Bacterial Culturing

Growth Factors

Microorganisms require specific nutrients and environmental conditions for growth. These factors are essential for cellular processes and survival.

• Energy Sources:

  • Autotrophs: Organisms that use inorganic carbon (CO2) as their carbon source.

  • Photoautotrophs: Use light energy to convert CO2 into organic compounds.

  • Chemoautotrophs: Obtain energy from inorganic chemical reactions.

  • Heterotrophs: Require organic compounds as both energy and carbon sources.

• Carbon: Essential for all organic molecules; autotrophs use CO2, heterotrophs use organic carbon.

• Nitrogen: Needed for amino acids, nucleic acids, and other cellular components.

• Sulfur & Phosphorus: Sulfur is required for some amino acids and vitamins; phosphorus is a component of nucleic acids and ATP.

• Trace Metals: Serve as enzyme cofactors (e.g., iron, zinc, copper).

• Vitamins: Often function as coenzymes in metabolic reactions.

Media: Functions and Types

Culture media provide nutrients and conditions for microbial growth and identification.

• Functions:

  • Support microbial growth

  • Enable identification based on metabolic properties

• Types of Media:

  • Nutrient/Basic/Complex Media: Contain a variety of nutrients; composition not precisely known (e.g., nutrient broth).

  • Enriched/Fortified Media: Supplemented with additional nutrients for fastidious organisms (e.g., blood agar).

  • Selective Media: Inhibit growth of some microbes while allowing others (e.g., MacConkey agar selects for Gram-negative bacteria).

  • Differential Media: Distinguish organisms based on metabolic reactions (e.g., lactose fermentation on MacConkey agar).

  • Assay/Defined/Synthetic Media: Exact chemical composition is known; used for specific research or assays.

Pure and Mixed Cultures

• Pure Culture: Contains only one species of microorganism.

• Mixed Culture: Contains two or more species.

Plating Methods

Plating methods are used to isolate and quantify microorganisms.

• Streaking: Used to isolate pure colonies.

  • Quadrant Streak: Sequential dilution across four sections.

  • Radiant Streak: Streaks radiate from a central point.

• Spread Plate: Evenly spreads diluted sample over agar surface.

• Pour Plate: Mixes sample with molten agar before solidification.

• Replica Plating: Transfers colonies from one plate to another in the same pattern for comparative studies.

• Qualitative vs. Quantitative:

  • Qualitative: Determines presence or absence of organisms.

  • Quantitative: Estimates number of organisms (e.g., colony-forming units, CFU).

Cultural Characteristics

Colony morphology helps in identification of microorganisms.

• Size: Small, medium, or large colonies.

• Color: Pigmentation varies by species.

• Elevation: Flat, raised, convex, umbonate, etc.

• Margin/Shape: Entire, undulate, lobate, filamentous, etc.

• Composition/Texture: Smooth, rough, mucoid, dry, etc.

• Consistency: Butyrous (buttery), viscid, friable, etc.

• Opacity: Transparent, translucent, opaque.

Sterilization

Definitions and Examples

• Sterilization: Complete destruction or removal of all forms of microbial life, including spores (e.g., autoclaving).

• Degerming: Mechanical removal of microbes from a limited area (e.g., handwashing).

• Disinfectant: Chemical used on inanimate objects to destroy pathogens.

• Antiseptic: Chemical used on living tissue to inhibit or kill microbes.

• Bacteriostasis: Inhibition of bacterial growth without killing.

• Sepsis: Presence of pathogenic microbes or their toxins in tissue or blood.

• Sanitization: Lowering microbial counts to safe public health levels.

• Germicide: Agent that kills microbes.

• Bacteriocide: Agent that kills bacteria.

• Fungicide: Agent that kills fungi.

• Algacide: Agent that kills algae.

• Viricide: Agent that inactivates viruses.

Thermal Death Measurements

• Thermal Death Point (TDP): Lowest temperature at which all microbes in a liquid suspension are killed in 10 minutes.

• Thermal Death Time (TDT): Minimum time required to kill all microbes in a liquid at a given temperature.

• Decimal Reduction Time (D-value): Time required to kill 90% of a microbial population at a given temperature. Equation: where is the initial number of organisms, is the number remaining after time .

Methods of Sterilization

Various physical and chemical methods are used to achieve sterilization, each with advantages and disadvantages.

• Moist Heat:

  • Denatures proteins and destroys membranes (e.g., autoclaving at 121°C, 15 psi, 15-20 min).

  • Kinetic vs. Potential Energy: Moist heat transfers energy more efficiently than dry heat.

  • Sterilizing Time: Determined by temperature, pressure, and load size.

  • Exhaust Cycle: Removal of air to allow steam penetration; fast cycles for small loads, slow for large loads.

• Pasteurization/Boiling: Reduces microbial load but does not sterilize; used for liquids like milk.

• Dry Heat:

  • Flaming: Direct exposure to flame (e.g., sterilizing inoculating loops).

  • Flambéing: Brief exposure to flame for surface sterilization.

  • Dry Heat Chamber: Hot air oven at 160-170°C for 2-3 hours.

• Low Temperatures: Inhibit microbial growth but do not sterilize.

• Desiccation: Removal of water inhibits growth; Lyophilization (freeze-drying) preserves cultures.

• Gas Sterilization: Uses gases like ethylene oxide for heat-sensitive materials.

• Electromagnetic Radiation:

  • Wave Theory: Energy is inversely proportional to wavelength; frequency and wavelength are related by (where is speed of light, is wavelength, is frequency).

  • Spectrum: Includes microwaves, UV, X-rays, gamma rays.

  • Ionizing Radiation: (e.g., X-rays, gamma rays) penetrates and damages DNA.

  • Non-ionizing Radiation: (e.g., UV) causes thymine dimers in DNA.

• Membrane Filtration: Physically removes microbes from liquids using filters with pore sizes typically 0.22 μm or smaller.

Chemical Disinfection

• Qualities of an Ideal Disinfectant:

  1. Broad-spectrum activity

  2. Effective in presence of organic matter

  3. Non-toxic to humans and animals

  4. Non-corrosive and non-staining

  5. Stable and long shelf-life

  6. Rapid action

  7. Penetrates well

  8. Inexpensive and easy to use

  9. Odorless or pleasant odor

  10. Does not damage materials

• How to Sterilize (Example): Use 70% ethanol to disinfect surfaces; autoclave for sterilizing media and instruments.

Mechanisms of Action

• Oxidation: Disrupts cell membranes and proteins (e.g., hydrogen peroxide).

• Protein Denaturation: Loss of protein structure and function (e.g., alcohols, heat).

• Salting: Draws water out of cells, inhibiting growth (osmotic effect).

• Poisons: Heavy metals (e.g., mercury, silver) inactivate enzymes.

• Permeability: Disrupts cell membrane integrity, causing leakage of cellular contents.

Additional info: The above notes expand on brief points with academic context, definitions, and examples to provide a comprehensive overview suitable for exam preparation in a college-level microbiology course.