Lesson
Read Morphology, Depth, and Physiology Together
The surface appearance is only the beginning. Purulence, tissue depth, progression, host reserve, and systemic response determine the first decision.
- Purulent versus nonpurulent
- Tissue depth
- Severity
- Mimics
- Disposition
Crust, follicle, diffuse erythema, pus, necrosis, and ulcer are different syndromes.
Pain, fluctuance, fascia, muscle, and bone change urgency.
Vitals, organs, progression, and host reserve set disposition.
Bilateral or chronic inflammatory disease can look infected.
Map the tissue plane
Separate epidermal impetigo, follicular disease, dermal cellulitis, subcutaneous abscess, fascial necrosis, muscle infection, and osteomyelitis.
Find pus
Fluctuance or ultrasound evidence of a collection shifts the plan toward drainage and staphylococcal ecology. Diffuse nonpurulent cellulitis usually centers streptococci.
Escalate severe physiology
SIRS, altered mental status, hypotension, rapid progression, immune compromise, or deep-tissue concern can require hospitalization, cultures, imaging, surgery, and parenteral therapy.
Challenge the diagnosis
Bilateral erythema, pruritus, chronic scale, joint-centered pain, thrombosis risk, ischemia, drug exposure, or poor response should reopen the differential.
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Lesson
Treat the Surface Without Overtreating the Patient
Impetigo and limited follicular infection often respond to local care. Lesion burden, depth, systemic findings, and transmission determine when oral therapy is justified.
- Impetigo
- Folliculitis
- Furuncle
- Carbuncle
- Transmission
Topical impetigo therapy can limit systemic exposure.
Oral therapy must address both common pathogens.
A carbuncle behaves like a source-control problem.
Coverage, hygiene, and laundering reduce spread.
Recognize impetigo
Papules become pustules and then honey-colored crusts. S. aureus and group A streptococci can look identical clinically.
Match treatment to burden
Use topical mupirocin or retapamulin for only a few lesions. Use an oral agent active against the expected pathogens for numerous lesions or outbreaks.
Respect increasing depth
Limited folliculitis or a small furuncle may need warmth and drainage support. A mature carbuncle or larger collection requires incision and drainage.
Stop spread
Cover lesions, clean hands, launder shared fabrics, avoid shared personal items, and follow local school, work, or team return guidance.
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Lesson
Keep Uncomplicated Cellulitis Focused
Typical nonpurulent cellulitis is primarily streptococcal. MRSA therapy, hospitalization, and longer duration require a reason.
- Streptococci
- MSSA
- MRSA gates
- Five days
- Edema and portals
Use a focused beta-lactam when risk and allergy fit.
Add coverage only for a credible exposure or severe phenotype.
Extend only when improvement is inadequate.
Treat edema, fissures, tinea, and skin breakdown.
Center streptococcal activity
Cephalexin 500 mg orally four times daily is a common adult outpatient option when allergy, organs, and local practice fit. Other appropriate beta-lactams can be used.
Open the MRSA gate selectively
Penetrating trauma, MRSA elsewhere or colonization, injection drug use, purulence, or severe systemic findings can justify activity against both MRSA and streptococci.
Use a five-day default
Treat improving uncomplicated cellulitis for five days. Extend only when improvement is inadequate or a deeper complication changes the diagnosis.
Prevent recurrence
Elevate the limb, control edema, treat interdigital fissuring or tinea, protect skin, and address venous or lymphatic disease.
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Lesson
Drain the Collection, Then Decide What Else It Needs
Antibiotics do not replace drainage. The value and intensity of adjunct therapy rise with systemic illness, host risk, lesion burden, and source-control failure.
- Incision and drainage
- Culture
- MRSA activity
- Oral options
- Recurrence
Examination or ultrasound identifies drainable pus.
Evacuate the protected infected space.
Systemic illness and host risk determine added therapy.
Find anatomy, colonization, and shared reservoirs.
Perform source control
Incise and drain a mature abscess, break loculations, and use ultrasound when examination does not reliably distinguish cellulitis from a collection.
Add antibiotics by risk
Systemic signs, multiple lesions, difficult anatomy, extensive cellulitis, immune compromise, age extremes, failed drainage, or severe disease support adjunct MRSA-active therapy.
Choose oral therapy deliberately
Trimethoprim-sulfamethoxazole, doxycycline, and clindamycin differ in streptococcal activity, pregnancy constraints, kidney effects, interactions, resistance, and C. difficile risk.
Investigate recurrence
Drain and culture early, then look for hidradenitis, pilonidal disease, foreign material, colonization, household spread, shaving, sports, and hygiene reservoirs.
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Lesson
Build Parenteral Therapy Around Source, Exposure, and Toxicity
Severe disease requires active initial exposure and aggressive source control. Definitive therapy and route transition should become narrower as evidence arrives.
- Vancomycin
- Daptomycin
- Linezolid
- Ceftaroline
- AUC monitoring
Drain, debride, and culture before relying on drug exposure.
Vancomycin, daptomycin, linezolid, and ceftaroline are not interchangeable.
Use current serious-MRSA vancomycin monitoring when applicable.
Cultures and stability should reduce route and spectrum burden.
Select beyond the spectrum list
Vancomycin, daptomycin, linezolid, ceftaroline, and other active agents differ in lung activity, bactericidal profile, kidney exposure, blood-count effects, CK risk, interactions, and oral availability.
Replace the old trough shortcut
For serious MRSA infection when vancomycin monitoring guidance applies, target AUC/MIC 400 to 600 mg-hour/L assuming MIC 1 mg/L rather than relying on the older universal trough 10 to 15 framing.
Look beyond the skin
Obtain blood cultures and evaluate endocarditis, bone, joint, device, vascular, or metastatic infection when bacteremia, persistent fever, focal symptoms, or host risk is present.
Transition on evidence
Use clinical stability, source control, active susceptibility, absorption, adherence, and follow-up to decide when oral therapy can complete the same total course.
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Lesson
Treat Suspicion as a Surgical Emergency
Necrotizing infection progresses faster than a complete diagnostic workup. Surgical exploration and debridement save tissue and life.
- Pain out of proportion
- Rapid progression
- Exploration
- Broad empiric therapy
- Toxin suppression
Rapid progression and systemic toxicity outweigh a reassuring surface.
Direct tissue assessment and debridement are primary therapy.
Start MRSA, streptococcal, gram-negative, and anaerobic activity.
Return until only viable controlled tissue remains.
Recognize the pattern
Disproportionate pain, rapid spread, profound toxicity, edema beyond erythema, bullae, ecchymosis, anesthesia, crepitus, or gas should trigger emergency action.
Do not let tools delay surgery
CT or MRI can help in selected stable uncertainty, but imaging and LRINEC must not delay exploration in a high-probability syndrome.
Cover the lethal range
Use an MRSA-active agent plus piperacillin-tazobactam, a carbapenem, or another complete regimen covering streptococci, gram-negative organisms, and anaerobes.
Narrow confirmed GAS correctly
Use high-dose penicillin plus clindamycin for confirmed group A streptococcal disease while continuing repeated debridement. Review local clindamycin susceptibility because resistance is increasingly common.
Quick check
Lesson
Diagnose Infection Clinically Before Reading the Culture
A diabetes-related foot ulcer is not automatically infected. Severity, tissue depth, perfusion, and specimen quality determine the antimicrobial plan.
- Clinical diagnosis
- IWGDF and IDSA severity
- Tissue culture
- Gram-positive focus
- Pseudomonas boundary
A positive swab alone does not make an infection.
Extent, depth, systemic signs, and morbidity set care.
Aseptic curettage or biopsy improves pathogen confidence.
Temperate-climate empiric therapy needs a specific risk.
Do not treat colonization
Antibiotics do not heal a clinically uninfected ulcer or prevent future infection. Culture growth alone is expected and is not an indication.
Classify severity
Use local inflammatory signs, erythema extent, tissue depth, systemic findings, and relevant morbidity to classify mild, moderate, or severe disease and disposition.
Prefer tissue over swab
After cleansing and debridement, obtain aseptic curettage or biopsy tissue when culture will change management. Superficial swabs overrepresent colonizers.
Avoid routine Pseudomonas coverage
In temperate climates such as Chicago, do not empirically target Pseudomonas without a recent affected-site isolate or another credible epidemiologic reason.
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Lesson
Keep Soft Tissue and Bone on Different Clocks
The diagnostic evidence, specimen, exposure, and duration change when infection reaches bone.
- Probe to bone
- Plain radiograph
- MRI
- Bone culture
- Duration
Combine probe-to-bone with radiograph and biomarkers.
Use advanced imaging when initial evidence remains uncertain.
Obtain a clean compartment specimen when definition matters.
Soft tissue, positive margins, and unresected bone use different durations.
Start with combined evidence
Use probe-to-bone, plain radiographs, and ESR, CRP, or PCT as the initial assessment. MRI is preferred when doubt remains after clinical, laboratory, and radiographic evaluation.
Culture the compartment
Obtain bone by a clean percutaneous or operative route when pathogen definition is needed. Do not treat a superficial wound isolate as definitive bone microbiology.
Treat soft tissue briefly
Use 1 to 2 weeks for most soft-tissue DFI. Ten days can be sufficient after debridement of moderate or severe soft-tissue disease, with low-certainty evidence.
Treat residual bone deliberately
Consider up to three weeks after minor amputation with a positive bone-margin culture and six weeks when osteomyelitis is treated without bone resection or amputation.
Quick check
Lesson
Preserve the Limb, Not Just the Culture
Antibiotics cannot replace blood flow, pressure relief, debridement, glycemic management, or wound care.
- Debridement
- Perfusion
- Revascularization
- Off-loading
- Interdisciplinary care
Debridement reduces burden and reveals viable anatomy.
Vascular assessment and revascularization support healing.
Off-loading stops repeated mechanical injury.
Wound, diabetes, nutrition, footwear, and follow-up act together.
Restore healing conditions
Clean and debride devitalized tissue, protect moisture balance, redistribute pressure, manage glucose and nutrition, and choose footwear that prevents repeated trauma.
Measure perfusion
Examine pulses and use appropriate vascular testing. Neuropathy, calcification, edema, and kidney disease can complicate interpretation of ankle measures.
Escalate threatening anatomy
Severe infection or moderate disease with gangrene, necrosis, deep abscess, compartment syndrome, or severe ischemia requires urgent surgical and vascular consultation.
Define remission and prevention
Assess osteomyelitis remission at least six months after therapy and continue lifelong frequent foot examination because recurrence and new ulceration remain common.
Quick check
Lesson
Close Every Portal, Exposure, and Follow-Up Gap
The safest regimen changes with allergy phenotype, organ function, pregnancy, age, interactions, recurrence pattern, and the patient's ability to carry out the plan.
- Allergy
- Organs
- Pregnancy and pediatrics
- Decolonization
- Follow-up
Separate intolerance from immediate and severe delayed injury.
Organs, weight, interactions, pregnancy, and age change safety.
Use a short strategy only after recurrence causes are addressed.
Rapid spread, severe pain, color change, or instability needs urgent care.
Clarify allergy precisely
Separate intolerance, remote low-risk history, immediate hypersensitivity, and severe delayed injury. Preserve effective beta-lactams through records, testing, or challenge when appropriate.
Monitor the chosen drug
Link kidney and liver trajectory, CBC, CK, potassium, QT risk, serotonergic medicines, anticoagulants, photosensitivity, pregnancy, and absorption to named dose or drug decisions.
Use decolonization selectively
For recurrent S. aureus disease after local causes and hygiene are addressed, consider a short mupirocin plus chlorhexidine or dilute-bleach strategy with household planning.
Teach early return
Rapid spread, disproportionate pain, color change, new numbness, crepitus, fever, hypotension, uncontrolled drainage, or worsening function requires urgent reassessment.
Quick check
Module test
Check the connections.
Each attempt draws 10 questions from the complete 170 question bank.
Each attempt draws a fresh set and rearranges the answer choices.
References
Current clinical foundation.
Lecture material was synthesized with the following contemporary guidance. Verify local policy and current guidance before applying clinical information.