Microbiology: An Introduction - Chapter 6 Notes

Overview of Microbial Growth

## Requirements for Growth

  • Microbial growth is influenced by various requirements which are broadly categorized into:

  • Physical Requirements

    • Temperature

    • Minimum growth temperature

    • Optimum growth temperature

    • Maximum growth temperature

    • pH

    • Osmotic Pressure

  • Chemical Requirements

    • Carbon

    • Nitrogen, Sulfur, and Phosphorus

    • Trace Elements

    • Oxygen

    • Organic Growth Factors

## Physical Requirements

Temperature
  • Organisms exhibit distinct temperature preferences:

  • Psychrophiles: Cold-loving; can grow below 20°C; found in polar regions and oceans.

  • Psychrotrophs: Grow at 0°C to 30°C; significant in food spoilage.

  • Mesophiles: Prefer temperatures between 20°C and 45°C; includes pathogens and normal microbiota.

  • Thermophiles: Optimal growth at 45°C to 80°C; found in hot springs and composts.

  • Hyperthermophiles: Optimal growth above 80°C; thrive in extreme heat environments.

pH
  • Bacterial growth optimal between pH 6.5 and 7.5.

  • Molds and yeasts prefer lower pH (5-6).

  • Certain preservation methods exploit acid production by bacteria (e.g., in sauerkraut and pickles).

  • Acidophiles thrive in acidic environments.

Osmotic Pressure
  • Hypertonic environments lead to plasmolysis (cell shrinkage).

  • Extreme halophiles require high salt concentrations (up to 30% NaCl).

  • Facultative halophiles tolerate salt concentrations (2-10% NaCl).

## Chemical Requirements

Carbon
  • Fundamental as a backbone for organic molecules.

  • Chemoheterotrophs: use organic molecules as carbon and energy sources.

  • Autotrophs: utilize carbon dioxide as their carbon source.

Nitrogen
  • Essential for proteins and nucleic acids (DNA, RNA).

  • Many bacteria decompose proteins to obtain nitrogen.

  • Some utilize nitrogen from organic compounds, and a few can perform nitrogen fixation.

Sulfur
  • Required for producing amino acids, thiamine, and biotin.

  • Acquired through decomposing proteins or directly from organic sources.

Phosphorus
  • Key element in DNA, RNA, ATP, and membranes (phospholipids).

Trace Elements
  • Inorganic elements needed in small amounts; serve as enzyme cofactors.

  • Include iron, copper, molybdenum, and zinc.

Oxygen
  • Types of Bacteria based on oxygen requirement:

  • Obligate aerobes: Require oxygen.

  • Facultative anaerobes: Grow with or without oxygen, utilizing respiration or fermentation.

  • Anaerobes: Cannot use oxygen; often harmed by it.

  • Aerotolerant anaerobes: Tolerate oxygen but do not use it.

  • Microaerophiles: Require lower oxygen levels than atmospheric concentration.

Organic Growth Factors
  • Organic compounds necessary for growth, including vitamins, amino acids, purines, and pyrimidines.

## Biofilms

  • Communities of microbes that form slime or hydrogels on surfaces.

  • Utilize quorum sensing for communication and nutrient sharing.

  • Provide protection against environmental factors and are involved in approximately 70% of infections (e.g., medical devices, dental issues).

## Culture Media

Types of Culture Media
  • Culture Medium: Nutrients prepared for microbial growth.

  • Sterile Media: Free from living microbes.

  • Inoculum: Introduction of microbes to the medium.

  • Agar: A solidifying agent derived from algae; used in Petri plates.

Types of Media Based on Composition
  • Chemically Defined Media: Exact chemical composition is known.

  • Complex Media: Composition varies; contains extracts from yeast or plants.

  • Selective Media: Encouage growth of certain microbes, suppressing others.

  • Differential Media: Differentiate between microbial colonies based on metabolic activity.

  • Enrichment Culture: Enhances growth of specific organisms from low initial concentrations.

## Obtaining Pure Cultures

  • A pure culture originates from a single species or strain, often isolated using the streak plate method.

## Bacterial Division

  • Growth represented by an increase in the number of cells (not size).

  • Binary Fission: Most common method of bacterial division.

  • Budding: Used by some species, primarily non-pathogenic.

## Growth Curves and Phases

  • Bacterial growth is observed in phases:

  1. Lag Phase: Initial adjustment; no increase in numbers but high metabolic activity.

  2. Log Phase: Rapid population increase; constant generation time.

  3. Stationary Phase: Growth slows; nutrient depletion and waste accumulation.

  4. Death Phase: Cell death exceeds division; population decreases logarithmically.

## Measuring Microbial Growth

Direct Measurements
  1. Plate Count: Counting colonies from diluted samples.

  2. Filtration: Bacteria collected on a membrane filter, then cultured.

  3. Direct Microscopic Count: Utilizes a Petroff-Hausser cell counter.

Indirect Methods
  • Turbidity: Measure cloudiness with a spectrophotometer.

  • Metabolic Activity: Measure production of metabolic products.

  • Dry Weight: Used for filamentous organisms after drying and weighing.