Overview
This lecture covers the principles of prokaryotic (bacterial) growth, factors influencing growth, methods to culture bacteria, and approaches to measure microbial populations in the lab.
Microbial Growth and Binary Fission
- Microbial growth refers to an increase in the number of cells, not cell size.
- Prokaryotes grow mainly via binary fission: one cell divides into two identical cells.
- Binary fission leads to exponential population growth.
- Generation time is the time it takes for a population to double, often 20–30 minutes for bacteria.
Calculating Growth
- Population after n generations: initial cells × 2^n, where n is the number of divisions.
- Example: 1 cell, 4 divisions, yields 16 cells (1→2→4→8→16).
- Exams focus on understanding multiplication patterns, not on memorizing the formula.
Culturing and Pure Cultures
- Pure culture: a population containing only one species.
- Obtaining a pure culture requires isolating a single cell and growing it on a solid medium (agar).
- Agar is a solidifying agent: not digested by bacteria, heat-stable, stays solid up to 95°C.
- Petri dishes are used to prevent contamination and allow air exchange.
- The streak-plate method spreads cells to isolate colonies (each from one cell).
Growth Curve in Closed Systems
- Four phases: lag (adjustment), log (exponential growth), stationary (growth equals death), death (death exceeds growth), prolonged decline (survival of the fittest).
- Closed systems (like petri dishes) have limited nutrients; open systems (with nutrients/waste managed) allow continual growth.
Environmental Growth Factors
- Temperature:
- Psychrophiles (cold-loving), mesophiles (moderate temps, e.g., human pathogens), thermophiles, hyperthermophiles (hot-loving).
- Oxygen:
- Obligate anaerobes (killed by oxygen), aerotolerant anaerobes (ignore oxygen), facultative anaerobes (can use but don’t require oxygen), microaerophiles (require low oxygen), obligate aerobes (require high oxygen).
- pH:
- Neutrophiles (neutral pH), acidophiles (acidic), alkaliphiles (basic).
- Salt:
- Halotolerant (can grow in high salt, e.g., Staphylococcus aureus).
Nutritional Requirements
- Autotrophs make organic compounds from CO₂; heterotrophs use existing organic compounds.
- Phototrophs use light for energy; chemotrophs use chemicals (organic = chemoorgano, inorganic = chemolitho).
- Most human pathogens are chemoorganoheterotrophs.
Growth Media Types
- Complex media: variable nutrient content, not precisely known.
- Chemically defined media: precise chemical composition known.
- Selective media: allows only specific organisms to grow.
- Differential media: distinguishes organisms based on appearance (e.g., blood agar showing hemolysis types).
Measuring Microbial Growth
- Total cell methods (count living + dead): microscope count, Coulter counter (electricity), flow cytometer (light/turbidity).
- Viable cell counts (living cells only): plate count, membrane filtration + plate count, most probable number (statistical estimate).
- Biomass measurement: turbidity (cloudiness), dry weight (mass doubles).
Key Terms & Definitions
- Binary fission — asexual cell division producing two identical cells.
- Generation time — time needed for a population to double.
- Pure culture — one bacterial species in a culture.
- Agar — non-digestible, heat-stable solid medium for culturing bacteria.
- Streak plate method — technique for isolating colonies on a petri dish.
- Lag/log/stationary/death phase — phases of bacterial growth in closed systems.
- Selective media — only certain organisms can grow.
- Differential media — all organisms grow but show visible differences.
Action Items / Next Steps
- Review and memorize terms and growth phases.
- Study the differences between types of media and oxygen requirements.
- Practice calculating cell numbers using generation times and binary fission.
- Refer to the handout on Blackboard for tables on measurement methods.