🌍 Microbial Worldview: Complexity Everywhere

Scientists study bacteria within complex, mixed communities. These communities contain countless interacting species, environmental gradients, and biochemical processes — from the smallest microbes to large ecosystems. This lecture explores how microbes live and organize themselves, focusing on biofilms — one of the dominant bacterial lifestyles.

🧫 Biofilms: How Bacteria Actually Live

What are they?

Biofilms are aggregates of microorganisms (bacteria, algae, fungi) that stick to surfaces and are embedded in a self-produced slimy matrix of polymers (EPS – extracellular polymeric substances). They can form on any surface — teeth, rocks, medical implants, water pipes, etc.

🦠 Planktonic vs. Biofilm

• Planktonic bacteria = free-floating, single cells.
• Biofilm bacteria = surface-attached, clustered communities. In nature, almost all bacteria live in biofilms, not as single free cells.

🌡️ Environmental Adaptability

Bacteria thrive everywhere — from icy oceans to boiling vents:

• Thermophiles / Hyperthermophiles – love heat.
• Psychrophiles – cold-loving.
• Acidophiles / Alkaliphiles – tolerate extreme pH.
• Halophiles – salt-tolerant. Such diversity makes bacteria valuable for biotechnology (e.g., enzymes for detergents that resist high temperature).

🧴 Everyday Examples of Biofilms

• Teeth plaque – a daily-forming bacterial biofilm.
• Contact lenses – if not cleaned, bacteria can form films causing eye infections (example: Pseudomonas causing blindness).
• Medical implants – artificial joints, catheters, or heart valves are hotspots for biofilm infections since they provide surfaces without immune defense.

🧱 Biofilms in Technology and Industry

Biofilms appear in wastewater treatment, paper manufacturing, drinking water systems, and industrial pipelines.

💧 In Wastewater

• Flocs: loose aggregates of microbes.
• Granules: compact microbial spheres.
• Biofilms: dense layers on reactor surfaces. All function in water purification but can cause clogging if uncontrolled.

⚙️ Problems

• Biofouling: unwanted biofilm buildup on surfaces (pipes, heat exchangers, membranes).
• Corrosion: some bacteria accelerate metal degradation by producing sulfides or acids.
• Economic losses: billions yearly (oil, maritime, and water industries).

💡 Benefits

• Biofilm polymers (EPS) can be harvested:
  • Used in fertilizers, bioplastics, flame retardants (potential PFAS replacements).
  • Even for art and jewelry, since EPS forms biopolymers with unique textures.

🧬 The Structure of a Biofilm

Think of a miniature city:

• Cells = inhabitants.
• EPS matrix = buildings and streets.
• Channels = nutrient highways.
• Pores = oxygen and waste diffusion zones.

EPS composition:

• Polysaccharides
• Proteins
• Extracellular DNA (eDNA)
• Lipids
• Humic substances (complex organic residues)

The EPS gives structural stability, traps nutrients, protects cells from toxins, and enables communication and gene transfer (including antibiotic resistance genes).

🧩 Composition Details

🧬 Extracellular DNA (eDNA)

Not just debris — eDNA helps cells attach to surfaces, transfer electrons, and stabilize the matrix.

🍬 Polysaccharides

Produced by genes in clusters (operons) that encode enzymes for synthesis and export. Examples: cellulose, alginates — both used industrially.

🧠 Amyloids

Protein fibers that provide mechanical strength to biofilms. In humans, amyloids are linked to diseases like Alzheimer’s, but in bacteria they act like biological glue (similar to spider silk).

⚖️ Environmental Factors Controlling Biofilm Growth

Biofilm thickness and stability depend on:

1. Nutrient availability – more food → faster growth.
2. Oxygen or electron acceptors – control microbial metabolism.
3. Bacterial species – different bacteria form different structures.
4. Flow/shear rate – high flow can wash biofilm off.
5. Temperature and pH – shape bacterial diversity.
6. Surface properties – rough or charged surfaces promote adhesion.
7. Predators – protozoa or nematodes graze on biofilms.
8. Time – older biofilms are thicker and more resistant.

⚗️ Gradients and Micro-Environments

Within a single biofilm:

• Top layers: aerobic (oxygen available).
• Bottom layers: anaerobic (fermenters).
• Middle zones: dormant or VBNC cells (viable but nonculturable).

This creates chemical gradients for nutrients, pH, and redox potential, forming tiny ecosystems inside the film.

🧠 Communication: Quorum Sensing

Bacteria "talk" via signaling molecules (e.g., N-acyl homoserine lactones). When concentration of these molecules increases, it signals “crowding.” → They trigger gene expression changes:

• EPS production
• Movement (swarming or dispersal)
• Sporulation or virulence

This process is essential for coordinated group behavior.

🧟‍♂️ Predation and Defense

Biofilms face predators like protozoa and bacteriophages (viruses). Defense strategies include:

• Producing killer vesicles (enzymatic bombs).
• Making antibiotics to inhibit competitors.
• Some bacteria act as “vampires” (e.g., Vampirococcus), attaching to others and consuming them.

🧮 Metabolic Cooperation

Different bacteria specialize in different tasks:

• One species breaks down proteins.
• Another eats nucleic acids.
• Another oxidizes ammonia or nitrite. Together, they form metabolic consortia — communities optimized for complete degradation of complex matter.

🧱 Resistance and Resilience

Biofilm bacteria are much more resistant to antibiotics and disinfectants than planktonic ones because:

• EPS acts as a physical barrier.
• Cells deep inside are metabolically slow.
• Some cells (persisters) survive treatment and re-grow later.

This means antibiotic testing on single-cell cultures underestimates real-world resistance. Biofilms must be tested directly.

🧩 Summary Table

🧠 Takeaway

Biofilms are the default lifestyle of bacteria — resilient, cooperative, and complex. They shape environments, drive biogeochemical cycles, cause industrial and medical issues, and hold huge potential for biotechnology.