Lecture 2
Microbial Growth Overview
Definition of Growth in Microbiology: Growth refers to the increase in the number of cells, not in size. Achieved through cell division (binary fission).
Binary Fission
Process of bacterial reproduction.
DNA replication
Cell elongation
Septum formation
Cell splits into two identical daughter cells
Can occur rapidly under ideal conditions.
Generation Time
Time taken for one cell to divide into two.
Examples:
E. coli: ~20 minutes
Mycobacterium tuberculosis: ~24 hours
Growth speed influenced by environment, nutrients, and species.
Bacterial Growth Curve
Observed in a closed system (e.g., test tube). Four phases:
Lag Phase: Cells are active but not dividing; preparing to grow.
Log Phase: Rapid, constant growth; population doubles regularly; cells most metabolically active and sensitive to antibiotics.
Stationary Phase: Growth equals death; nutrient depletion and waste accumulation; population size constant.
Death Phase: Cells die as conditions worsen; death rate exceeds growth rate; not all cells die simultaneously.
Factors Affecting Microbial Growth
Environmental Factors:
Temperature
pH
Oxygen availability
Water availability
Nutrients
Temperature Categories
Psychrophiles: Cold
Mesophiles: Moderate (human pathogens)
Thermophiles: Hot
Hyperthermophiles: Extremely hot
Oxygen Requirements
Types of Microbes:
Obligate aerobes: require O₂
Obligate anaerobes: O₂ is toxic
Facultative anaerobes: can use O₂ or not
Microaerophiles: require low levels of O₂
Clinical Relevance
Understanding microbial growth aids in:
Infection progression analysis
Antibiotic effectiveness
Food safety protocols
Laboratory cultivation of microbes.
Review Questions
What does "growth" refer to in microbiology?
What is the primary method of reproduction in most bacteria?
Define generation time.
Which phase of the bacterial growth curve features the most metabolic activity?
Why does the population size remain constant in the stationary phase?