Microbial Growth
Microbial Growth
Overview of Microbial Growth
Growth primarily refers to an increase in the number of organisms rather than their size.
Bacteria achieve full size shortly after division.
Growth is dependent on several factors.
Growth Requirements
Bacteria have varying growth requirements, categorized as:
Physical Requirements
Chemical Requirements
Physical Requirements
Categories of Physical Requirements
Temperature
Temperature ranges from near freezing to beyond boiling points.
Every bacterium has specific temperature ranges for growth:
Minimum Growth Temperature
Optimum Growth Temperature
Maximum Growth Temperature
Temperatures outside of these ranges can inhibit growth without necessarily killing the bacteria.
pH
The acidity or alkalinity scale, significant for bacteria, usually ranges from 2 to 12.
Most bacteria thrive within a pH range of 6.5-7.5 (where 7.0 is neutral).
Acidophiles can tolerate low pH levels and have industrial applications, e.g., in coal mining.
Bacterial fermentation may produce acid, necessitating buffers in commercial media to stabilize pH.
Osmotic Pressure
Bacterial cells consist of 70-80% water.
Bacteria must maintain this balance, or cell lysis or plasmolysis may occur.
High salt concentrations can preserve food by creating a hypertonic environment.
Halophiles:
Obligate Halophiles: Require high salt concentrations (survive up to 30% NaCl).
Facultative Halophiles: Tolerate moderate salt concentrations (up to 2% NaCl).
Chemical Requirements
Essential elements and compounds bacteria utilize include:
Carbon: Constitutes 50% of the dry weight; obtained from CO₂ (autotrophs) or organic compounds (heterotrophs).
Hydrogen: Critical for energy generation; typically supplied by water.
Nitrogen: Accounts for about 14% of dry mass; used in amino acids and nucleic acids.
Sources include decomposed matter, ammonium (NH₄), and nitrates.
Certain bacteria can fix atmospheric nitrogen (N₂) into ammonium.
Sulfur: Used for amino acids and vitamins; usually derived from hydrogen sulfide or organic breakdown products.
Phosphorus: Integral for DNA, RNA, and ATP structure; typically required in phosphate form (PO₄³⁻).
Trace Elements: Required in small amounts (e.g., potassium, magnesium, calcium) as enzyme cofactors.
Organic Growth Factors: Similar to vitamins; some bacteria can synthesize them; others must obtain them from their environment.
Oxygen: Essential for aerobic life but toxic to cells in certain forms (e.g., superoxide radicals).
Categories of Oxygen Utilization
Bacteria are classified based on oxygen requirements:
Obligate Aerobes: Require oxygen for growth.
Facultative Anaerobes: Can grow with or without oxygen but prefer aerobic conditions.
Microaerophiles: Require low concentrations of oxygen.
Aerotolerant Anaerobes: Only grow anaerobically but can tolerate oxygen.
Obligate Anaerobes: Cannot grow in the presence of oxygen.
Effects of Oxygen on Growth
Specific forms of oxygen are harmful:
Singlet Oxygen: High-energy state of O₂, highly reactive.
Superoxide Free Radicals: Formed during respiration and can damage DNA and proteins.
Hydrogen Peroxide: Produced from metabolic activities; highly toxic to cells.
Hydroxyl Radical: Another reactive oxygen species that disrupts bonds.
Ozone: Can oxidize cellular components; useful as an antimicrobial agent.
Mechanisms to Mitigate Oxygen Toxicity
Superoxide Dismutase (SOD): Converts superoxide radicals to hydrogen peroxide.
Catalase: Decomposes hydrogen peroxide into water and oxygen:
2 H₂O₂
ightarrow 2 H₂O + O₂
Culture Media
Culture media allow microbial growth in controlled conditions, including:
Chemically Defined: Exact composition is known.
Complex: Contains unknown amounts of nutrients from biological sources.
Solid or Liquid: Solid (e.g., with agar) or liquid (broth).
Aerobic or Anaerobic: Tailored for oxygen presence or absence.
Media Requirements
The choice of media depends on:
Nutrient requirements,
Moisture,
Oxygen tolerance.
Must be sterile to avoid contamination.
Techniques for Isolating Bacteria
Streak for Isolation: Used to isolate individual colonies from a mixture to achieve pure cultures.
Selective Media: Encourages growth of target bacteria while suppressing others.
Differential Media: Allows distinction between different bacteria based on biochemical properties.
Enrichment Media: Increases the number of desired bacteria from a low-density sample.
Phases of Microbial Growth
Lag Phase: Initial adaptation to culture conditions; cells are active but not dividing.
Log Phase: Exponential growth occurs; constant generation time.
Stationary Phase: Nutrient depletion and waste accumulation lead to a growth plateau.
Death Phase: Mortality exceeds growth, leading to population decline.
Methods to Measure Microbial Growth
Direct Methods: Count actual cells using plating, microscopy, or filtration.
Indirect Methods: Estimate population size through turbidity, metabolic activity, or dry weight measurements.
Turbidity: Measure cloudiness using a spectrophotometer.
Metabolic Activity: Commonly monitored by acid or CO₂ production.
Dry Weight: Harvest culture, dry it, and measure the weight.
Preservation Technologies
Deep Freezing: Samples mixed with cryoprotectants and stored at very low temperatures to prevent ice crystal formation.
Lyophilization: Samples quick-frozen and dehydrated under vacuum, allowing long-term storage at room temperature.
Biofilms
Complex communities of microbes embedded in extracellular polymeric substances (EPS).
Form through cell attachment, communication via quorum sensing, and structured growth—leading to enhanced nutrient access and mutual protection.
Biofilm Impact on Health
Biofilms can complicate infections and are resistant to treatment due to the protective EPS matrix.
Conclusion
Understanding microbial growth and its regulating factors is essential for applications in health, industry, and environmental sciences.