Growth

Prokaryote Growth

Introduction to Growth

  • Growth in microbiology refers to several processes, including and not limited to:

    • Increase in size of individual cells.

    • Increase in population numbers, which entails the division of cells to create more cells.

Survival and Growth

Liebig’s Law of the Minimum:

•Amount of growth of any organism is determined by the nutrient present in the lowest concentration in relation to the requirements of that organism

Shelford’s Law of Tolerance:

•For an organism to grow in a given environment, each of a complex set of conditions must remain within the tolerance range of that organism

•Conditions can interact

Microbial Growth Process

  • The microbial growth process can be summarized as follows:

    • A mother (parent) cell doubles in size.

    • The mother cell then divides into two daughter cells.

  • Definition of Microbial Growth: An increase in the number of cells by cell division over time.

Binary Fission

  • The key mechanism of prokaryotic cell division is binary fission, which includes the following stages:

    • The nucleoid (region of the bacterial chromosome) elongates and divides.

    • The cell wall and membrane begin to form a transverse septum, which eventually completes and separates daughter cells.

  • This process results in two daughter cells that are genetically identical to the parent cell.

Studying Microbes in a Laboratory

  • To study microbes effectively, it is necessary to grow them in a controlled laboratory environment by:

    • Providing an environment that supports growth.

    • Removing unwanted organisms to ensure purity.

    • Using monitoring methods to track growth and conditions.

Techniques for Culture Purity

  • Aseptic Technique: Essential for keeping cultures uncontaminated by unwanted organisms which ensures purity in microbial studies.

Measuring Microbial Growth

  1. Direct Microscopic Counts

    • Utilizes a counting chamber where a bacterial suspension is placed to count cells manually within calibrated areas to determine the concentration of cells per mL.

  2. Petroff-Hausser Counting Chamber

    • A specialized slide with shallow wells and an inscribed grid that allows for the counting of bacterial cells using a microscope.

  3. Direct Counting with Haemocytometer

    • A tedious method that requires significant effort for counting small or motile cells.

Plate Count Method

  • Involves counting viable cells by diluting a sample and spreading it on a plate to incubate.

  • After incubation, colonies are counted, and the number of viable cells in the original sample is calculated.

Serial Dilution Process

  • A method often used in microbiology to determine the concentration of cells in a culture through numerous dilutions leading to distinct counts at each stage.

Methods of Colony Counts

  • Spread Plate Method: Involves adding a diluted bacterial culture to the surface of a solid agar and spreading it evenly.

  • Pour Plate Method: A technique where bacterial cultures are mixed into melted agar, poured into a Petri dish, and incubated to form colonies.

Growth-Related Measurements

  • Optical density (OD) measurements are another way to estimate bacterial growth in cultures as a function of the number of cells present, providing an indirect count by measuring how much light is absorbed by the bacterial culture.

The Bacterial Growth Curve

  • Represents the growth phases of bacteria over time, which encompasses:

    • Lag Phase: Cells adapt without significant increase in numbers but prepare for growth.

    • Exponential Phase: Active growth at a constant rate, maximum rate of cell division.

    • Stationary Phase: Nutrient limitations and waste accumulation leads to a decrease in growth rate.

    • Death Phase: Cells start to die; total count may remain high, but viable counts decrease.

Factors Affecting Bacterial Growth

  • Nutritional factors such as:

    • Carbon, nitrogen, sulfur, phosphorus, and sometimes vitamins.

  • Physical factors such as:

    • pH, temperature, oxygen concentration, and moisture.

Growth Curve

Lag Phase

• grow in size

• synthesise enzymes,

• incorporate molecules from medium

• produce energy (ATP)

Exponential Phase

  • Organisms have adapted

  • Growth occurs at exponential rate

  • Organisms divide at maximum rate

  • regular, genetically determined interval

  • generation time

Stationary Phase

  • Cell division rate decreases

  • Nutrient limitation

  • Build-up of toxic products

  • Oxygen limitation

Death Phase

  • Cell lose their ability to divide

  • Cells die

  • Total count mat remain high

  • Viable count decreases

Types of Culture Media

  1. Complex Medium: Flexible, cheap, and variable for growth.

  2. Synthetic Medium: Requires extensive knowledge; well-defined composition but more expensive.

  3. Natural Medium: Utilizes natural extracts or samples, leading to variability in results.

Special Media

  • Selective Media: Designed to inhibit certain organisms while supporting others (e.g., MacConkey agar).

  • Differential Media: Allows differentiation of microbes based on biochemical properties and visible changes.