Wound Healing and Surgical Site Infections

Normal Wound Healing: Four Overlapping Phases

Phase 1: Hemostasis (Minutes to Hours)

• Immediately upon injury, platelets adhere to exposed collagen and tissue factor via von Willebrand factor and integrin receptors, forming the primary platelet plug
• Platelet activation triggers release of dense granules (ADP, serotonin, calcium) and alpha granules (fibrinogen, von Willebrand factor, thrombospondin, platelet-derived growth factor [PDGF])
• Coagulation cascade activation (tissue factor/Factor VII initiation) generates thrombin, converting fibrinogen to fibrin and creating a three-dimensional clot matrix
• This fibrin scaffold serves dual purpose: hemostasis and temporary matrix for subsequent cell migration
• Thrombin also activates platelets and endothelial cells, releasing additional growth factors (PDGF, transforming growth factor-beta [TGF-β], vascular endothelial growth factor [VEGF])

Phase 2: Inflammation (Hours to Days; Peaks at 24-48 Hours)

• Neutrophils are first responders (within 6-24 hours), recruited via chemotactic factors (complement fragments C5a, bacterial lipopolysaccharides, fibrin degradation products, cytokines IL-1, IL-6, TNF-α)
• Neutrophils perform debridement by phagocytosing bacteria, dead tissue, and cellular debris; peak at 24-48 hours then decline as macrophages predominate
• Macrophages (derived from circulating monocytes recruited by chemokines including MCP-1, TNF-α, and fibronectin fragments) arrive at 48-72 hours and persist for weeks
• Macrophages release critical pro-healing cytokines: TGF-β (fibroblast recruitment and activation), VEGF (angiogenesis), PDGF (smooth muscle cell and fibroblast recruitment), and IL-6
• Inflammatory phase is characterized by increased vascular permeability (histamine and bradykinin release from mast cells and basophils), resulting in edema; vasodilation increases blood flow (accounting for erythema)
• Complement cascade activation generates C5a and C3a, potent chemotactic agents amplifying neutrophil and macrophage recruitment
• Persistent inflammation beyond normal window suggests infection, foreign body, or impaired resolution; chronic inflammation drives pathologic scarring and keloid formation

Phase 3: Proliferation (Days 3 to Weeks 3; Peaks at 5-7 Days)

• Fibroblasts differentiate from circulating fibrocytes and resident mesenchymal cells, recruited by TGF-β, PDGF, and fibroblast growth factor (FGF)
• Fibroblasts synthesize extracellular matrix components: Type III collagen (initial predominance, provides scaffold), proteoglycans (hold moisture, provide cushioning), and fibronectin (guides cell migration and matrix organization)
• Matrix cross-linking occurs via lysyl oxidase-mediated formation of aldehyde groups on lysine and hydroxylysine residues; cross-linked collagen provides tensile strength
• Angiogenesis is essential: hypoxia-driven HIF-1α signaling upregulates VEGF; endothelial cells sprout from existing capillaries, forming new microvasculature to restore blood supply
• Epithelialization begins at wound edges: keratinocytes migrate across the wound bed, stimulated by epidermal growth factor (EGF), hepatocyte growth factor (HGF), and mechanical signals from the provisional matrix
• Myofibroblasts (activated fibroblasts expressing alpha-smooth muscle actin) appear and contribute to wound contraction (mediated by actin-myosin interaction), reducing wound size by up to 40%
• Proliferation is maximal at 5-7 days (corresponding to peak vascularity and collagen deposition) and gradually declines

Phase 4: Remodeling/Maturation (Weeks 3 Onwards; Continues for Months to Years)

• Collagen undergoes remodeling: Type III collagen (abundant in proliferation phase) is progressively replaced by Type I collagen (stronger, more organized), resulting in increased tensile strength
• Matrix metalloproteinases (MMPs) and their tissue inhibitors (TIMPs) regulate collagen turnover; imbalance in MMP/TIMP ratios contributes to excessive scarring (elevated MMPs) or keloid formation (elevated TIMPs)
• Angiogenesis regresses: excessive capillaries are pruned via apoptosis; vascular density decreases, reducing erythema
• Myofibroblast apoptosis occurs, reducing contraction; however, persistent myofibroblasts drive pathologic fibrosis in hypertrophic scars and keloids
• Tensile strength increases: at day 5-7, wound strength is 5% of unwounded skin; at 2 weeks, 30%; at 3 weeks, 50%; at 6 weeks, 80%; achieving maximum strength of ~80% of original tissue at 12 months (100% strength is never fully regained)
• Scar maturation continues for 12-24 months; initial red, raised scars gradually blanch and flatten as collagen remodeling completes

Surgical Site Infection: Microbial and Host Pathogenesis

Bacterial Colonization Pathway

• Bacteria enter the surgical site via skin flora (most common, endogenous source), direct inoculation from surgical instruments or environment (exogenous), or hematogenous seeding from distant sites
• Biofilm formation is critical: within hours, bacteria produce extracellular polysaccharide matrix, creating a protected niche resistant to antibiotics and immune cells; biofilms can reduce antibiotic susceptibility 100-1000 fold
• Bacteria adhere via adhesins (pili, fimbriae, surface proteins) to exposed tissue and foreign materials (sutures, implants); this is enhanced by operative trauma and inflammation

Host Defense Impairment

• Impaired neutrophil function occurs in diabetes (hyperglycemia impairs chemotaxis, phagocytosis, respiratory burst), extremes of age, obesity, malnutrition, immunosuppression
• Reduced tissue perfusion from hypotension, vasoconstriction, anesthesia, or chronic hypoxia critically impairs antibiotic delivery and oxygen-dependent neutrophil killing
• Oxidative stress and inflammation dysregulation: excessive or prolonged inflammatory response impairs epithelialization and promotes pathologic matrix degradation
• Impaired angiogenesis prevents adequate vascular ingrowth, perpetuating hypoxia and neutrophil dysfunction
• Seroma and hematoma formation create fluid collections where bacteria proliferate beyond antibiotic reach

Classification of SSIs

Patient Risk Factors (Modifiable and Non-Modifiable)

Non-Modifiable Factors

• Age extremes: very young and elderly patients have impaired immune response; age >65 significantly increases risk
• Male gender: some studies suggest modestly increased SSI rates in males, possibly related to higher baseline inflammatory state
• Genetic predisposition: polymorphisms in IL-6, TNF-α, and innate immune genes increase infection susceptibility in some populations

Modifiable Host Factors

• Obesity (BMI >30): hyperglycemia impairs neutrophil function; excess adipose tissue is hypoxic with reduced perfusion; mechanical complications increase operative time
• Diabetes mellitus: hyperglycemia (>180 mg/dL perioperatively) impairs neutrophil chemotaxis, phagocytosis, and T-cell function; increased risk independent of glycemic control, but tighter glucose control (target 80-180 mg/dL) perioperatively reduces SSI rates by ~30%
• Malnutrition and hypoalbuminemia: inadequate protein impairs collagen synthesis and immune function; albumin <3.0 g/dL significantly increases risk
• Chronic corticosteroid use: impairs early inflammatory phase and Th1 immunity; SSI risk increases with dose and duration
• Immunosuppressive therapy (chemotherapy, biologics, antiretrovirals): impairs all phases of wound healing and immune response
• Active infection at remote site: bacteremia seeds surgical field; however, asymptomatic bacteriuria does not increase SSI risk and routine screening is not indicated
• Preoperative anemia: hemoglobin <7 g/dL significantly impairs wound healing via reduced oxygen-carrying capacity and oxidative burst capability; transfusion to hemoglobin 7-9 g/dL may improve outcomes
• Smoking: impairs angiogenesis via increased carboxyhemoglobin and nitric oxide synthase inhibition; smokers have 2-fold increased SSI risk; cessation even 2-4 weeks preoperatively reduces risk

Operative/Technical Risk Factors

Operative Duration and Complexity

• Prolonged operative time: each additional hour increases SSI risk (often logarithmically); duration varies by procedure type; risk increases significantly beyond 2-3 hours for most procedures
• Surgical complexity: more complex procedures (longer duration, more tissue trauma, greater blood loss) carry higher inherent risk
• Repeat surgery: revision procedures have higher infection rates due to altered tissue planes and increased operative time

Type of Surgery (CDC Surgical Classification)

• Clean surgeries (elective, non-traumatic, primary closure, no major break in sterile technique, no entry into GI, biliary, GU, or respiratory tracts): baseline SSI risk ~1-2%; examples include total joint replacement, cardiac surgery, vascular surgery
• Clean-contaminated surgeries (controlled entry into GI, biliary, GU, respiratory tracts with minimal spillage; gynecologic or biliary procedures): SSI risk ~3-5%; examples include cholecystectomy, hysterectomy
• Contaminated surgeries (major break in sterile technique, gross spillage from GI tract, trauma <4 hours old, entry into biliary/GU tracts with infection, or major operative violation of sterility): SSI risk ~10-15%
• Dirty/infected surgeries (perforated viscus, old trauma >4 hours, preoperative infection): SSI risk >20%; examples include perforated appendix, infected wound debridement

Intraoperative Factors

• Hypothermia (core temperature <36°C): impairs vasoconstriction-mediated retention of heat and oxygen delivery; increases SSI risk by 2-3 fold; maintenance of normothermia (36.5-37.5°C) is critical
• Hypoxia (inadequate FiO2 or ventilation): tissue oxygen tension <50 mmHg impairs neutrophil killing; maintaining PaO2 80-100 mmHg reduces SSI risk
• Hyperglycemia: perioperative glucose >180 mg/dL increases SSI risk; tight glucose control (target 80-180 mg/dL) is recommended
• Excessive blood loss and transfusion: allogeneic transfusion may induce immunosuppression; RBC transfusion is associated with increased infection risk, though causality unclear
• Inadequate hemostasis: hematoma formation creates hypoxic, fluid-filled space favoring bacterial growth
• Surgical technique: excessive tissue trauma, inadequate perfusion of wound edges, inappropriate suture material or tension increases SSI risk

Microbial Factors

Bacterial Species (Site-Dependent)

• Skin flora: Staphylococcus aureus (most common cause of SSI overall, ~20-30% of cases), coagulase-negative staphylococci (S. epidermidis), Propionibacterium acnes, streptococci
• GI tract flora: Escherichia coli, Klebsiella, Proteus, Bacteroides, Clostridium species; common in abdominal surgeries
• GU tract flora: E. coli, Proteus, Klebsiella, Enterococcus; risk increases with preoperative instrumentation or catheterization
• Respiratory tract flora: Streptococcus pneumoniae, Hemophilus influenzae, anaerobes; relevant in cardiothoracic surgery
• **Methicillin-resistant S. aureus (MRSA)**: increasing prevalence (10-30% of S. aureus isolates in many centers); requires alternative antibiotics (vancomycin, linezolid)
• Gram-negative rods and anaerobes: more common in deep/organ-space infections and abdominal procedures; increasingly multidrug-resistant (extended-spectrum beta-lactamase [ESBL]-producing organisms, carbapenem-resistant enterobacteria [CRE])

Biofilm Virulence and Antibiotic Resistance

• Biofilm-forming organisms (notably Pseudomonas aeruginosa, Acinetobacter, S. aureus) establish protected niches reducing antibiotic penetration by 100-1000 fold
• Biofilm-embedded bacteria exhibit altered gene expression (SOS response, alginate upregulation in Pseudomonas), slowing metabolism and reducing antibiotic susceptibility

Environmental and Healthcare-Associated Factors

Surgical Environment

• Operating room air contamination: laminar airflow reduces SSI rates in clean surgeries; positive pressure, HEPA filtration reduces risk
• Traffic and personnel: each additional person in OR increases contamination risk

Antibiotic-Related Factors

• Inadequate prophylactic antibiotic dosing: undersized patients receiving standard doses achieve subtherapeutic levels
• Prolonged prophylaxis (beyond single preoperative dose or <24 hours postoperatively): increases resistance without reducing SSI
• Delayed antibiotic administration: each hour delay increases SSI risk; target is <60 minutes before incision (120 minutes for vancomycin/clindamycin)
• Prior antibiotic exposure: selects for resistant organisms (MRSA, ESBL-producers)

Timing and Classification of Presentation

• Onset: typically days 5-10 postoperatively, but can present within 30 days
• Drainage: purulent discharge (thick, discolored, often with odor) from incision
• Erythema and warmth: surrounding skin demonstrates acute inflammatory changes
• Cellulitis: spreading erythema, edema, warmth extending beyond immediate incision
• Pain and tenderness: loc

Initial assessment

• Bedside wound examination: probe the incision for fluctuance, undermining, or fascial disruption. Purulent drainage from the incision satisfies CDC/NHSN criteria on its own — culture is not required to make the diagnosis.
• Systemic markers: leukocytosis with left shift supports infection, but CRP normally rises and peaks in the first few postoperative days; failure of CRP to fall after that expected peak is more informative than a single value. Lactate and blood cultures are obtained when sepsis physiology is present.
• Glucose: perioperative hyperglycemia both predisposes to and is worsened by SSI; check in all patients, diabetic or not.

Microbiology

• Deep tissue or aspirated fluid culture is the specimen of choice. Superficial swabs of an open wound sample colonizers and are a classic distractor.
• Gram stain guides empiric therapy (gram-positive cocci in clusters → S. aureus; mixed flora with gram-negative rods and anaerobes → enteric/perineal source).

Imaging

• Ultrasound for a suspected superficial or subcutaneous collection; CT with IV contrast is the test of choice for suspected organ/space SSI (abscess, anastomotic leak) and can show soft-tissue gas.
• Soft-tissue gas or fascial-plane fluid raises concern for necrotizing infection.

Necrotizing soft tissue infection

• The LRINEC score (CRP, WBC, hemoglobin, sodium, creatinine, glucose) may raise suspicion, but per the IDSA skin and soft tissue infection guideline a low score never excludes it — surgical exploration is the diagnostic gold standard, with "dishwater" gray fluid, absent fascial resistance to blunt finger dissection, and non-bleeding, non-contracting muscle.

Immediate priorities

• Assess for necrotizing infection or sepsis: hypotension, pain out of proportion, crepitus, bullae, or rapidly advancing erythema mandates emergent operative debridement plus broad empiric coverage — an antipseudomonal beta-lactam (piperacillin-tazobactam or a carbapenem) plus an anti-MRSA agent (vancomycin or linezolid) plus clindamycin for ribosomal toxin suppression, per the IDSA skin and soft tissue infection guideline (clindamycin is added for toxin suppression when vancomycin is the anti-MRSA agent; linezolid is itself a protein-synthesis inhibitor and already suppresses toxin production). Antibiotics never substitute for debridement.
• Fascial dehiscence/evisceration is a surgical emergency: cover bowel with saline-moistened gauze, do not attempt reduction, and return to the operating room.

Source control is first-line for ordinary SSI

• Open the incision, evacuate pus, debride devitalized tissue, and pack the wound to heal by secondary intention. For an uncomplicated superficial incisional SSI without systemic signs, IDSA guidance is that opening and drainage alone suffices — antibiotics add nothing.
• Adjunctive antibiotics are indicated when there is significant surrounding cellulitis or systemic response (fever, tachycardia, leukocytosis). Choose by operative site, following the IDSA scheme:
• Trunk, head/neck, extremity (excluding axilla and perineum): a first-generation cephalosporin (cefazolin) or an antistaphylococcal penicillin; vancomycin if MRSA risk or known colonization.
• Axilla, perineum, GI tract, female genital tract: these sites harbor gram-negative and anaerobic flora, so cover both — ceftriaxone plus metronidazole, cefoxitin or cefotetan, ciprofloxacin plus metronidazole, or piperacillin-tazobactam for severe disease.
• Vancomycin is dosed to a 24-hour AUC with a target AUC/MIC of 400–600 (2020 IDSA/ASHP/PIDS/SIDP consensus); trough-only targeting has been retired.

Definitive/adjunct management

• Organ/space SSI: percutaneous image-guided drainage when a discrete collection exists; operative washout for multiloculated collections, anastomotic leak, or failure of drainage.
• Infected prosthetic material frequently requires removal or staged exchange because biofilm defeats antibiotics.
• Negative-pressure wound therapy and delayed primary closure for contaminated wounds; optimize glycemia, normothermia, tissue oxygenation, and nutrition per the CDC 2017 SSI prevention guideline.

Contraindicated/low-value

• Continuing prophylactic antibiotics after closure — CDC 2017 recommends against additional prophylactic doses once the incision is closed, even with drains in place.
• Topical antibiotics applied to the surgical incision, primary closure over undrained pus, and treating culture data from superficial swabs.

Emergencies

• Necrotizing soft tissue infection: bacterial toxins and thrombosis of perforating vessels cause fascial-plane necrosis faster than the overlying skin changes. Signals are pain out of proportion, crepitus, bullae, skin anesthesia, and gas on imaging. Presentation within 24–48 hours of surgery points to Streptococcus pyogenes or Clostridium perfringens. Requires immediate debridement.
• Fascial dehiscence and evisceration: failure of the fascial closure (technical, raised intra-abdominal pressure, or impaired collagen deposition) typically around postoperative days 5–8, heralded by salmon-colored serosanguineous drainage. Evisceration is a surgical emergency.
• Sepsis/septic shock from an undrained organ/space collection — the finding is persistent fever and leukocytosis despite appropriate antibiotics, meaning source control has not been achieved.
• Toxic shock syndrome: superantigen-driven polyclonal T-cell activation with diffuse erythroderma and shock, sometimes from a deceptively benign-appearing wound.

Disease-related, non-emergent

• Chronic non-healing wound: persistent inflammatory phase with elevated MMP activity degrading matrix faster than it is deposited.
• Incisional hernia: late sequela of deep incisional infection or occult dehiscence; presents as a reducible bulge months later.
• Enterocutaneous fistula: deep infection eroding into bowel; feculent or bilious drainage from the incision.
• Hypertrophic scar vs. keloid: hypertrophic scars stay within the original wound margins and often regress; keloids extend beyond the original borders, do not regress, and recur after simple excision.
• Wound contracture from persistent myofibroblasts, limiting joint range of motion.
• Marjolin ulcer: squamous cell carcinoma arising in a chronic wound or burn scar — suspect with a new heaped-up, non-healing edge.

Treatment-related

• Clostridioides difficile colitis from antibiotic disruption of colonic flora — new watery diarrhea and leukocytosis.
• Acute kidney injury, notably with vancomycin plus piperacillin-tazobactam; monitor creatinine and AUC-guided vancomycin exposure.
• Vancomycin infusion reaction (red man syndrome): histamine release, not IgE-mediated; slow the infusion rather than abandoning the drug.
• Antimicrobial resistance selected by prolonged prophylaxis.

• Timing tells you the organism: a wound infection appearing within the first 24–48 hours is Streptococcus pyogenes or Clostridium perfringens until proven otherwise; typical staphylococcal SSI presents around postoperative days 5–10. The very early wound infection demands opening and inspection, not observation.
• Single best next step for a draining, fluctuant incision: open the wound and drain it. Antibiotics alone for undrained pus is the most commonly tested wrong answer, and per IDSA an uncomplicated superficial SSI without systemic signs needs no antibiotic at all.
• Fascial dehiscence (detailed under Complications): the tested move is operative repair; if bowel is visible, cover with saline-moistened gauze and go to the OR — do not reduce it at the bedside.
• Site drives empiric coverage: IDSA groups the axilla with the perineum, GI, and genitourinary tract — these need gram-negative plus anaerobic coverage, not cefazolin alone. Only trunk, head/neck, and extremity incisions outside those zones get antistaphylococcal monotherapy.
• Collagen switch: type III collagen dominates the proliferative phase and is replaced by type I during remodeling. Vitamin C deficiency blocks prolyl/lysyl hydroxylation (weak, easily disrupted scars and old wounds reopening); zinc is a cofactor for MMPs; vitamin A partially reverses the healing impairment caused by chronic corticosteroids.
• Keloid vs. hypertrophic scar: keloid extends beyond the original wound margins, favors earlobe, sternum, and shoulder, is more common in patients with darker skin, and recurs after excision alone — treat with intralesional corticosteroid, not simple excision.
• Prophylaxis facts examiners love: cefazolin within 60 minutes of incision (vancomycin and fluoroquinolones within 120 minutes because of infusion time), redose for prolonged operations or large blood loss, and stop at wound closure — CDC 2017 recommends no further prophylactic doses even with drains in place.
• Skin preparation: clip hair, never shave with a razor (microabrasions inoculate flora); alcohol-based chlorhexidine antisepsis is preferred over aqueous povidone-iodine.
• Culture pitfall: send deep tissue or aspirated fluid, never a superficial swab of an open wound. And if vancomycin is used, remember it is now dosed to AUC/MIC 400–600, not to a 15–20 mcg/mL trough.