2460 Chapter 9

Chapter 9: Microbial Growth

9.1 Binary Fission

Definition: Most common form of bacterial reproduction.4 Basic Steps:

  1. Growth of cell size and increase in cell components.

  2. Replication of DNA.

  3. Division of the cytoplasm (cytokinesis).

  4. Septum formation and division of daughter cells.

9.1.1 Cytokinesis and Chromosomal Division

FtsZ Protein: Directs cytokinesis by assembling the Z ring to form the divisome. The divisome activates the production of peptidoglycan and drives septum formation.Cell Elongation: Takes place prior to septum formation and daughter cell separation, allowing for an increase in size before division.

9.1.2 Generation Time

Definition: The duration required to double a population.

Varies by Species:

  • Example Generation Times:

    • E. coli: 20 minutes

    • S. aureus: 30 minutes

    • B. subtilis: 120 minutes

    • M. tuberculosis: 15-20 hours.

9.1.3 Calculating Population Size

Exponential Growth: This growth is calculated based on the initial size (N0) and the number of generations (n):Formula: Nn = N0 * 2^n.Example Calculation: For a 30-minute generation time over 8 hours:

  • 8 hours equals 16 generations, allowing calculation of the final population size using the formula.

9.1.4 Growth Curve

Closed Cultures: Here, finite resources lead to predictable growth phases:

  • Lag phase: Adjustment period, no change in population size occurs.

  • Log (exponential) phase: Rapid division of cells, outpacing cell death.

  • Stationary phase: Depletion of resources; cell replication equals cell death, and endospore formation begins.

  • Death phase: Cell death outstrips replication, remaining endospores persist for survival.

Open Systems:Offer infinite resources, which is advantageous for industrial microbiological processes.

9.1.5 Measuring Growth

Importance: This is crucial in clinical diagnostics, contamination monitoring, and understanding infection dynamics.Methods for Quantifying Populations:

  1. Microscopic cell count.

  2. Fluorescent staining to assess viability.

  3. Coulter counter: Measures electrical resistance changes as cells pass through.

  4. Optical density measurements: Determined through turbidity of the culture.

  5. Viable cell counts: Expressed as CFU/ml (colony-forming units per milliliter).

9.1.6 Viable Cell Counts

Countable Range: Traditionally set between 30 - 300 CFU/ml to ensure statistical accuracy.

Techniques: Utilized include serial dilution methods (pour plate or spread plate techniques).

Most Probable Number (MPN):A statistical method suited for very low counts (<30 CFU/ml), involving multiple dilutions compared against reference tables to estimate cell populations.

9.2 Oxygen Requirements

Microbial Grouping: Based on oxygen tolerance, categorizing microbes into:

Oxygen Requirements

Microbial Grouping: Based on oxygen tolerance, categorizing microbes into:

  • Obligate Aerobes: Requires oxygen for survival; cannot grow without it.

  • Obligate Anaerobes: Cannot survive in the presence of oxygen; oxygen is toxic to them.

  • Facultative Anaerobes: Can grow in the presence or absence of oxygen. They prefer oxygen but can switch to anaerobic metabolism when it's unavailable.

  • Aerotolerant Anaerobes: Do not require oxygen for growth and are not affected by its presence. They utilize fermentation exclusively, regardless of oxygen availability.

  • Microaerophiles: Require a reduced level of oxygen for growth; they thrive in environments with lower oxygen concentrations than is present in the atmosphere (about 2-10% oxygen).

  • Fluid Thioglycolate Medium (FTM): Designed to evaluate microbial aerotolerance by observing growth in an oxygen gradient.

9.3 pH Requirements

Effects on Growth: pH levels can significantly affect macromolecular functions, particularly proteins.Microbial Preferences:

  • Acidophiles (<5.5)

  • Neutrophiles (~7)

  • Alkaliphiles (8-10.5).

9.4 Temperature Requirements

Grouping by Temperature:

  • Psychrophiles: Thrive at <0°C

  • Psychotrophs: Grow well at 4-20°C

  • Mesophiles: Optimal at 20-40°C

  • Thermophiles: Prefer 50-80°C

  • Hyperthermophiles: Exist at 80-110°C.

9.5 Nutritional Requirements & Media

Selective Media: Formulated to promote the growth of specific microorganisms.

  • Mannitol Salt Agar (MSA): Selectively differentiates Staphylococci.

  • MacConkey Agar: Selectively differentiates between Gram-negative bacteria and lactose fermenters.