What antibiotics should be given for a bacterial infection?

Antibiotic Selection for Bacterial Infections

The choice of antibiotic depends critically on the infection site, severity, and local resistance patterns—with skin/soft tissue infections treated with dicloxacillin or cephalexin for outpatient cases, vancomycin or linezolid for hospitalized MRSA cases, and amoxicillin-clavulanate for bite wounds. 1

General Principles Before Prescribing

• Obtain microbiological cultures before starting antibiotics whenever possible, as this allows for targeted therapy and prevents treating colonization rather than true infection 2
• Avoid prescribing antibiotics to treat fever alone—investigate the source and confirm bacterial infection before initiating therapy 2
• Consider the infection site, patient risk factors for multidrug-resistant organisms, and local susceptibility patterns when selecting empiric therapy 2, 3

Skin and Soft Tissue Infections

Outpatient/Uncomplicated Cases

• For impetigo: Oral dicloxacillin, cephalexin, erythromycin, clindamycin, or amoxicillin-clavulanate 1
• For purulent infections (likely S. aureus): Dicloxacillin, cefazolin, clindamycin, cephalexin, doxycycline, or trimethoprim-sulfamethoxazole 1
• For non-purulent cellulitis: Benzylpenicillin, phenoxymethylpenicillin, clindamycin, nafcillin, cefazolin, or cephalexin 1
• Treatment duration: 5-10 days 1

Hospitalized/Complicated Cases

• For suspected or confirmed MRSA: Vancomycin 30-60 mg/kg/day IV divided in 2-4 doses (with loading dose of 25-30 mg/kg in seriously ill patients), teicoplanin 6-12 mg/kg IV q12h for 3 doses then daily, linezolid 600 mg IV/PO q12h, or daptomycin 4 mg/kg IV daily 1
• Treatment duration: 7-14 days 1
• Important caveat: Linezolid shows superior clinical success compared to vancomycin for skin/soft tissue infections (OR 1.40,95% CI 1.01-1.95) 1

Necrotizing Fasciitis (Life-Threatening)

• Combination therapy required: Vancomycin or linezolid PLUS piperacillin-tazobactam or a carbapenem, OR ceftriaxone plus metronidazole 1
• Source control with surgical debridement is essential 2

Bite Wounds

Animal Bites

• Oral therapy: Amoxicillin-clavulanate 1
• IV therapy: Ampicillin-sulbactam, piperacillin-tazobactam, second/third-generation cephalosporins (cefuroxime, cefoxitin, ceftriaxone, cefotaxime), or carbapenems 1
• Add metronidazole or clindamycin for anaerobic coverage if needed 1

Human Bites

• First-line: Amoxicillin-clavulanate or ampicillin-sulbactam 1
• Alternatives: Carbapenems or doxycycline 1
• For multidrug-resistant organisms: Vancomycin, daptomycin, linezolid, or colistin 1

Diabetic Wound Infections

Mild Infections

• Dicloxacillin, clindamycin, cephalexin, levofloxacin, amoxicillin-clavulanate, or doxycycline 1
• For suspected/confirmed MRSA: Trimethoprim-sulfamethoxazole 1

Moderate to Severe Infections

• Levofloxacin, cefoxitin, ceftriaxone, ampicillin-sulbactam, moxifloxacin, ertapenem, tigecycline, or ciprofloxacin plus clindamycin 1
• For MRSA: Linezolid, daptomycin, or vancomycin 1
• For Pseudomonas risk: Piperacillin-tazobactam, ceftazidime, cefepime, aztreonam, or carbapenems 1
• Critical: Clinically uninfected wounds require NO antibiotics—only infected wounds warrant treatment, supported by debridement and wound care 1

Surgical Site Infections

Intestinal/Genitourinary Tract Surgery

• Single-drug regimens: Ticarcillin-clavulanate, piperacillin-tazobactam, or carbapenems (imipenem, meropenem, ertapenem) 1
• Combination regimens: Ceftriaxone plus metronidazole, fluoroquinolone (ciprofloxacin/levofloxacin) plus metronidazole, or ampicillin-sulbactam plus gentamicin/tobramycin 1

Trunk/Extremity Surgery (Away from Axilla/Perineum)

• Oxacillin, nafcillin, cefazolin, cephalexin, trimethoprim-sulfamethoxazole, or vancomycin 1

Axilla/Perineum Surgery

• Ceftriaxone or fluoroquinolone (ciprofloxacin/levofloxacin) plus metronidazole 1

Critical Dosing and Duration Principles

• Prescribe optimal dosing adapted to patient characteristics including renal function, weight, and infection severity 2, 4
• Vancomycin dosing: 30-60 mg/kg/day IV in 2-4 divided doses; loading dose of 25-30 mg/kg for seriously ill patients 1
• Amoxicillin suspension: May be taken every 8 or 12 hours depending on prescribed dose; refrigeration preferable but not required; discard unused portion after 14 days 5
• De-escalate therapy based on culture results and clinical response—switch to narrow-spectrum oral agents when appropriate 2, 4
• Stop antibiotics once absence of infection is confirmed 2

Common Pitfalls to Avoid

• Do not combine teicoplanin with doxycycline—this combination lacks guideline support and is never recommended 6
• Avoid prolonged metronidazole courses beyond 10-14 days due to neurotoxicity risk, especially in liver disease 7
• Do not use doxycycline in chronic kidney disease due to nephrotoxicity 6
• Avoid empiric fluoroquinolones in patients with MRSA history due to high resistance rates 8
• Recognize that antibiotic overuse promotes resistance and secondary infections with resistant organisms, which spread to other patients and the environment 9, 4
• Combination therapy with vancomycin and piperacillin-tazobactam increases acute kidney injury risk 10

When Combination Therapy Is Appropriate

• Use antibiotic combinations only where evidence suggests benefit 2
• For necrotizing fasciitis: Glycopeptide plus beta-lactam/carbapenem or ceftriaxone plus metronidazole 1
• For teicoplanin combinations: Rifampin, gentamicin, or trimethoprim-sulfamethoxazole are guideline-recommended—not tetracyclines 6
• Metronidazole can be safely combined with teicoplanin for polymicrobial infections, as they have complementary mechanisms without antagonism 7