Microbial Culture and Media

Definitions and Types of Media

Microbial culture involves growing microorganisms under controlled laboratory conditions to study their properties, physiology, and genetics. The choice of media and culturing technique is fundamental to successful microbiological work.

• Culture: A method of multiplying microbial organisms by letting them reproduce in predetermined culture media.

• Growth media/culture medium: A liquid or solid substance designed to support the growth of microorganisms or cells.

• Agar plate: A Petri dish containing a growth medium (typically agar plus nutrients) used to culture microorganisms or small plants.

• Defined vs. undefined media: Defined media have known quantities of all ingredients; undefined media contain complex ingredients (e.g., yeast extract, casein hydrolysate) with unknown chemical composition.

• Selective media: Media that favor the growth of certain microorganisms while inhibiting others, often by including antibiotics or specific nutrients.

• Differential media: Media that distinguish one microorganism type from another based on observable changes (e.g., color change, hemolysis).

Inoculation and Transfer Techniques

Inoculation is the process of introducing microorganisms into a culture medium. Proper technique ensures pure cultures and reliable results.

• Inoculation: Placement of something that will grow or reproduce into a medium or other suitable environment.

• Inoculating tool: Instruments (e.g., loop, needle, pipette) used to transfer microorganisms.

• Culture transfer: Moving inoculum from one source to a new location, often to establish new cultures.

Colony Morphology and Isolation

Colony Morphology Terms

Colony morphology describes the visible characteristics of microbial colonies on solid media, which aids in identification.

Isolated colony: A bacterial colony growing on a solid medium that is physically separate from all other bacterial growth.

Isolation Streak Technique

The isolation streak is a method used to obtain pure cultures by spreading microorganisms over the surface of an agar plate.

• Use an inoculating loop to dilute an inoculum over the surface of a solid medium.

• Streaking allows for the separation of individual cells, which grow into isolated colonies.

Methodology Steps:

1. Identify the colony to sample.

2. Flame loop, lift plate lid, cool loop on agar edge.

3. Pick up inoculum and deposit on plate periphery.

4. Streak loop smoothly over plate surface (avoid cutting agar).

5. Flame loop, cool, streak second sector.

6. Repeat for additional sectors.

7. Invert plates to avoid condensation.

8. Label plates with relevant information.

9. Incubate at 30°C for 24–48 hours.

Quantifying Microbial Growth

CFU Calculation

Colony-forming units (CFU) are used to estimate the number of viable microorganisms in a sample.

• CFU/mL or CFU/colony: Number of colonies counted on a plate × dilution factor of the plate sample.

Formula:

Example: If 32 colonies are counted on a plate with a dilution factor of 1:1,000,000, then: CFU/mL

Bacterial Growth Curve

Bacterial populations exhibit distinct phases during growth in batch culture.

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

• Exponential (log) phase: Rapid cell division and population growth.

• Stationary phase: Growth rate slows as nutrients deplete and waste accumulates; cell division equals cell death.

• Death (decline) phase: Cell death exceeds cell division due to harsh conditions.

Microscopy in Microbiology

Compound Light Microscope

Microscopes are essential tools for observing microorganisms. Proper use ensures clear visualization and accurate identification.

• Start with lowest power objective (4X).

• Secure slide on stage; center specimen.

• Use coarse focus for initial viewing, then fine focus.

• Adjust iris-diaphragm for brightness.

• Switch to higher magnification as needed.

• For oil immersion (100X), use immersion oil and ensure objective contacts oil.

• Clean lenses with lens paper and cleaning solution only.

Key Laboratory Terms and Concepts

Culture Types and Sterilization

• Axenic or pure culture: Population of cells or organisms growing in the absence of other species.

• Sterilization: Process that eliminates or kills all forms of microbial life, including transmissible agents (fungi, bacteria, viruses, spores, etc.).

Indicators and Tubes

• C-source (carbon source): Carbohydrate or polymer used for growth and energy.

• pH indicator: Chemical compound added to media to indicate pH changes visually.

• Fermentation: Catabolic process converting sugars to acids, gases, or alcohol.

• Durham tube: Inverted glass tube in fermentation tube to capture gas produced during fermentation.

• Sugar fermentation tube: Tube with single sugar type, protein source, and Durham tube to test for fermentation.

Antimicrobial Agents

Antibiotics, Antiseptics, and Disinfectants

Understanding the differences between these agents is crucial for proper laboratory and clinical application.

• Antibiotic: Chemical agent that inhibits microbial cell functions; used to treat infections.

• Antiseptic: Agent that opposes sepsis; inhibits microbial multiplication, applied to animate objects (e.g., skin).

• Disinfectant: Agent that opposes sepsis; inhibits microbial functions, applied to inanimate objects (e.g., surfaces).

Osmotic Pressure

Definition and Importance

Osmotic pressure is the minimum pressure required to prevent the flow of solvent molecules through a semipermeable membrane. It is important in maintaining cell integrity and function.

Practice and Application

Gram Stain and Morphology

Gram staining is a differential staining technique used to classify bacteria as Gram-positive or Gram-negative based on cell wall properties. Morphological analysis under the microscope aids in identification.

• Prepare, interpret, and record results of slides under a microscope.

• Perform isolation streaks and label appropriately.

• Identify phases of bacterial growth curve: Lag, Exponential, Stationary, Death.

Summary Table: Antibiotics vs. Antiseptics vs. Disinfectants

Additional info: The notes also include practical instructions for using the microscope, performing isolation streaks, and interpreting Gram stains, which are essential laboratory skills in microbiology.