Wound healing represents a complex physiological process where the microbiome plays a critical and often underappreciated role. Understanding wound microbiota dynamics illuminates why some wounds heal smoothly while others progress toward chronicity, and how bacterial biofilms perpetuate non-healing wounds affecting millions of individuals.
Wound microbiota undergo predictable ecological succession. Acute wounds begin with a phase of contamination: bacteria from skin and environment enter the wound but are not yet causing inflammation. Early colonization follows, where bacteria proliferate and adapt to the wound environment without yet overwhelming host defences—acute wounds show 10^3-10^4 bacteria per gram of tissue. This phase is characterized by planktonic bacteria, not yet forming biofilms. Most acute wounds progress through these phases toward healing without developing infections, a testament to wound antimicrobial defences.
The critical bacterial load threshold—approximately 10^5-10^6 bacteria per gram of tissue—distinguishes colonization from infection. When bacterial burden exceeds this threshold, bacteria overwhelm local immune responses and inflammatory responses escalate, impeding wound healing. Interestingly, this threshold can vary between wound types and individual immune status—a surgical patient with intact immunity might tolerate 10^6 bacteria per gram, while an immunocompromised individual might develop clinical infection at lower burdens.
Chronic wounds show dramatically different microbiota patterns than acute wounds. Approximately 90% of chronic wounds harbor biofilms, compared to fewer than 6% of acute wounds. Biofilms are structured bacterial communities embedded in polysaccharide matrices with multiple phenotypic states including some cells in stationary phase with reduced metabolic activity and enhanced survival. Within biofilms, bacterial cells show altered antibiotic susceptibility—increased resistance to both antibiotics and antimicrobial peptides. Biofilm-embedded cells communicate through quorum sensing, coordinated gene expression triggered by bacterial density-dependent signals, allowing synchronized virulence factor production.
Chronic wounds typically contain polymicrobial communities, not single pathogens. The diversity of bacteria in chronic wounds is substantial, with 20-50 distinct species commonly found. This polymicrobial ecology includes both pathogenic species (Pseudomonas aeruginosa, Staphylococcus aureus) and seemingly commensal species (Corynebacterium, Propionibacterium). However, the notion that some wound bacteria are "commensal" and others "pathogenic" oversimplifies chronic wound microbiology. Often, commensal species have specific metabolic roles in the biofilm community: some might degrade biofilm matrix for nutrient acquisition; others might produce metabolites that other species utilize; still others might produce antimicrobial compounds against competing bacteria while being tolerated by biofilm-embedded cells through resistance mechanisms.
S. aureus deserves special mention as a particularly damaging wound pathogen. Wound colonization with S. aureus correlates strongly with wound infection and impaired healing. S. aureus produces multiple virulence factors: α-hemolysin creates pores in immune and epithelial cells; Panton-Valentine leukocidin kills phagocytes; tissue-degrading enzymes destroy wound matrix. However, S. aureus's primary impact in chronic wounds may come from its biofilm-forming capacity and quorum sensing-mediated virulence production rather than individual virulence factors.
S. epidermidis, traditionally viewed as a commensal, can paradoxically improve or impair wound healing depending on context. In acute wounds, S. epidermidis competitively excludes S. aureus and other pathogenic organisms, supporting normal healing. In chronic wounds, however, S. epidermidis can form biofilms that perpetuate wound chronicity, producing polysaccharide matrices that prevent immune cell infiltration and epithelial cell migration necessary for healing.
Anaerobic bacteria predominate in deep chronic wounds, particularly in lower extremity wounds and wounds with compromised vascular supply. Species like Bacteroides fragilis, Peptostreptococcus, and Clostridium difficile utilize the anaerobic environment and oxygen-depleted wound core, producing metabolites including ammonia and hydrogen sulfide that impair local immune function and epithelial cell function.
The biofilm-perpetuated chronic wound state becomes self-sustaining. Biofilm-driven inflammation triggers persistent Th2 responses (IL-4, IL-13 elevation) rather than Th1 responses needed for pathogen clearance. TGF-β elevation drives fibroblasts toward myofibroblasts (wound contraction phenotype) that don't support epithelialization. Proteases from neutrophils and biofilm bacteria degrade extracellular matrix and growth factors faster than they can be replaced. The result is a chronic inflammatory state where healing phase progression halts indefinitely.
Clinical management approaches target multiple microbial mechanisms. Sharp debridement removes necrotic tissue and biofilm. Antimicrobial dressings use silver, iodine, or other antimicrobial compounds to suppress biofilms. Negative pressure wound therapy creates vacuum that removes exudate and biofilm material while promoting granulation tissue formation. Maggot therapy—intentional colonization with sterile Lucilia sericata larvae—accomplishes remarkable biofilm disruption: larvae produce antimicrobial compounds, mechanically disrupt biofilms through their movement, and selectively consume necrotic tissue while avoiding healthy tissue.
Understanding wound microbiota as complex ecological communities rather than simple infections supports more sophisticated management. Strategies promoting beneficial microbiota communities while suppressing pathogenic biofilms, or supporting innate immune function rather than simply killing bacteria, represent emerging approaches to chronic wound healing.