Bacterial Survival Under Stress ppt

1. Introduction: Why Stress Responses Matter

Bacteria constantly experience fluctuating and hostile environments in:

  • Soil

  • Water

  • Host organisms

  • Clinical settings

Stress responses are active, regulated adaptations, not passive reactions.

Key idea: Bacteria shift from growth mode → survival mode through coordinated global gene regulation.

2. Types of Stress Encountered by Bacteria

A. Physical Stress

  • Heat shock (high temperature)

  • Cold shock

  • UV radiation

  • Osmotic pressure changes

  • Desiccation

B. Chemical Stress

  • Oxidative stress (ROS)

  • Acidic or alkaline pH

  • Toxic compounds

  • Antibiotics

C. Nutrient Stress

  • Carbon starvation

  • Nitrogen limitation

  • Phosphate limitation

  • Iron restriction

Each stress affects core cellular systems:

  • Proteins (misfolding, denaturation)

  • DNA (mutations, strand breaks)

  • Membranes (fluidity changes)

  • Metabolism (energy imbalance)

3. Stress Sensing and Signal Transduction

Bacteria detect environmental changes rapidly using:

A. Two-Component Systems

  • Sensor kinase detects stress.

  • Response regulator alters gene expression.

Allows fast, specific responses.

B. Alternative Sigma Factors

  • Redirect RNA polymerase to stress-response genes.

  • Enable global transcriptional shifts.

C. Small Signalling Molecules

  • e.g., alarmones such as (p)ppGpp.

  • Coordinate metabolic adaptation.

4. Sigma Factors and Global Regulation

Sigma factors determine which genes are transcribed.

A. Housekeeping Sigma Factor

  • Active during normal growth.

  • Maintains metabolic and replication functions.

B. Alternative Sigma Factors

Activated during stress.

RpoS (σS) – General Stress Response

  • Induced in stationary phase and under multiple stresses.

  • Controls genes for:

    • Oxidative resistance

    • Osmotic protection

    • Acid resistance

    • Starvation survival

Acts as a master regulator of survival mode.

5. Heat Shock Response

Trigger:

  • Elevated temperature.

  • Protein misfolding and aggregation.

Cellular Effects:

  • Loss of protein structure.

  • Enzyme dysfunction.

Protective Mechanisms:

  • Molecular chaperones → refold damaged proteins.

  • Proteases → degrade irreversibly damaged proteins.

Purpose:
Maintain proteostasis (protein homeostasis).

6. Oxidative Stress Response

Causes:

  • Host immune attack.

  • Aerobic respiration.

  • Environmental exposure.

Reactive Oxygen Species (ROS):

  • Superoxide

  • Hydrogen peroxide

  • Hydroxyl radicals

Damage:

  • DNA strand breaks

  • Protein oxidation

  • Lipid membrane damage

Defense Mechanisms:

  • Superoxide dismutase

  • Catalase

  • Peroxidases

  • DNA repair systems

Regulatory proteins detect redox imbalance and activate detox genes.

7. Stringent Response (Nutrient Limitation)

Trigger:

  • Amino acid starvation.

  • General nutrient depletion.

Key Molecule:

  • (p)ppGpp (alarmone).

Effects:

  • Decreases ribosomal RNA synthesis.

  • Suppresses growth-related genes.

  • Activates survival genes.

Outcome:
Cell conserves energy and prioritises maintenance over growth.

8. Stationary Phase Adaptation

Occurs when nutrients become limiting.

Characteristics:

  • Reduced growth rate.

  • Increased stress resistance.

  • Activation of RpoS-dependent genes.

Stationary-phase cells are more resistant to:

  • Heat

  • Oxidative stress

  • Osmotic stress

  • Acid stress

Represents long-term survival physiology.

9. DNA Damage and SOS Response

Trigger:

  • DNA damage (e.g., UV radiation).

Mechanism:

  1. DNA damage activates RecA.

  2. RecA promotes cleavage of LexA repressor.

  3. DNA repair genes are expressed.

Consequences:

  • DNA repair enzymes produced.

  • Error-prone polymerases may increase mutation rate.

Trade-off:
Survival vs increased mutagenesis.

10. Osmotic Stress Adaptation

Problem:

Water movement disrupts turgor pressure.

Solutions:

  • Accumulation of compatible solutes (osmoprotectants).

  • Regulation of membrane composition.

  • Controlled solute transport.

Goal:
Maintain cellular integrity and prevent lysis or dehydration.

11. Biofilms as a Survival Strategy

Biofilm Features:

  • Surface-attached communities.

  • Extracellular matrix protection.

  • Altered gene expression.

Advantages:

  • Increased antibiotic tolerance.

  • Protection from immune responses.

  • Reduced environmental stress exposure.

Biofilm cells often exhibit slower growth and stress-adapted phenotypes.

12. Sporulation (Extreme Survival Mechanism)

Triggered by severe nutrient limitation in certain bacteria.

Process:

  • Asymmetric cell division.

  • Formation of endospore.

Spore Characteristics:

  • Metabolically dormant.

  • Highly resistant to:

    • Heat

    • Radiation

    • Desiccation

    • Chemicals

Spores germinate when favourable conditions return.

13. Cross-Protection

Exposure to one mild stress can increase resistance to other stresses.

Example:

  • Mild heat shock can improve oxidative resistance.

Reason:

  • Overlapping regulatory networks.

  • Shared protective proteins.

14. Clinical and Evolutionary Relevance

Stress responses contribute to:

  • Antibiotic tolerance

  • Persistence

  • Chronic infection

  • Increased mutation rates

  • Virulence regulation

Important concept:
Stress adaptation enhances survival without necessarily increasing growth.

15. Integrated Big Picture

Bacterial stress survival relies on:

  1. Rapid environmental sensing.

  2. Global transcriptional reprogramming.

  3. Protein repair and degradation systems.

  4. DNA repair pathways.

  5. Metabolic slowdown.

  6. Community-level protection (biofilms).

  7. Dormancy (sporulation).

Key takeaway: Stress responses form an interconnected regulatory network that balances growth, repair, and long-term survival.