Biofilm Formation in Orthopaedic Infections

Extracellular Polymeric Substance (EPS) Matrix

The matrix is mostly water, and the bacteria are a small minority of what is there. By volume, water is 80-90% and bacterial cells only 10-15%. The dry weight divides as:

• Polysaccharides: 40-50% (the structural backbone)
• Proteins: 20-30% (enzymes, adhesins; in staphylococci the cell wall also carries teichoic acids)
• eDNA (extracellular DNA): 10-20%
• Lipids: 5-10%

Polysaccharides. The polysaccharide differs by organism:

• PIA (polysaccharide intercellular adhesin) in S. epidermidis, encoded by the ica genes
• PNAG (poly-N-acetylglucosamine) in Staphylococcus species
• Alginate, Pel and Psl in Pseudomonas aeruginosa

Whichever the species, the polysaccharide is the structural scaffold, mediates adhesion, and protects against desiccation and immune cells.

The ica operon. The icaADBC operon in S. epidermidis encodes the enzymes for PIA synthesis, and icaA and icaD are essential. Bacteria lacking the ica genes cannot form biofilm and are less virulent in prosthetic infection. Biofilm-negative strains exist but are uncommon among clinical isolates.

Extracellular DNA. eDNA is released from lysed bacteria or actively secreted. It scaffolds the matrix, it binds cationic antibiotics (aminoglycosides, polymyxins) and so reduces their penetration, and it carries antibiotic resistance genes for horizontal transfer within the biofilm. DNase treatment can disrupt a young biofilm, so far a research application.

Proteins. Adhesins bind host proteins and the implant surface; the enzymes include beta-lactamases, proteases and nucleases; amyloid fibrils give structural support in Staphylococcus and E. coli.

Three-Dimensional Biofilm Architecture

The structure. A mature biofilm is not a uniform slab. A dense basal layer adheres to the surface, mushroom-like towers of bacterial aggregates extend into the fluid, and water channels 10-50 micrometres wide run between them carrying nutrients in and waste out, a primitive circulatory system that reaches the deep layers. Density, pH and oxygen all vary from place to place.

Microenvironments. Oxygen falls with depth: the outer layers are aerobic with active growth, the middle layers microaerobic with moderate metabolism, the deep layers anaerobic with slow growth. Bacterial metabolism creates pH gradients (pH 5-7) and the depths are nutrient-limited. The oxygen gradient creates niches for different metabolic states, and the nutrient limitation in the depths is what drives dormancy.

Persister Cells - The Achilles' Heel of Treatment

Definition. Persister cells are phenotypic variants that are dormant, non-growing and tolerant to all antibiotics despite being genetically identical to their susceptible neighbours. They make up 0.1-1% of biofilm bacteria and sit deep in the biofilm where nutrients are limited. The state is reversible: when conditions improve they return to active growth.

Mechanisms of persistence. Dormancy is induced by:

• Toxin-antitoxin systems (HipA-HipB, MazE-MazF)
• Stringent response: the (p)ppGpp alarmone slows metabolism
• SOS response: DNA damage response with growth arrest
• Low ATP: insufficient energy for active transport of antibiotics

Quorum Sensing - Bacterial Communication

Definition. Quorum sensing is cell-to-cell communication through small diffusible signal molecules (autoinducers) that coordinate group behaviour once bacterial density reaches a threshold. It lets the population act as a multicellular organism rather than as individuals.

Mechanism.

1. Bacteria produce autoinducer molecules (AI)
2. AI concentration rises as bacterial density rises
3. At the threshold (the quorum), AI binds its receptor
4. The receptor activates gene expression
5. Coordinated behaviour follows: biofilm formation, virulence, dispersal

The systems.

• Gram-positive (Staphylococcus): autoinducing peptides (AIP) and the accessory gene regulator (agr) system
• Gram-negative (Pseudomonas): acyl-homoserine lactones (AHL), the LasI/LasR and RhlI/RhlR systems
• Universal: AI-2, used by both gram-positive and gram-negative bacteria

What it regulates. EPS production begins once density is sufficient; toxins and proteases are activated at high density; dispersal and planktonic spread are triggered, which is how quorum sensing signals can precipitate an acute exacerbation; and efflux pumps and other resistance mechanisms are induced.

View larger

Therapeutic target. Quorum sensing inhibitors (QSI) such as halogenated furanones and anti-AIP antibodies block bacterial communication, preventing biofilm formation and reducing virulence. They are experimental and not yet clinically available, but a promising direction for preventing implant infection.

Biofilm in Prosthetic Joint Infections

On the implant. Bacteria attach within hours of contamination, whether intraoperative or haematogenous, and a mature biofilm is established by 48-72 hours. Coagulase-negative staphylococci, S. epidermidis above all, are the master biofilm formers. Once formed, the biofilm is a sanctuary from antibiotics and immune cells.

View larger

The clock. Acute infection is defined by symptom duration, not by time since surgery. The clock that matters is how long the joint has been symptomatic, conventionally under about 3 weeks, because that is what approximates biofilm maturity. Time since implantation is a separate and weaker limb: IDSA additionally allows retention within about 30 days of implantation even where symptoms are harder to date. These are alternatives, not a conjunction.

Acute infection. The biofilm is not yet mature or well established, so DAIR (debridement, antibiotics, implant retention) is possible in a well-fixed implant with no sinus tract. It requires aggressive debridement, exchange of the modular parts and biofilm-active antibiotics (rifampicin). Infection-free survival at about 2 years on the retained implant is roughly 50-70% in reported series, but this is not one number: it falls with S. aureus, a retained sinus tract, a loose implant, delay to debridement, and where rifampicin cannot be used, and rises with streptococci and early, well-selected cases.

Chronic infection. After 3 weeks of symptoms the biofilm is mature and implant removal is mandatory for cure. Two-stage exchange removes the implant and places an antibiotic spacer, with reimplantation after 6-12 weeks. Suppressive antibiotics without removal give temporary symptom control and eventual failure.

Biofilm-active antibiotics. Penetration differs by drug:

• Rifampicin: best biofilm penetration
• Fluoroquinolones: moderate penetration, bactericidal
• Linezolid: good penetration, oral bioavailability
• Daptomycin: biofilm activity against Staphylococcus
• Avoid vancomycin: poor biofilm penetration despite intravenous use

Biofilm in Chronic Osteomyelitis

The sequestrum. Dead bone provides the surface for biofilm attachment, and because the tissue is avascular there is no blood flow to deliver antibiotics or immune cells to it. The biofilm on it is impenetrable to antibiotics and protects the bacteria from host defences, and its persisters survive every antibiotic course and reactivate when the antibiotics stop, which is why chronic osteomyelitis relapses.

Treatment principles.

• Debridement of all necrotic tissue, to remove the biofilm substrate
• Sequestrum removal: it cannot be sterilised with antibiotics
• Preservation of viable tissue while necrotic bone is excised
• Dead space management: antibiotic beads, muscle flap, bone graft
• Prolonged antibiotics, 6-12 weeks intravenous and oral, to suppress the remaining bacteria

Preventing Biofilm Formation

The race for the surface. Gristina's concept (1987): an implant surface is contested between host tissue cells, seeking osseointegration or tissue integration, and bacteria, seeking colonisation and biofilm formation, and whichever reaches and covers the surface first tends to win. If host cells integrate first, the surface is covered by living, immunocompetent tissue that resists colonisation. If bacteria attach first, they lay down EPS and become a mature biofilm before host cells and the immune system can act.

Why implants are uniquely vulnerable. An implant is dead, avascular and immune-privileged, so even a tiny inoculum can establish infection: the implant lowers the infective dose by orders of magnitude, and a small number of bacteria that native tissue would clear causes infection on an implant. Prevention therefore concentrates on helping the host win the race, through correctly timed prophylaxis and antiseptic technique in the critical early window, and through surface engineering (silver or antibiotic coatings, anti-fouling or nanotextured and pro-osteointegration surfaces) that biases the surface toward host-cell coverage.

Antibiotic prophylaxis. Given within 60 minutes before incision (optimal: 30 minutes), so that therapeutic tissue levels are present before bacterial contamination and initial attachment (Stage 1) is prevented. Given too early, over 2 hours before, levels drop before closure; given after incision, the bacteria are already attached. Cefazolin 2 g intravenously (3 g if over 120 kg) is standard for arthroplasty, redosed if surgery runs over 4 hours (its half-life is about 2 hours) or blood loss exceeds 1500 mL.

Surgical technique.

• Minimise operative time (less contamination exposure)
• Gentle tissue handling (less necrosis and haematoma)
• Copious irrigation (removes bacteria before attachment)
• Wound closure without dead space
• Minimise implant surface area (only what is needed)

Implant modifications (research and emerging).

• Antibiotic-loaded bone cement: PMMA with gentamicin or vancomycin
• Silver-coated implants: an antimicrobial surface (Ag+ ions are toxic to bacteria)
• Chlorhexidine coatings: prevent bacterial attachment
• Nanotextured surfaces: too small for bacteria to attach (research)

View larger

Patient optimisation.

• Glycaemic control: HbA1c under 7.5% (diabetes impairs immunity)
• Smoking cessation: 4-6 weeks preoperatively (improves vascularity)
• Weight loss: BMI under 40 (fewer wound complications)
• Nutritional optimisation: albumin over 3.5 g/dL
• MRSA decolonisation: nasal mupirocin and chlorhexidine baths if colonised

Diagnosing Biofilm Infections

Why culture fails. Standard culture methods target planktonic bacteria, but biofilm bacteria are dormant and slow-growing and may never be released from the implant surface, so they are difficult to culture. The culture-negative rate in PJI is 10-30%. Sonication and multiple tissue samples are the most practical ways to improve the yield.

Sonication. The explanted implant is placed in saline and ultrasonic waves disrupt the biofilm and release the bacteria; the fluid is then cultured. This increases culture yield by 20-30% compared with tissue samples alone, and greater than 50 CFU/mL is the threshold for infection (Trampuz criteria).

Multiple tissue samples. Take 5-7 samples from different sites (MSIS criteria); two or more positive cultures of the same organism means infected. More samples give higher sensitivity for biofilm bacteria.

Extended incubation. Standard incubation is 5 days. Slow-growing biofilm bacteria need longer to grow in culture, so incubation is extended to 7-14 days for fastidious organisms (Cutibacterium, Kingella, fungi).

Molecular diagnostics. 16S rRNA PCR detects bacteria in culture-negative samples, and next-generation sequencing identifies every organism present, including the unculturable. Both are expensive and not routine, but helpful in culture-negative PJI.

Biomarkers.

• Synovial fluid analysis: WBC over 3000/μL, PMN over 80% suggests infection
• Alpha-defensin: an antimicrobial peptide elevated in PJI (sensitivity 80-97%)
• Leucocyte esterase: a point-of-care colorimetric strip