Microbial Growth Basics

Microbial Growth

• Microbes grow by increasing in number and accumulating into colonies. Many bacteria survive and grow in nutrient-poor environments by forming biofilms.

Learning Outcomes

• Identify microbial growth requirements
• Determine how bacteria grow exponentially, using generation time
• Describe different microbial growth phases

Course Material

• Growth Factors: Microbes may require small amounts of specific organic compounds they can't synthesize.
• Physical Requirements:
  • Temperature: Microbes are categorized by their optimal growth temperatures:
    • Psychrophiles: grow at or near freezing temperatures (0°C)
    • Mesophiles: grow at moderate temperatures (25-40°C), most common
    • Thermophiles: grow at relatively high temperatures (50-60°C)
    • Hyperthermophiles: grow at extremely high temperatures (80°C or higher), some up to 121°C
  • pH: Most bacteria grow best near neutral pH (6.5-7.5).
  • Osmotic Pressure: Microbes require water (80-90% of their composition). High osmotic pressure removes water from a cell, which causes plasmolysis (shrinkage of cell cytoplasm).
• Chemical Requirements:
  • Carbon
  • Sulfur, Nitrogen, and Phosphorus
  • Trace elements (e.g., iron, copper, molybdenum, zinc)
  • Organic growth factors

Types of Oxygen Requirements

• Obligate aerobes: Require oxygen to live (examples: Pseudomonas aeruginosa, Mycobacterium tuberculosis, Bacillus)
• Facultative anaerobes: Can survive with or without oxygen (examples: E. coli, yeast)
• Obligate anaerobes: Cannot use oxygen for energy. They are harmed by oxygen (examples: Clostridium)
• Aerotolerant anaerobes: Can't use oxygen for energy, but can tolerate it (examples: Lactobacilli)
• Microaerophiles: Need oxygen but at lower levels than in the air (examples: Borrelia burgdorferi, Helicobacter pylori)

Types of Halophiles

• Extreme Halophiles: Require high salt concentration
• Obligate Halophiles: Require significant salt (e.g. 30% salt) for growth.
• Facultative Halophiles: Tolerate some salt (e.g. 15% salt) for growth.

Cell Division

• Binary Fission: A common method in bacteria; one cell divides into two identical daughter cells, retaining the mother cell's identity.
• Budding Division: Another method; a new cell grows out from the parent cell, eventually separating and becoming an independent cell.

Microbial Growth and Quantification

• Growth: Increase in the number of cells.

• Culture Medium: Liquid or gel used to support microbial growth, in the lab.

• Culture Media types:

  • Defined media: Precise amounts of inorganic and organic chemicals are added to distilled water.
  • Complex media: Made from microbial, animal, or plant digests

• Microbial Growth Cycle:

  • Lag Phase: Initial period where cell numbers remain static. The cells adapt to the new environment.
  • Exponential or Log Phase: Rapid cell increase; cells reproduce at a constant rate.
  • Stationary Phase: Growth rate levels off due to resource limitation or accumulation of waste products.
  • Death Phase: Cells die off faster than being created.

Measuring Microbes

• Microscopic Counting: Method to estimate cell numbers; involves counting cells in a calibrated chamber or counting cells in a stained sample.
• Plate Count: Frequent method to estimate bacterial populations using dilutions to count colony-forming units (CFUs) and calculating viable cell counts from plate counts.
• Serial Dilution: Often used with plate counts to dilute original samples.
• Filtration: Technique to detect and enumerate coliform bacteria (indicator of fecal contamination).
• Most Probable Number (MPN) method: Used when microbes won't grow on solid media or the growth rate is very slow in the liquid medium.
• Direct Microscopic Counts: Bacterial suspensions' measured volume placed in a microscopic slide area.
• Turbidity: The cloudiness of a liquid sample due to cells; increase in cell count increase the turbidity of liquid medium. Spectrophotometers/colorimeters measure turbidity.

Metabolic Activity

• Measuring metabolic products (e.g., acid or CO2) can be proportional to bacterial number.
• Dry Weight: Remove the fungus from the growth medium, filter any extraneous materials, then weigh the dried material.

Explore the fundamentals of microbial growth, including the essential requirements for various microbes. Understand the concept of exponential growth as well as the different phases microbes undergo during their life cycle. Learn how factors such as temperature, pH, and osmotic pressure influence microbial development.