Microbiology Lecture: Chapter 9 - Microbial Growth

Lecture Objectives

• Understand biofilms and microbial division
• Explore different growth factors and requirements
• Techniques for culturing microbes
• Comparison of agar vs. liquid media
• Preparation of media
• Selective vs. differential media
• Measuring growth and calculating cell concentrations

Biofilms

• Biofilms: Structured communities of bacteria adhering to surfaces and each other
• Formation Steps:
  1. Reversible attachment of planktonic cells
  2. Irreversible attachment
  3. Growth and division
  4. Production of exopolysaccharide (EPS)
  5. Attachment of secondary colonizers
• Quorum sensing: Regulation of activities based on cell density using autoinducers

Microbial Growth

• Microbial growth refers to an increase in cell number, not size
• Binary Fission:
  • DNA replication, cell elongation, septum formation, cytokinesis
• Other replication methods: Budding (asymmetric division)

Growth Requirements

• Factors: Temperature, pH, osmotic pressure, oxygen levels, nutrients
• Temperature growth categories:
  • Psychrophiles: Cold-loving
  • Mesophiles: Moderate temperature
  • Thermophiles: Heat-loving
  • Hyperthermophiles: Extremely hot environments

pH and Growth

• pH affects cellular macromolecules
• Categories:
  • Acidophiles: pH 1-5.5
  • Neutrophiles: pH 5.5-8.5
  • Alkalophiles: pH 7.5-11.5

Oxygen Requirements

• Types of Microbes:
  • Obligate aerobes: Require oxygen
  • Facultative anaerobes: Prefer oxygen but can grow without it
  • Obligate anaerobes: Harmed by oxygen
  • Aerotolerant anaerobes: Tolerate but do not use oxygen
  • Microaerophiles: Require low oxygen
• Toxic forms of oxygen: Superoxide anion and hydrogen peroxide

Culturing Microbes

• Media Types: Solid (agar) vs. liquid (broth)
• Preparation involves sterilization (autoclaving)
• Media Categories:
  • Selective media: Suppress unwanted microbes
  • Differential media: Distinguish between microbes

Growth Measurement

• Methods:
  • Plate counts: Colony forming units (CFUs)
  • Serial dilutions to avoid overcrowding
  • Direct microscopic count: Using hemocytometers
• Growth phases:
  1. Lag phase
  2. Log phase (exponential growth)
  3. Stationary phase (equilibrium)
  4. Death phase
• Exponential growth model and impact on population size

Calculations

• Doubling Time: Time for population to double
• Formula: Final number of cells = Initial cells x 2^number of generations
• Dilution Factors: Used for original concentration calculations

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Important Terms

• Biofilm, quorum sensing, binary fission, budding, psychrophiles, mesophiles, thermophiles, acidophiles, neutrophiles, alkalophiles, obligate aerobes, facultative anaerobes, obligate anaerobes, aerotolerant anaerobes, microaerophiles, CFU, serial dilution, hemocytometer

Graph Analysis

• Understand trends between minimum, optimum, and maximum growth conditions for given microbes

Note: Practice math problems related to microbial growth and calculations are available for further practice. Contact instructor for any questions or clarification.