenem or cefazolin/cefuroxime plus metronidazole; for healthcare-associated infections, use meropenem or piperacillin-tazobactam. 2

• Community-acquired: Ertapenem (1 g IV daily), ampicillin-sulbactam, cefazolin plus metronidazole 2
• Healthcare-associated/high-risk: Meropenem (1 g IV q8h), imipenem-cilastatin, piperacillin-tazobactam (4.5 g IV q6h) 2

Pathogens: E. coli, Klebsiella, Enterococcus, Bacteroides fragilis 2

Necrotizing Fasciitis

Vancomycin or linezolid plus piperacillin-tazobactam or a carbapenem provides comprehensive coverage for polymicrobial necrotizing infections. 2

• Regimen: Vancomycin (15 mg/kg IV q8-12h) or linezolid (600 mg IV q12h) PLUS piperacillin-tazobactam (4.5 g IV q6h) or meropenem (1 g IV q8h) 2
• Alternative: Ceftriaxone (1 g IV q12h) plus metronidazole (500 mg IV q8h) 2

Pathogens: Streptococcus pyogenes, Staphylococcus aureus, Clostridium species, mixed aerobes/anaerobes 2