What are the recommended antimicrobial treatments for common skin, Sexually Transmitted and Blood-Borne Infections (STBBI), and urinary infections in an adult patient with no significant underlying medical conditions, according to Canadian guidelines?

Common Skin, STBBI, and Urinary Infections: Antimicrobial Treatment Guide

Skin and Soft Tissue Infections

Uncomplicated Cellulitis (Non-purulent)

Beta-lactam monotherapy is the standard of care, successful in 96% of patients, as MRSA is uncommon in typical cellulitis even in high-prevalence settings 1.

Oral regimens (first-line):

• Cephalexin 500 mg every 6 hours for 5 days 1
• Dicloxacillin 250-500 mg every 6 hours for 5 days 1
• Amoxicillin 500 mg three times daily for 5 days 1
• Penicillin V 250-500 mg four times daily for 5 days 1

IV regimens (hospitalized patients):

• Cefazolin 1-2 g IV every 8 hours 1
• Oxacillin 2 g IV every 6 hours 1

Treatment duration: Exactly 5 days if clinical improvement occurs; extend only if symptoms have not improved 1.

Purulent Cellulitis or MRSA Risk Factors

Add MRSA coverage when specific risk factors are present: penetrating trauma, injection drug use, purulent drainage/exudate, known MRSA colonization, or systemic inflammatory response syndrome 1.

Oral regimens:

• Clindamycin 300-450 mg every 6 hours for 5 days (if local MRSA resistance <10%) 2, 1
• Trimethoprim-sulfamethoxazole (TMP-SMX) 1-2 double-strength tablets twice daily PLUS a beta-lactam (cephalexin or amoxicillin) for 5 days 2, 1
• Doxycycline 100 mg twice daily PLUS a beta-lactam for 5 days 2, 1, 3

IV regimens:

• Vancomycin 15-20 mg/kg IV every 8-12 hours (A-I evidence) 2, 1
• Linezolid 600 mg IV twice daily (A-I evidence) 2, 1
• Daptomycin 4 mg/kg IV once daily (A-I evidence) 2, 1

Critical caveat: Never use doxycycline or TMP-SMX as monotherapy for cellulitis—streptococcal coverage is inadequate 1.

Severe Cellulitis with Systemic Toxicity or Suspected Necrotizing Fasciitis

Mandatory broad-spectrum combination therapy is required for patients with fever, hypotension, tachycardia, altered mental status, or suspected necrotizing infection 1.

Recommended regimens:

• Vancomycin 15-20 mg/kg IV every 8-12 hours PLUS piperacillin-tazobactam 3.375-4.5 g IV every 6 hours 1
• Linezolid 600 mg IV twice daily PLUS piperacillin-tazobactam 1
• Vancomycin PLUS a carbapenem (meropenem 1 g IV every 8 hours) 1

Duration: 7-14 days based on clinical response 1.

Skin Abscesses

Incision and drainage is the primary treatment; antibiotics play only a subsidiary role 1.

Antibiotics indicated only if:

• Surrounding cellulitis present 2
• Systemic signs of infection 2
• Immunocompromised host 2
• Multiple lesions 2

If antibiotics needed, use MRSA-active agents as above 2.

Bite Wounds (Animal and Human)

Amoxicillin-clavulanate 875/125 mg twice daily for 5-7 days provides single-agent coverage for polymicrobial oral flora 2, 1.

Alternative for penicillin allergy:

• Doxycycline 100 mg twice daily PLUS metronidazole 500 mg three times daily 2
• Moxifloxacin 400 mg daily (covers both aerobes and anaerobes) 2

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Sexually Transmitted and Blood-Borne Infections (STBBI)

Gonorrhea (Urethritis/Cervicitis)

Ceftriaxone 250 mg IM as a single dose is first-line therapy 2.

Alternative (if ceftriaxone unavailable):

• Azithromycin 2 g orally as a single dose 4

Note: Fluoroquinolones are no longer recommended due to widespread resistance 2.

Chlamydia (Urethritis/Cervicitis)

Azithromycin 1 g orally as a single dose 2, 4.

Alternative:

• Doxycycline 100 mg orally twice daily for 7 days 2, 3

Non-Gonococcal Urethritis (NGU)

Doxycycline 100 mg orally twice daily for 7 days 2, 3.

Alternative:

• Azithromycin 1 g orally as a single dose 2, 4

Syphilis (Early, <1 year duration)

Benzathine penicillin G 2.4 million units IM as a single dose is first-line 2.

For penicillin allergy:

• Doxycycline 100 mg orally twice daily for 2 weeks 2, 3

Syphilis (Late, >1 year duration)

Benzathine penicillin G 2.4 million units IM weekly for 3 weeks 2.

For penicillin allergy:

• Doxycycline 100 mg orally twice daily for 4 weeks 2, 3

Chancroid (Genital Ulcer Disease)

Azithromycin 1 g orally as a single dose 4.

Alternatives:

• Ceftriaxone 250 mg IM as a single dose 2
• Ciprofloxacin 500 mg orally twice daily for 3 days 2

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Urinary Tract Infections

Uncomplicated Cystitis (Lower UTI)

First-line agents:

• Nitrofurantoin 100 mg twice daily for 5 days 5
• TMP-SMX 1 double-strength tablet twice daily for 3 days 6, 5

Alternatives (if resistance patterns permit):

• Fosfomycin 3 g as a single dose 5
• Amoxicillin-clavulanate 875/125 mg twice daily for 5-7 days 5

Avoid fluoroquinolones as first-line due to increasing resistance and adverse effects 7, 5.

Complicated Urinary Tract Infections (cUTI)

Fluoroquinolones should not be used as first-line for serious cUTIs when risk factors for resistant organisms exist 5.

Mild-to-moderate cUTI (oral therapy):

• TMP-SMX 1 double-strength tablet twice daily for 7-14 days 6, 5
• Ciprofloxacin 500 mg twice daily for 7-14 days (if local resistance <10%) 5
• Levofloxacin 750 mg daily for 5-7 days (if local resistance <10%) 5

Severe cUTI or risk factors for resistant organisms (IV therapy):

• Piperacillin-tazobactam 3.375 g IV every 6 hours 5
• Carbapenems (ertapenem 1 g IV daily, meropenem 1 g IV every 8 hours) 5
• Ceftriaxone 1-2 g IV daily (if susceptibility confirmed) 5

Duration: 7-14 days for cUTI; up to 4 weeks for complicated pyelonephritis with abscess 5.

Acute Pyelonephritis (Uncomplicated)

Outpatient oral therapy:

• Ciprofloxacin 500 mg twice daily for 7 days 5
• Levofloxacin 750 mg daily for 5 days 5
• TMP-SMX 1 double-strength tablet twice daily for 14 days (if local resistance <20%) 6, 5

Hospitalized patients:

• Ceftriaxone 1-2 g IV daily 5
• Ciprofloxacin 400 mg IV every 12 hours 5
• Piperacillin-tazobactam 3.375 g IV every 6 hours (if severe or septic) 5

Transition to oral therapy once clinically stable, typically after 24-48 hours 5.

Acute Bacterial Prostatitis

Empiric therapy (pending cultures):

• Ciprofloxacin 500 mg orally twice daily for 4-6 weeks 8
• Levofloxacin 500 mg orally daily for 4-6 weeks 8
• TMP-SMX 1 double-strength tablet twice daily for 4-6 weeks 8, 6

Severe/hospitalized patients:

• Piperacillin-tazobactam 4.5 g IV every 6-8 hours 8
• Ceftriaxone 1-2 g IV daily PLUS gentamicin 5 mg/kg daily 8

Critical caveat: Do NOT perform prostatic massage in acute bacterial prostatitis—risk of bacteremia and sepsis 8.

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Key Principles Across All Infections

Antibiotic Stewardship

• Avoid fluoroquinolones as first-line for mild infections where other options exist due to serious adverse effects and resistance 7, 5
• Reserve carbapenems, linezolid, and daptomycin for life-threatening situations or confirmed resistant organisms 9
• Use shortest effective duration: 5 days for uncomplicated cellulitis, 3 days for uncomplicated cystitis 1, 5

Common Pitfalls

• Do not add MRSA coverage reflexively to cellulitis without specific risk factors 1
• Do not use doxycycline or TMP-SMX as monotherapy for cellulitis—inadequate streptococcal coverage 1
• Do not continue ineffective antibiotics beyond 48 hours—reassess for complications or resistant organisms 1
• Do not prescribe antibiotics for simple abscesses without surrounding cellulitis—drainage is primary treatment 2, 1