Mutation & Diversification in Biofilms lecture

1. Introduction to Biofilms

Definition

  • Biofilms are surface-attached, structured communities of bacteria encased in an extracellular polymeric matrix.

  • Represent a fundamentally different lifestyle from planktonic (free-swimming) bacteria.

  • Biofilm formation is a developmental and regulated process, not random surface accumulation.

Key Themes of the Lecture

  • Differences between planktonic vs biofilm growth

  • Biofilm lifecycle

  • Differentiation and division of labour

  • Mutation and phenotypic variation

  • Clonal selection in microcolonies

  • Dispersal mechanisms

  • Implications for control strategies

2. Biofilm Lifecycle

Biofilm formation follows a developmental sequence:

2.1 Initial Attachment

  • Planktonic cells approach surface.

  • Reversible attachment via weak interactions.

  • Transition to irreversible attachment.

2.2 Matrix Production

  • Production of extracellular polymeric substances (EPS).

  • Matrix functions:

    • Structural support

    • Adhesion

    • Protection

    • Diffusion barrier

2.3 Microcolony Formation

  • Cells proliferate on surface.

  • Clusters form → microcolonies.

  • Begin developing 3D architecture.

2.4 Maturation

  • Complex, heterogeneous 3D structures.

  • Channel formation.

  • Nutrient and oxygen gradients develop.

2.5 Dispersal

  • Subpopulations regain motility.

  • Cells escape and recolonise new surfaces.

  • Biofilm lifecycle restarts.

3. Biofilm Structure

3D Architecture

Observed using confocal laser scanning microscopy.

Characteristics:

  • Heterogeneous structure

  • Microcolonies separated by channels

  • Not uniform “slime layer”

  • Spatial differentiation of cells

Microcolony Architecture

  • Aggregated bacterial clusters.

  • Important for:

    • Survival

    • Evolutionary selection

    • Antibiotic tolerance

4. Clinical Relevance – Cystic Fibrosis (CF)

Example: Pseudomonas aeruginosa

In CF lungs:

  • Bacteria grow as microcolony aggregates in sputum.

  • Presence of both:

    • Single cells

    • Aggregated biofilm clusters

Importance

Biofilm growth contributes to:

  • Chronic infection

  • Antibiotic tolerance

  • Difficulty in eradication

5. Mechanisms of Antibiotic Tolerance in Biofilms

Tolerance is multifactorial.

5.1 Matrix Barrier

  • Physical obstruction of antibiotic diffusion.

  • Charge interactions reduce antibiotic penetration.

  • Antibiotics may bind to matrix components.

5.2 Enzymatic Degradation

  • Secretion of antibiotic-degrading enzymes.

  • Enzymes retained within matrix.

5.3 Nutrient and Oxygen Gradients

  • Interior cells experience:

    • Low oxygen

    • Low nutrient levels

  • Leads to slow growth or dormancy.

5.4 Dormancy and Persister Cells

  • Subpopulation becomes metabolically inactive.

  • Many antibiotics require active cell division.

  • Dormant cells survive treatment.

5.5 Physiological Alterations

  • Biofilm growth alters gene expression.

  • Changes stress responses and metabolism.

6. Clonal Microcolony Formation Experiment

Experimental Setup

  • Two genetically identical P. aeruginosa strains.

  • One tagged with blue fluorescent protein.

  • One tagged with yellow fluorescent protein.

  • Mixed in 1:1 ratio.

  • Grown in flow cell biofilm system.

Observation

  • Microcolonies were single-colour.

  • Aggregates were not mixed.

  • Surface “carpet layer” may contain mixed cells.

Interpretation

  • Microcolonies likely arise from:

    • Single founding cell.

    • Clonal expansion.

  • Suggests:

    • Competitive advantage of certain cells.

    • Selection occurring within biofilm.

7. Phenotypic Variation in Biofilms

Observations from Plating Biofilm-Derived Cells

Compared to planktonic cultures:

  • Colonies show:

    • Size variation

    • Surface texture differences

    • Altered motility

    • Smooth vs wrinkled morphologies

Metabolic Changes

Using Biolog carbon utilisation plates:

  • 96 carbon sources tested.

  • Respiratory dye turns purple if substrate utilised.

  • Biofilm-derived bacteria:

    • Gain ability to metabolise new substrates.

    • Lose ability to metabolise others.

Conclusion

Biofilm growth promotes:

  • Phenotypic diversity

  • Metabolic variation

  • Adaptive flexibility

8. Role of Mutation in Biofilm Formation

Hypothesis

Genetic mutation and variation contribute to biofilm development.

Mutator Strains

  • Mutations introduced in mismatch repair genes.

  • Leads to higher mutation frequency.

Findings

Compared to wild-type:

  • Increased biofilm biomass.

  • Larger microcolonies.

  • Greater heterogeneity.

Correlation

Higher mutation frequency → increased biofilm formation.

9. Whole Genome Sequencing of Biofilms

Method

  • Biofilms grown for 8–9 days.

  • DNA extracted.

  • High-throughput sequencing (50× coverage).

  • Compared to reference genome.

Findings

  • Mutations distributed across genome.

  • Mostly:

    • Single nucleotide polymorphisms (SNPs).

  • Occurred in:

    • Coding regions

    • Non-coding regions

Affected Functional Categories

  • Metabolism

  • Gene regulation

  • Transport systems

  • Stress response genes

Conclusion:

  • Rapid genetic diversification occurs in biofilms.

10. Mutation Localisation – GFP Reversion System

Design

  • Inserted non-functional GFP gene (frameshift mutation).

  • If spontaneous mutation restores reading frame → GFP fluoresces.

Observations

  • GFP expression localised within microcolonies.

  • Some entire colonies fluorescent → early mutation followed by clonal expansion.

Quantification

Mutation frequency:

  • 100–1000× higher in microcolonies than elsewhere.

Conclusion:
Microcolonies are mutation hotspots.

11. Evolutionary Model of Biofilm Growth

Proposed model:

  1. Initial mutation

  2. Clonal expansion

  3. Secondary mutation

  4. Further selection and expansion

Analogy to Cancer

  • Mutation

  • Selection

  • Clonal succession

  • Formation of structured tumour mass

Implication:
Biofilms undergo Darwinian evolution within structure.

12. Biofilm Dispersal

Concept

Biofilm formation is not terminal.
Subpopulations differentiate and escape.

Observations

  • Interior cells become motile.

  • Microcolonies hollow out.

  • Leaves structural voids.

Seen in:

  • Laboratory systems

  • Oral biofilms

  • Wastewater granules

13. Marine Example – Pseudoalteromonas tunicata

Observed Pattern

  1. Microcolonies form.

  2. Cells in centre begin dying.

  3. Extensive lysis occurs.

  4. Large-scale detachment.

  5. Few cells remain attached.

Mechanism

Production of autolytic protein (AlpP):

  • ~190 kDa protein.

  • Induces hydrogen peroxide production from lysine.

  • Hydrogen peroxide causes cell death.

Evidence

  • Deletion mutant lacking AlpP:

    • No lysis.

    • No dispersal.

Functional Interpretation

  • Some cells sacrifice themselves.

  • Lysis destabilises biofilm.

  • Facilitates dispersal of surviving cells.

Represents:

  • Division of labour.

  • Population-level benefit.

  • Primitive programmed cell death.

14. Pseudomonas aeruginosa Dispersal Mechanism

Reactive Oxygen & Nitrogen Species

  • Accumulation in microcolonies.

  • Detection using fluorescent dyes.

  • Peroxynitrite formed from:

    • Superoxide

    • Nitric oxide

15. Role of Nitric Oxide (NO)

Nitrite Reductase

  • Produces nitric oxide.

  • Upregulated in late-stage biofilm.

  • Expressed in microcolonies.

Genetic Evidence

  • Deletion of nitrite reductase:

    • No NO.

    • Theicker biofilms.

  • Deletion of NO reductase:

    • Excess NO.

    • Increased lysis and dispersal.

Conclusion:
NO regulates dispersal.

16. Exogenous Nitric Oxide Treatment

Using Chemical NO Donor

Results:

  • Low-dose NO → induces dispersal.

  • High-dose NO → promotes biofilm formation (possible stress response).

Combined Therapy Strategy

NO donor + antibiotic:

  • Loosens biofilm.

  • Converts cells toward planktonic state.

  • Enhances antibiotic killing.

Represents:
Adjunctive treatment strategy.

17. Cyclic-di-GMP Regulation

Master Regulator of Lifestyle

  • High c-di-GMP → biofilm formation.

  • Low c-di-GMP → planktonic state & dispersal.

Enzymes Involved

  • Diguanylate cyclases (GGDEF domains):

    • Synthesize c-di-GMP.

  • Phosphodiesterases (EAL domains):

    • Degrade c-di-GMP.

Nitric oxide influences:

  • Expression of these enzymes.

  • Alters intracellular c-di-GMP.

  • Triggers dispersal.

OVERALL TAKE-HOME MESSAGES

  1. Biofilms are structured, heterogeneous communities.

  2. Microcolonies arise through clonal expansion.

  3. Mutation frequency is elevated in biofilms.

  4. Microcolonies act as evolutionary hotspots.

  5. Biofilm growth involves Darwinian selection.

  6. Dispersal is regulated and genetically controlled.

  7. Nitric oxide is a key dispersal signal.

  8. Targeting dispersal pathways may enhance treatment.