Microbial Growth

Microbial Growth Notes

9.1 How Microbes Grow

  • Binary Fission: The most common form of bacterial reproduction involving four basic steps:

  • Growth of cell size and increase in cell components

  • Replication of DNA

  • Division of the cytoplasm (cytokinesis)

  • Septum formation and division of daughter cells

  • Z ring Assembly: Involves FtsZ protein to help direct cytokinesis by assembling a Z ring, forming a divisome that activates peptidoglycan production leading to cell division.

  • Generation Time: Time it takes for a population to double. It varies among species:

  • E. coli: 20 min

  • S. aureus: 30 min

  • B. subtilis: 120 min

  • M. tuberculosis: 15-20 hrs

  • Calculating Population Size: Population growth is exponential. For any starting size Nn = N0 * 2^n, where Nn is the number of cells at generation n, N0 is the initial number of cells, and n is the number of generations.

  • Example: With a generation time of 30 min, one initial cell will result in over 281 trillion cells after 24 hours!

Growth Curve

  • Closed Cultures:

  • Lag Phase: Cells adjust to culture medium; no change in population.

  • Log Phase: Binary fission occurs; cell replication exceeds cell death.

  • Stationary Phase: Nutrients are depleted; cell replication equals cell death.

  • Death Phase: Toxic waste accumulates; cell death exceeds replication.

  • Lag Phase: Duration affected by genetic make-up, media composition, and inoculum size.

  • Log Phase: Characterized by constant growth, good for industrial applications, and susceptibility to disinfectants and antibiotics.

  • Stationary Phase: Cells enter survival mode, expressing less virulence and synthesizing fewer components due to resource depletion.

  • Death Phase: Endospore formation; cells lyse and release nutrients for surviving cells.

  • Open System Cultures: Continuous supply of nutrients and removal of waste; beneficial for industrial microbiology.

Measuring Growth

  • Methods:

  • Direct Microscopic Cell Count: Manual counting of cells; does not distinguish live/dead.

  • Fluorescence Staining: Distinguishes live (green) vs. dead (red) cells.

  • Coulter Counter: Measures cell density via electric resistance; does not differentiate live/dead.

  • Viable Plate Counts: Requires culturing cells; limited to easily cultured species.

  • Optical Density: Measures turbidity to estimate population; includes live and dead cells.

Viable Cell Counts

  • Serial dilutions aim to achieve a countable range of 30-300 CFU/ml, calculated from dilution factor.

  • Membrane Filtration: Used for very dilute samples; involves filtering known volume to count colonies.

  • Most Probable Number (MPN): Statistical method for low counts; uses multiple dilutions to estimate CFU.

Biofilm Formation

  • Microecosystem comprised of clusters of microbes in a protective matrix of extracellular polymeric substances (EPS).

  • Steps:

  1. Planktonic cells attach to the substrate.

  2. Attachment becomes irreversible; cells grow and divide.

  3. Production of EPS; formation of water channels.

  4. Secondary colonizers attach and disperse.

Environmental Factors that Affect Growth

  1. Oxygen Level:

  • Optimal concentration, minimum and maximum permissive concentrations.

  • Grouping microbes based on oxygen requirements: obligate aerobes, obligate anaerobes, facultative anaerobes, aerotolerant anaerobes, microaerophiles.

  1. pH:

  • Microbial growth affected by pH; optimal, minimum, and maximum pH levels.

  • Groups: Neutrophiles (~pH 7), acidophiles (<5.5), alkaliphiles (8-10.5).

  1. Temperature:

  • Microbes classified by optimal, minimum, and maximum temperatures: mesophiles (20-45 °C), psychrotrophs (4-20 °C), psychrophiles (<0 °C), thermophiles (50-80 °C), hyperthermophiles (80-110 °C).

  1. Osmotic Pressure:

  • Influence of solute concentrations; halophiles require high salt, halotolerant tolerate it.

  1. Barometric Pressure:

  • Barophiles require high pressure, often found in deep-sea environments.

  1. Light:

  • Various forms of photosynthetic organisms; their growth depends on light available in environments.

Media Used for Bacterial Growth

  • Types of media:

  • All-Purpose Media (e.g., TSA)

  • Enriched Media: Growth factors for fastidious organisms.

  • Chemically Defined Medium: Complete known composition.

  • Complex Medium: Unknown composition based on extracts.

  • Selective Media: Promotes growth of specific organisms while inhibiting others.

  • Differential Media: Distinguishes colonies based on color change.

  • Understand each media type's role in supporting or inhibiting microbial growth, and implications for research and clinical settings.