Microbial Growth: Physical and Chemical Requirements, Culture Methods, and Measurement

Chapter 6 – Microbial Growth

Physical Requirements for Microbial Growth

Microbial growth refers to an increase in the number of cells, not the size of individual cells. The physical environment plays a crucial role in determining whether and how fast microorganisms can grow. The main physical factors are temperature, pH, and osmotic pressure.

• Temperature: Microorganisms are classified based on their preferred temperature ranges:

  • Psychrophiles (cold-loving): Capable of growth at 0°C.

  • Mesophiles (moderate-temperature-loving): Optimum growth at 25–40°C; includes most pathogenic bacteria, which grow best at 37°C.

  • Thermophiles (heat-loving): Optimum growth at 50–60°C.

• pH: The pH scale measures acidity or alkalinity (7 is neutral; below 7 is acidic; above 7 is basic).

  • Most bacteria grow best at pH 6.5–7.5.

  • Molds and yeasts prefer pH 5–6.

  • Phosphate salts are often used as buffers in culture media to maintain proper pH.

• Osmotic Pressure: High salt concentrations cause water to leave bacterial cells, leading to plasmolysis (shrinkage of the plasma membrane), which inhibits growth.

  • Obligate halophiles require high salt concentrations to live.

  • Facultative halophiles can tolerate salt concentrations up to 2% but do not require it.

Example: Salt is used to preserve foods by increasing osmotic pressure, inhibiting bacterial growth.

Chemical Requirements for Microbial Growth

Microorganisms require various chemical elements for growth, which are essential for building cellular components and supporting metabolic processes.

• Carbon: Backbone of all organic molecules.

• Nitrogen: Needed for proteins, DNA, RNA, and ATP synthesis.

• Sulfur: Required for some amino acids and vitamins (thiamine, biotin).

• Phosphorus: Essential for DNA, RNA, ATP, and phospholipids.

• Potassium, Magnesium, Calcium: Serve as enzyme cofactors.

Oxygen Requirements: Microorganisms vary in their need for oxygen:

• Obligate aerobes: Require oxygen for growth.

• Facultative anaerobes: Can grow with or without oxygen (use fermentation or anaerobic respiration when oxygen is absent).

• Obligate anaerobes: Cannot tolerate oxygen (e.g., Clostridium).

• Aerotolerant anaerobes: Do not use oxygen but are not harmed by it.

• Microaerophiles: Require low oxygen concentrations.

Culture Media

A culture medium is a nutrient material prepared for the growth of microorganisms in the laboratory. The microbes that grow are called a culture. Agar, a polysaccharide from seaweed, is often used to solidify media.

• Media must provide nutrients, moisture, proper pH, and oxygen levels, and must be sterile and incubated at the correct temperature.

• Chemically defined media: Exact chemical composition is known; used for specific research.

• Complex media: Contains extracts from yeasts, meats, or plants; composition varies.

• Nutrient broth: Liquid complex medium; nutrient agar: Solidified with agar.

• Anaerobic growth media: Special media (reducing media) are used to remove oxygen for anaerobes.

• Some bacteria (e.g., those causing leprosy and syphilis) cannot be grown on artificial media and require living hosts.

• Capnophiles: Organisms that grow better at high CO2 concentrations.

Types of Media

Type of Media	Main Purpose	Example
Selective	Suppress unwanted microbes, encourage desired ones	MacConkey agar
Differential	Distinguish colonies of different microbes	Blood agar
Enrichment	Increase small numbers of desired microbes	Soil or fecal sample enrichment

Additional info: MacConkey agar is both selective (for Gram-negative bacteria) and differential (lactose fermenters vs. non-fermenters).

Isolation of Pure Cultures

Pure cultures are essential for studying specific microorganisms. Most samples contain mixed populations, so isolation techniques are used.

• Streak plate method: A sterile loop is used to spread bacteria over the surface of solid media in a pattern that thins out the sample, resulting in isolated colonies.

• Each colony arises from a single cell or group of identical cells.

• If the organism is present in low numbers, enrichment culture may be needed before isolation.

Preservation of Microbial Cultures

Microbial cultures can be preserved for short or long periods using various methods:

• Refrigeration: Short-term storage.

• Deep-freezing: Microbes are suspended in liquid and frozen at –50 to –95°C for years.

• Lyophilization (freeze-drying): Water is removed by high vacuum after freezing at –54 to –72°C; allows long-term storage.

Bacterial Growth and Generation Time

Bacteria reproduce mainly by binary fission, resulting in population growth.

1. Cell elongates and DNA replicates.

2. Cell wall and membrane grow inward.

3. Cross-wall forms, dividing the cell.

4. Two identical daughter cells are produced.

Some bacteria reproduce by budding or fragmentation.

• Generation time: Time required for a cell to divide and its population to double (typically 1–3 hours).

• Bacterial growth is often represented logarithmically due to rapid population increases.

Equation for exponential growth:

$N = N_0 \times 2^n$ Where: $N$ = final number of cells $N_0$ = initial number of cells $n$ = number of generations

Phases of Microbial Growth

When bacteria are grown in a closed system (batch culture), their population follows a characteristic growth curve with four phases:

Phase	Description
Lag	Little or no cell division; intense metabolic activity; adaptation to new environment
Log (Exponential)	Rapid cell division; population doubles at a constant rate; cells are most sensitive to adverse conditions
Stationary	Growth rate slows; number of new cells equals number of deaths; nutrients deplete, waste accumulates
Death	Number of deaths exceeds new cells; population declines logarithmically

Direct Methods of Measuring Microbial Growth

Direct methods involve counting cells or colonies to estimate population size.

• Plate counts: Measures viable cells by counting colonies formed on agar plates after serial dilution. Results expressed as colony-forming units (CFU) per mL.

• Filtration: Used for small bacterial populations; water is filtered, and bacteria are retained on a membrane, then cultured.

• Most Probable Number (MPN): Statistical estimation based on dilution series; useful for bacteria that do not grow on solid media.

• Direct microscopic count: Cells are counted under a microscope using a defined volume; includes both live and dead cells.

Indirect Methods of Measuring Microbial Growth

Indirect methods estimate cell numbers based on other measurable properties.

• Turbidity: Cloudiness of a culture is measured with a spectrophotometer; higher turbidity indicates more cells.

• Metabolic activity: Amount of a metabolic product (e.g., acid, CO2) is proportional to the number of cells.

• Dry weight: Used for filamentous organisms; cells are filtered, dried, and weighed.

Additional info: Turbidity measurements are rapid but only reliable for high cell densities; metabolic activity is useful for slow-growing or non-culturable organisms.