Microbiology: Growth Factors Overview

Growth Factors

• Some microorganisms rely on external sources for essential components due to lacking specific enzymes.
• Growth factors are organic compounds vital for cellular functions that cannot be synthesized by certain organisms.
• Major classes of growth factors include:
  • Amino acids
  • Purines and pyrimidines
  • Vitamins such as thiamine, biotin, cobalamin, and pyridoxine.
• Other growth factors include heme (present in cytochromes) and cholesterol.
• Understanding growth factor needs aids in:
  • Conducting bioassays to identify specific microbial growth factors.
  • Utilizing growth-response assays that are specific, sensitive, and quantitative.
  • Industrial production of growth factors (example: vitamin D by Saccharomyces).

Physical Factors Influencing Growth

• Temperature impacts microbial growth; cardinal temperatures include:
  • Minimum temperature: below which cell activity halts.
  • Optimum temperature: the highest growth rate occurs, close to maximum.
  • Maximum temperature: above which growth ceases due to protein inactivation or membrane disruption.
• Cardinal temperatures differ among organisms and can range from 0°C to 75°C, with some capable of surviving up to 113°C (e.g., Pyrolobus fumarii).
• Microbes can be categorized based on temperature optima:
  • Psychrophiles: thrive at 0°C - max at ~20°C.
  • Psychrotolerant: grow well at 0°C; optimal at 20°C - 30°C.
  • Mesophiles: optimal temperatures of 25°C - 40°C; common microbes.

Culture Media Types

• Chemically Defined Media:

  • Exact chemical composition is known; used for controlled experiments.
  • Example: BM+G medium with defined amounts of glucose, salts, and amino acids.

• Complex Media:

  • Composition varies between batches, less control over nutrient composition.
  • Example: Luria Bertani (LB) and Tryptic Soy Broth (TSB) used for general growth needs.

Media Consistency and Application

• Media can be liquid or solidified, essential for pure culture establishment and cell enumeration.
• Solidifying agent: agar, extracted from red algae, melts at 90°C, solidifies at 45°C, resistant to microbial hydrolysis.
• Types of culture media:
  • General-purpose (supportive) media sustain a variety of microbes.
  • Enriched media contains additional nutrients for fastidious organisms.
  • Selective media favor certain microbes while inhibiting others.
  • Differential media allow for distinguishing metabolic activities.

Enrichment Technique

• Developed by Beijerinck to promote the growth of specific microbial groups over others in a mixed sample.

Notes on Culturing Microbes

• Not all microbes are cultivable in the lab; selective conditions may be needed to foster particular groups.
• Chemostat allows control of growth rate and density independently, differing from batch cultures.
  • Can maintain populations in exponential phase and study growth under low nutrient conditions.

Bacterial Growth Curve

• Monitored growth of batch cultures can be represented as a growth curve divided into four phases:
  • Lag Phase: cells adapt and prepare for growth; no increase in count.
  • Exponential Phase (Log phase): maximum growth rate occurs; cells divide rapidly; uniform properties.
    • Balanced growth: proportionate synthesis of cellular components.
    • Unbalanced growth results from nutrient/environmental changes.