What antibiotic class is typically used for each type of infection, such as skin and soft tissue infections, urinary tract infections (UTIs), community-acquired pneumonia (CAP), and sepsis?

Antibiotic Selection by Infection Type and Class

For empiric antibiotic selection, match the antibiotic class to the most likely pathogen based on infection site: beta-lactams (penicillins/cephalosporins) for skin/soft tissue and respiratory infections, fluoroquinolones or aminoglycosides combined with beta-lactams for urinary and intra-abdominal infections, and broad-spectrum coverage including anti-pseudomonal agents for sepsis. 1, 2

Skin and Soft Tissue Infections (SSTIs)

Mild Infections

• First-line: Anti-staphylococcal penicillins or first-generation cephalosporins 1, 2
  • Cloxacillin/dicloxacillin, cefalexin, or amoxicillin-clavulanic acid target Staphylococcus aureus and Streptococcus species 1
  • These cover the most common community-acquired pathogens in uncomplicated cellulitis 3

Purulent Infections (Suspected S. aureus)

• Dicloxacillin, cefazolin, or cefalexin for methicillin-susceptible strains 1, 2
• Add doxycycline or sulfamethoxazole-trimethoprim for outpatient MRSA coverage 1, 2

MRSA Infections

• Vancomycin, linezolid, or daptomycin for severe cases 1, 2
• Linezolid shows superior clinical cure compared to vancomycin (OR 1.41) in MRSA SSTIs 1, 2
• Ceftaroline is FDA-approved for MRSA SSTIs 1, 4

Necrotizing Fasciitis

• Requires polymicrobial coverage: clindamycin + piperacillin-tazobactam (with or without vancomycin) 1, 2
• Alternative: ceftriaxone + metronidazole + vancomycin 1
• For streptococcal necrotizing fasciitis: clindamycin + penicillin 1
  • Clindamycin suppresses toxin production and is superior to beta-lactams alone 1

Diabetic Wound Infections

• Mild: dicloxacillin, cefalexin, or amoxicillin-clavulanic acid 1
• Moderate-severe: levofloxacin, ceftriaxone, ampicillin-sulbactam, or ertapenem 1
• Add MRSA coverage (linezolid, daptomycin, vancomycin) if suspected 1
• For Pseudomonas risk: piperacillin-tazobactam, ceftazidime, or carbapenems 1

Urinary Tract Infections (UTIs)

Uncomplicated UTIs

• First-line: nitrofurantoin or trimethoprim-sulfamethoxazole (based on general medical knowledge and stewardship principles)
• Fluoroquinolones (ciprofloxacin, levofloxacin) reserved for complicated cases 1

Complicated UTIs/Pyelonephritis

• Fluoroquinolones or third-generation cephalosporins (ceftriaxone) target gram-negative organisms including E. coli 5
• Consider local resistance patterns for empiric selection 2, 5

Community-Acquired Pneumonia (CAP)

Non-Severe CAP

• First-line: amoxicillin or amoxicillin-clavulanate + macrolide (clarithromycin) 1, 2
• Alternative: second/third-generation cephalosporin + macrolide 1, 2
• Doxycycline monotherapy acceptable for outpatients without comorbidities 1

Severe CAP (ICU-Level)

• Third-generation cephalosporin (ceftriaxone or cefotaxime) + macrolide 1, 2
• Alternative: beta-lactam/beta-lactamase inhibitor + macrolide 1
• No mortality difference between beta-lactam monotherapy, beta-lactam-macrolide combination, or fluoroquinolone monotherapy 1

CAP with Pseudomonas Risk

• Anti-pseudomonal cephalosporin (ceftazidime) or piperacillin-tazobactam 1, 2
• Combination with ciprofloxacin or aminoglycoside recommended 1

CAP with MRSA Risk

• Add vancomycin or linezolid to beta-lactam regimen 1, 2

Intra-Abdominal Infections

Mild-Moderate Community-Acquired

• First-line: amoxicillin-clavulanic acid 1, 2
• Alternative: ampicillin + gentamicin + metronidazole (especially pediatrics) 1, 2

Severe Community-Acquired

• Cefotaxime or ceftriaxone + metronidazole 1, 2
• Alternative: piperacillin-tazobactam monotherapy 1, 2

Hospital-Acquired/Critically Ill

• Piperacillin-tazobactam, tigecycline, or carbapenem (meropenem, imipenem) 1, 2
• These provide broad gram-negative and anaerobic coverage 1

Sepsis/Severe Infections

Empiric Sepsis Coverage

• Broad-spectrum anti-pseudomonal beta-lactam (piperacillin-tazobactam, cefepime, or carbapenem) + fluoroquinolone or aminoglycoside 5, 6
• This combination covers gram-negatives including Pseudomonas, gram-positives, and prevents resistance emergence 5

Sepsis with MRSA/Catheter Risk

• Add vancomycin, daptomycin, or linezolid to beta-lactam regimen 1, 6
• Glycopeptides indicated when catheter cannot be removed and patient is hemodynamically unstable 1

Neutropenic Sepsis

• Broad-spectrum monotherapy: carbapenems, anti-pseudomonal cephalosporins, or piperacillin-tazobactam 1
• Add vancomycin if catheter-associated infection, hemodynamic instability, or skin findings present 1

Key Antibiotic Classes by Primary Targets

Beta-Lactams (Penicillins/Cephalosporins)

• Primary use: Skin/soft tissue, respiratory, and intra-abdominal infections 1
• Cover Streptococcus, methicillin-susceptible S. aureus, and many gram-negatives 1, 3

Glycopeptides (Vancomycin)

• Primary use: MRSA infections, severe penicillin allergy, catheter-related sepsis 1
• Also first-line for severe C. difficile colitis (oral formulation) 1

Fluoroquinolones

• Primary use: Complicated UTIs, severe CAP, gram-negative sepsis 1, 5
• Avoid as first-line due to resistance concerns and adverse effects 1, 2

Carbapenems

• Primary use: Severe hospital-acquired infections, polymicrobial sepsis, resistant gram-negatives 1, 5
• Reserve for critically ill patients or documented resistant organisms 2, 5

Macrolides

• Primary use: Atypical pneumonia coverage, combination therapy for CAP 1, 2
• Clarithromycin preferred over erythromycin due to fewer adverse events 1

Critical Pitfalls to Avoid

• Never use fluoroquinolones as first-line for simple infections due to resistance and serious adverse effects 1, 2
• Do not delay anti-pseudomonal coverage in neutropenic patients as gram-negative infections have highest mortality 1
• Avoid vancomycin monotherapy for necrotizing infections without adequate gram-negative and anaerobic coverage 1
• Check local susceptibility before using clindamycin for anaerobes as resistance varies 1, 2
• Modify therapy within 48-72 hours based on cultures; initial inadequate therapy cannot be salvaged by later modification 5, 6
• Duration: 5-7 days sufficient for most community-acquired infections; 10-14 days for severe cases 2