Microbial Growth & Cultures

Microbial Growth

Bacterial Nutrition

Bacteria require various chemical elements for growth and cellular function. These elements serve as building blocks for macromolecules and as sources of energy.

• Carbon: Essential for constructing cell structures; forms the backbone of organic molecules.

• Nitrogen: Required for synthesis of amino acids, proteins, and nucleic acids.

• Hydrogen: Integral to organic molecules and ATP production.

• Sulfur: Needed for certain amino acids (e.g., cysteine, methionine) and vitamins (e.g., biotin, thiamine).

• Phosphorus: Component of ATP, DNA, RNA, and phospholipids.

• Oxygen: Required for cellular respiration and energy production in many organisms.

Types of Microbial Nutrition

• Chemoheterotrophs: Obtain energy from chemical compounds and carbon from organic sources.

• Chemoautotrophs: Derive energy from chemical compounds and carbon from CO2.

• Photoheterotrophs: Use light as an energy source and organic compounds as a carbon source.

• Photoautotrophs: Utilize light for energy and CO2 for carbon.

• Lithoautotrophs: Obtain energy from inorganic chemicals and carbon from CO2.

Culture Media

Types of Media

Culturing microbes requires specific media that provide necessary nutrients and environmental conditions.

• Chemically Defined Media: Exact chemical composition is known; used for precise nutritional studies.

• Complex Media: Contains extracts (e.g., yeast extract); exact composition is not fully known.

• Agar: A solidifying agent not digested by most bacteria; melts at approximately 100°C and solidifies at 40–45°C.

Specialized Media

• Selective Media: Inhibits growth of some microbes while allowing others to grow.

• Differential Media: Distinguishes microbes based on biochemical reactions (e.g., color changes).

• Enrichment Media: Enhances growth of specific microbes, often those present in low numbers.

Example: Blood agar is both differential (shows hemolysis) and enrichment (supports fastidious organisms).

Oxygen Requirements of Microbes

Classification by Oxygen Use

Microbes vary in their oxygen requirements and their ability to detoxify reactive oxygen species.

Type	Uses O2?	Grows without O2?	Catalase	SOD	Peroxidase
Obligate aerobe	Yes	No	Yes	Yes	No
Facultative anaerobe	Yes	Yes	Yes	Yes	No
Microaerophile	Yes (low levels)	No	Yes	Yes	No
Aerotolerant anaerobe	No	Yes	No	Yes	Yes
Obligate anaerobe	No	Yes	No	No	No

Catalase and superoxide dismutase (SOD) are enzymes that detoxify harmful oxygen species.

Environmental Factors Affecting Growth

Osmotic Pressure

• Halophiles: Require high salt concentrations for growth.

• Obligate Halophiles: Must have salt to survive.

• Osmotolerant: Can tolerate high salt but do not require it.

High salt environments cause water to leave the cell (plasmolysis), inhibiting growth. This principle is used in food preservation.

pH Groups

• Acidophiles: Grow optimally at low pH.

• Alkalophiles: Grow optimally at high pH.

• Neutrophiles: Grow best near neutral pH (around pH 7).

Temperature Groups

• Psychrophiles: Cold-loving; optimal growth at 0–15°C.

• Mesophiles: Moderate temperature; optimal at 20–45°C. Most human pathogens are mesophiles because human body temperature is 37°C.

• Thermophiles: Heat-loving; optimal growth above 45°C.

Microbial Growth Patterns

Binary Fission

Bacteria reproduce by binary fission, a process resulting in two genetically identical daughter cells.

1. DNA replicates.

2. Cell elongates.

3. Septum forms.

4. Cell divides into two identical cells.

Phases of Bacterial Growth Curve

• Lag Phase: Cells adapt to new environment; little or no cell division.

• Log (Exponential) Phase: Rapid cell division; population increases exponentially.

• Stationary Phase: Nutrient depletion; cell growth rate equals cell death rate.

• Death Phase: Cells die at a rate faster than new cells are produced.

• Prolonged Decline: A small number of cells survive for extended periods.

Biofilms

Structure and Importance

Biofilms are communities of microbes attached to surfaces and embedded in a self-produced slime matrix. They are significant in medical and industrial contexts due to their resistance to antimicrobial agents.

• Quorum Sensing: Chemical communication between bacteria within a biofilm, coordinating gene expression and behavior.

• Examples: Dental plaque, catheters, medical implants, industrial pipelines.

Control of Microbial Growth

Targets and Resistance

• Cell Membrane: Most common target of disinfectants.

• Prions: Most resistant to microbial chemicals.

• Order of Resistance (most to least):

  1. Prions

  2. Endospores

  3. Mycobacteria

  4. Gram-negative bacteria

  5. Fungi

  6. Protozoan cysts

  7. Gram-positive bacteria

  8. Viruses

  9. Enveloped viruses (least resistant)

Types of Chemical Agents

Agent	Mechanism	Use
Phenolics	Damage membranes and proteins	Surface disinfectant
Biguanides	Membrane disruption	Antiseptic
Halogens	Damage proteins and enzymes	Antiseptic or disinfectant
Quats (quaternary ammonium compounds)	Alter membrane permeability	Mouthwash or antiseptic
Alcohols	Protein denaturation	Surface disinfectant

• Soaps and Detergents: Act as degerming agents by physically removing microbes.

Physical Methods of Microbial Control

• Autoclave: Uses steam under pressure to sterilize and destroy endospores.

• Pasteurization: Reduces pathogens in food and beverages without sterilizing.

• Filtration: Removes microbes from liquids by passing through a filter.

• Radiation: Damages microbial DNA, leading to cell death.

Measuring Effectiveness: Disk Diffusion Test

The disk diffusion method assesses the efficacy of antimicrobial agents.

1. Disks containing chemicals are placed on a bacterial culture plate.

2. The chemical diffuses into the agar.

3. A zone of inhibition (clear area) forms where bacteria cannot grow.

4. Larger zones indicate more effective antimicrobials.

Additional info: The disk diffusion test is commonly used in clinical microbiology to determine antibiotic susceptibility of pathogens.