Recording-2025-03-13T20:15:37.405Z chapter 9

Overview of Culture Media

• Nutrient Broth and Agar: Used in labs to culture microbes with varying compositions.

• Complex Media: Less specific. Ingredients like peptones and beef extract are added without precise quantifications.

  • Example: Complex media provides nutrients but does not specify exact components.

• Chemically Defined Media: More specific than complex media, ideal for strict growth requirements.

  • Analogy: Comparable to a vegan needing specific dietary restrictions.

Types of Media

• Reducing Media: Used for culturing anaerobic bacteria by excluding oxygen.

  • Selective for specific microbes, for example, using bismuth sulfide to inhibit gram-positive bacteria and most gram-negative bacteria except Salmonella typhi.

• Differential Media: Helps differentiate species based on observable changes, e.g., color changes on plates.

  • Blood Agar: Used to identify the ability of different strains of streptococci to lyse red blood cells.

    • Alpha Hemolysis: Partial lysis (greenish appearance).

    • Beta Hemolysis: Complete lysis (clear zone around colonies).

    • Gamma Hemolysis: No lysis (no change).

Selective and Differential Media Combined

• Mannitol Salt Agar (MSA): Selects for staphylococci due to high salt levels.

• Differentiates staphylococcal species.

  • Staphylococcus aureus: Ferments mannitol, producing acid, changing the indicator (phenol red) to yellow.

  • Staphylococcus epidermidis: Unable to ferment mannitol, remains red.

Enrichment Culture

• Provides specific nutritional needs for particular microbes to grow where they might not otherwise thrive.

Pure Culture and Colony Formation

• A pure culture contains a single species of microbe.

• Colonies are clusters of cells on an agar plate considered colony-forming units (CFUs).

• Streak Plate Method: Technique used to isolate pure cultures from mixed samples.

Bacterial Growth

• Binary Fission: Primary method of bacterial reproduction, involves DNA replication followed by cell division.

  • Stages of binary fission: cell elongation, DNA replication, and cell division.

  • Each division results in a doubling of the population (vertical transmission of genetic information).

Generation Time and Cell Growth Calculation

• Generation Time: Time taken for a cell to divide; varies among species (e.g., E. coli vs. staphylococci).

• Typically expressed as powers of two due to binary fission.

  • Example: Starting with one cell, after one generation (2^1) there are 2 cells, after two generations (2^2) four cells, and so on.

• To calculate total cells:

  • Formula: ( n = n_0 \times 2^g ) (where ( n_0 ) is starting cells, g is the number of generations).

  • Example problem: Starting with 5 streptococcus, a generation time of 30 minutes, after 2 hours (4 generations) results in 80 streptococci.

Stages of Bacterial Growth Curve

• Lag Phase: Cells adapt to growth conditions before replication starts.

• Log Phase: Rapid growth and division, high metabolic activity.

• Stationary Phase: Nutrient depletion leads to balance between new cell growth and cell death.

• Death Phase: Nutrients are exhausted, and the population declines.

Metabolism in Growth Phases

• Most metabolic activity occurs in the log phase due to rapid cell division.

• Less activity during lag and death phases.