Microbial Nutrition and Growth: Chapter 6 Study Guide

Microbial Nutrition and Growth

Classification of Organisms by Carbon, Energy, and Electron Sources

Microorganisms are classified based on their sources of carbon, energy, and electrons, which determine their metabolic strategies.

• Carbon Source:

  • Autotrophs: Use inorganic carbon (CO2) as their carbon source.

  • Heterotrophs: Use organic compounds as their carbon source.

• Energy Source:

  • Phototrophs: Obtain energy from light.

  • Chemotrophs: Obtain energy from chemical compounds.

• Electron Source:

  • Organotrophs: Use organic molecules as electron donors.

  • Lithotrophs: Use inorganic molecules as electron donors.

• Example: Cyanobacteria are photoautotrophs; Escherichia coli is a chemoheterotroph.

Classification by Oxygen Requirements

Microorganisms differ in their need for and tolerance of oxygen, which affects their growth and survival.

• Obligate Aerobes: Require oxygen for growth.

• Obligate Anaerobes: Cannot tolerate oxygen; it is toxic to them.

• Facultative Anaerobes: Can grow with or without oxygen.

• Microaerophiles: Require low levels of oxygen.

• Aerotolerant Anaerobes: Do not use oxygen but can tolerate its presence.

• Toxic Forms of Oxygen: Include superoxide radicals, peroxide anions, and hydroxyl radicals. Some organisms produce enzymes (e.g., superoxide dismutase, catalase) to neutralize these.

Nitrogen Cycle and Nitrogen Fixation

Nitrogen is essential for microbial growth, but most organisms cannot use atmospheric nitrogen (N2).

• Atmospheric Nitrogen (N2): Inert and unusable by most organisms.

• Usable Nitrogen (NH3): Produced by nitrogen-fixing bacteria, which convert N2 to ammonia (NH3).

• Nitrogen Fixing Bacteria: Examples include Rhizobium and Azotobacter.

• Importance: Nitrogen is used for synthesis of amino acids, nucleotides, and other cellular components.

Trace Elements and Growth Factors

Trace elements and growth factors are required in small amounts but are essential for microbial metabolism and growth.

• Trace Elements: Minerals such as iron, zinc, copper, and magnesium.

• Growth Factors: Organic compounds (e.g., vitamins, amino acids) that some microbes cannot synthesize and must obtain from the environment.

• Limiting Factors: Absence of these can restrict microbial growth.

Temperature and Microbial Growth

Temperature affects enzyme activity and membrane fluidity, influencing microbial growth rates.

• Minimum Temperature: Lowest temperature for growth.

• Optimal Temperature: Temperature at which growth rate is highest.

• Maximum Temperature: Highest temperature for growth.

Temperature-Based 'Philes'

Microorganisms are classified by their preferred temperature ranges.

• Psychrophiles: Grow best at 0–15°C; found in cold environments.

• Mesophiles: Grow best at 20–40°C; most human pathogens.

• Thermophiles: Grow best at 45–70°C; found in hot springs.

• Hyperthermophiles: Grow best above 80°C; found in deep-sea vents.

• Psychrotrophs: Grow at low temperatures but have higher optimum than psychrophiles.

pH-Based 'Philes'

Microorganisms are also classified by their pH preferences.

• Acidophiles: Grow best at pH below 5.5; found in acidic environments.

• Neutrophiles: Grow best at pH 6.5–7.5; most human pathogens.

• Alkalinophiles: Grow best at pH above 8; found in alkaline environments.

Osmotic Pressure and Microbial Growth

Osmotic pressure affects water movement across cell membranes, influencing cell survival.

• Hypotonic Environment: Water enters cell; risk of cell bursting (lysis).

• Hypertonic Environment: Water leaves cell; risk of plasmolysis (cell shrinkage).

• Halophiles: Thrive in high salt concentrations.

Barophiles

Barophiles are microorganisms adapted to high-pressure environments.

• Environment: Deep-sea trenches and ocean floors.

• Reason: Their cellular structures are adapted to withstand extreme pressure.

Microbial Relationships

Microorganisms can form close relationships with each other.

• Antagonistic: One organism harms another.

• Synergistic: Both benefit, but relationship is not essential.

• Symbiotic: Both benefit and relationship is essential for survival.

Biofilms and Quorum Sensing

Biofilms are complex communities of microorganisms attached to surfaces, regulated by quorum sensing.

• Biofilm: Structured microbial community encased in a self-produced matrix.

• Quorum Sensing: Cell-to-cell communication that regulates gene expression based on population density.

• Healthcare Impact: Biofilms increase resistance to antibiotics and complicate infections.

Terminology of Culturing Microorganisms

Understanding key terms is essential for laboratory work.

• Inoculum: Sample introduced into a culture medium.

• Medium: Nutrient material for microbial growth.

• Broth: Liquid medium.

• Pure Culture: Population of cells derived from a single cell.

• CFU (Colony Forming Unit): Estimate of viable cells.

• Incubation: Maintaining cultures under controlled conditions.

Universal Precautions in Sample Transport

Universal precautions are critical to prevent contamination and infection during sample handling.

• Importance: Protects laboratory personnel and ensures sample integrity.

Obtaining Pure Cultures

Pure cultures are necessary for accurate identification and study of microorganisms.

• Clinical Specimens: Pure cultures prevent misidentification and allow for proper testing.

Streak Plate and Pour Plate Techniques

These techniques are used to isolate pure cultures from mixed populations.

• Streak Plate: Involves spreading inoculum over the surface of an agar plate to separate individual cells.

• Pour Plate: Involves mixing inoculum with molten agar and pouring into a plate; colonies form within the agar.

• Goal: To obtain isolated colonies for pure culture.

Types of Culture Media

Different media are used for various purposes in microbial cultivation.

• Defined Media: Exact chemical composition is known.

• Complex Media: Contains extracts; composition is not precisely known.

• Selective Media: Favors growth of specific microbes.

• Differential Media: Distinguishes between organisms based on reactions.

• Anaerobic Media: Supports growth of anaerobes.

• Transport Media: Preserves specimens during transport.

Arithmetic vs. Logarithmic Growth

Microbial populations grow exponentially, not arithmetically.

• Arithmetic Growth: Adds a constant number of cells per time interval.

• Logarithmic (Exponential) Growth: Doubles the population each generation.

• Formula: $N = N_0 \times 2^n$ Where $N$ = final cell number, $N_0$ = initial cell number, $n$ = number of generations.

Phases of Microbial Growth

Microbial cultures progress through distinct growth phases.

• Lag Phase: Cells adjust to environment; no increase in cell number.

• Log (Exponential) Phase: Rapid cell division; population doubles at regular intervals.

• Stationary Phase: Growth rate slows; nutrients deplete, waste accumulates.

• Death Phase: Cells die faster than they divide.

Methods of Measuring Microbial Growth

Microbial growth can be measured directly or indirectly.

• Direct Methods:

  • Plate counts (CFU)

  • Microscopic counts

  • Membrane filtration

• Indirect Methods:

  • Turbidity (optical density)

  • Metabolic activity

  • Dry weight measurement