Ch.9

Chapter 9: Microbial Growth

Key Concepts

1. Microbial Growth

• Definition:
Microbial growth refers to an increase in the number of cells, not the size of individual cells. Growth occurs through cell division, typically by binary fission in prokaryotes.

Drawing: Draw a diagram showing binary fission in a bacterial cell.
![Binary Fission Drawing]

2. The Growth Curve

• Phases of the Growth Curve:

1. Lag Phase: Cells are adjusting to the environment; no cell division occurs.

2. Log (Exponential) Phase: Rapid cell division and growth, where cells are most sensitive to antibiotics.

3. Stationary Phase: Nutrients become limited, and waste products accumulate, leading to a balance between cell division and death.

4. Death (Decline) Phase: Cells die due to depletion of nutrients and accumulation of toxic wastes.

Drawing: Draw the growth curve with labeled phases (Lag, Log, Stationary, Death).
![Growth Curve Drawing]

3. Factors Affecting Microbial Growth

• Physical Factors:

1. Temperature:

▪ Psychrophiles: Cold-loving organisms.

▪ Mesophiles: Moderate-temperature-loving organisms (human pathogens are often mesophiles).

▪ Thermophiles: Heat-loving organisms.

▪ Hyperthermophiles: Thrive in extremely hot environments (e.g., hydrothermal vents).

▪ Optimal Growth Temperature: The temperature at which the organism grows best.

Drawing: Create a graph showing growth rates of microorganisms at different temperatures.
![Temperature Growth Graph]

2. pH:

▪ Microorganisms grow best at different pH levels.

▪ Acidophiles: Prefer acidic environments (pH < 5.5).

▪ Neutrophiles: Prefer neutral pH (pH 6-8).

▪ Alkaliphiles: Prefer basic environments (pH > 8.5).

Drawing: Show the range of pH tolerance for different groups of microbes.
![pH Tolerance Chart]

3. Oxygen:

▪ Microorganisms have varying needs for oxygen.

▪ Obligate Aerobes: Require oxygen for growth.

▪ Obligate Anaerobes: Cannot grow in the presence of oxygen.

▪ Facultative Anaerobes: Can grow with or without oxygen.

▪ Aerotolerant Anaerobes: Tolerate oxygen but do not use it.

▪ Microaerophiles: Require oxygen but at lower concentrations than in the atmosphere.

Drawing: Draw a chart showing different oxygen requirements (e.g., Obligate Aerobes, Facultative Anaerobes, etc.).
![Oxygen Requirements Chart]

4. Water Availability:

▪ Microorganisms require water to grow, but some are adapted to dry conditions.

▪ Halophiles: Salt-loving organisms that thrive in high salt concentrations.

▪ Xerophiles: Organisms that grow in dry environments.

4. Measuring Microbial Growth

• Methods of Measuring Growth:

1. Direct Methods:

▪ Plate Counts: Counting colonies on agar plates.

▪ Filtration: Used for low numbers of organisms in liquid samples.

▪ Microscopic Counts: Counting cells under a microscope (using a hemocytometer or other counting chambers).

Drawing: Draw a petri dish with colonies to illustrate plate count.
![Plate Count Drawing]

2. Indirect Methods:

▪ Turbidity: Measuring the cloudiness of a culture with a spectrophotometer. The more cells present, the more light is scattered.

▪ Metabolic Activity: Measuring the byproducts of microbial metabolism (e.g., CO₂ production).

▪ Dry Weight: Weighing the microbial biomass after drying.

5. The Role of Nutrients in Growth

• Essential Nutrients:

1. Macronutrients: Required in large quantities for microbial growth.

▪ Examples: Carbon, Nitrogen, Phosphorus, Sulfur, Potassium, Magnesium, Calcium, Iron.

2. Micronutrients: Required in trace amounts, often as cofactors for enzymes.

▪ Examples: Zinc, Copper, Manganese, Molybdenum, Cobalt.

• Nutrient Media:

o Complex Media: Contain complex materials like beef extract or yeast extract (e.g., nutrient broth).

o Defined Media: Have a precise chemical composition.

o Selective Media: Encourage the growth of specific microorganisms and inhibit others.

o Differential Media: Allow distinguishing between different microorganisms based on appearance (e.g., color changes)