Biofilms & Quorum Sensing

1. The Biofilm Life Cycle

A typical biofilm progresses through four stages:

1. Reversible attachment — planktonic cells bump into a surface; weak van der Waals and hydrophobic interactions hold them briefly.
2. Irreversible attachment — pili and adhesins lock cells in place; flagellar genes turn off, EPS genes turn on.
3. Maturation — cells divide, secrete extracellular polymeric substances, build mushroom- or pillar-shaped structures penetrated by water channels.
4. Dispersal — under nutrient depletion or stress, cells release matrix-degrading enzymes, return to the planktonic state, and seed new biofilms elsewhere.

Mature biofilms are heterogeneous: oxygen and nutrient gradients lead to spatially segregated metabolic states (active aerobic at the surface, anaerobic and slow at the base). This heterogeneity contributes to the famous antibiotic tolerance of biofilms (Module 6).

2. The Extracellular Polymeric Matrix

The biofilm matrix typically accounts for ~90% of the biofilm dry mass and consists of:

• Polysaccharides — e.g., PIA in Staphylococcus, alginate in Pseudomonas aeruginosa, cellulose in Salmonella.
• Proteins — amyloid-like curli fibres in E. coli, BAP family adhesins.
• Extracellular DNA (eDNA) — released by cell lysis or active secretion. Mechanically stabilises the matrix; serves as a reservoir for horizontal gene transfer.
• Water — up to 97% of the matrix volume; channels deliver nutrients to the interior.

The matrix is a viscoelastic gel of measurable rheology — biofilms can be characterised by storage modulus G′ (typically 10–100 Pa). They are physically tough and chemically permselective, slowing diffusion of antibiotics by ~10× compared to bulk water.

3. Quorum Sensing: V. fischeri and the lux Operon

The original quorum-sensing system is in Vibrio fischeri, the bioluminescent symbiont of the Hawaiian bobtail squid Euprymna scolopes. The cells:

• Produce a small autoinducer, 3-oxo-C6 homoserine lactone (3-oxo-C6-HSL), by the LuxI synthase.
• Detect HSL with the LuxR receptor, which dimerises on binding and activates transcription of the luxICDABE operon.
• The operon contains luxI itself (positive feedback) and the bioluminescence genes luxAB (luciferase) and luxCDE (substrate synthesis).

At low cell density HSL diffuses away; at high density it accumulates, LuxR activates, and the population “decides” collectively to produce light. In the squid’s light organ, where cell density reaches ~10¹¹/mL, the squid’s ventral camouflage from moonlight depends on it.

4. Two Quorum-Sensing Languages

• AHLs in Gram-negatives — acyl-homoserine lactones with varying acyl-chain lengths and modifications. ~70 species use them; LuxR-type receptors are mostly cytoplasmic.
• AIPs in Gram-positives — cyclic autoinducer peptides. Staphylococcus aureus Agr system: AIP secreted, detected by membrane sensor kinase AgrC, response regulator AgrA activates virulence (RNAIII).

AI-2, a furanone-borate-based molecule, is proposed as a universal cross-species signal. Its actual ecological role is debated but it is found across both Gram-positives and Gram-negatives.

5. Vibrio cholerae and Inverted Quorum Sensing

V. cholerae, the cholera pathogen, uses quorum sensing in reverse: at high cell density it represses virulence genes and activates dispersal (escape from the host gut); at low density (early infection) it activates cholera toxin and the toxin-coregulated pilus. The signalling pathway converges on the master regulator HapR. Bonnie Bassler’s lab dissected this circuit; small-molecule HapR agonists are being explored as anti-virulence drugs — the idea that you can disarm the bacterium without killing it.

6. Biofilms in Disease

~80% of chronic bacterial infections are biofilm-associated: cystic fibrosis lungs (Pseudomonas), urinary catheters (E. coli, Staphylococcus), dental plaque (Streptococcus mutans), endocarditis (S. aureus on valves), prosthetic-joint infections, chronic wounds. Biofilm cells display tolerance — not genetic resistance — to antibiotics, requiring concentrations 10–1000× higher than planktonic minimum inhibitory concentrations. Anti-biofilm strategies include matrix-degrading enzymes (DNase), quorum-quenching molecules, and cyclic-di-GMP-modulating compounds that promote dispersal.