Microbial Growth

Microbial Growth Study Notes

The Growth of Bacterial Cultures

  • Definition: Growth of bacterial cultures is defined as the increase in the number of cells, NOT an increase in cell size.

  • Three Main Mechanisms of Division:

    • Budding

    • Fragmentation

    • Binary Fission: This is the most common method of bacterial cell division.

Mechanisms of Division Illustrations

  • Fragmentation: Observed in filamentous cyanobacteria, which replicate by the process of fragmentation.

  • Budding: Example shown with Gemmata obscuriglobus, where a larger mother cell buds off a smaller daughter cell. The structure labeled includes nucleoids (N) and a forming nuclear envelope (NE).

Binary Fission Process

  1. Cell Elongation: The cell increases in size and replicates its DNA.

  2. Constriction: The cell wall and plasma membrane start to constrict at the center of the cell.

  3. Formation of Cross-wall: A cross-wall forms that completely separates the two DNA copies.

  4. Separation: The two daughter cells separate.

  • Role of FtsZ Proteins: These proteins assemble to form a Z ring anchored to the plasma membrane, which helps to pinch the cell envelope and separate the cytoplasm of the new cells.

Generation Time and Bacterial Growth

  • Generation Time: Refers to the time required for a cell to divide and form two cells.

    • Range: This typically spans from 20 minutes to 24 hours in bacteria.

    • Doubling: During binary fission, the number of cells doubles with each generation.

    • Calculation: Total number of cells can be calculated as 2^n, where n is the number of generations.

  • Visual Representation: An increase in bacterial numbers over five generations can be represented visually, showing how the population doubles each generation. The superscripts represent the number of generations (e.g., 2^5 = 32).

Graphing Bacterial Growth

  • Two Ways to Graph Growth:

    1. Arithmetic Scale: Growth rate appears as a curve.

    2. Semilogarithmic Scale: y-axis represented logarithmically, making the growth rate appear linear.

Phases of Bacterial Growth

  • The growth curve can be divided into four distinct phases:

    1. Lag Phase: Preparing for population growth, no increase in cell numbers yet.

    2. Log Phase: Characterized by a logarithmic, or exponential, increase in the population.

    3. Stationary Phase: Balance between microbial deaths and production of new cells, leading to no net increase in population.

    4. Death Phase: Population declines at a logarithmic rate.

  • Implication: Understanding the bacterial growth curve is critical for managing infections, food preservation, and industrial microbiology applications.

Biofilms

  • Definition: Biofilms are communities of microorganisms that form slime or hydrogels and adhere to surfaces.

  • Cell Communication: Bacteria communicate through quorum sensing, attracting other cells using signaling chemicals.

  • Benefits: Biofilms share nutrients and provide protection against environmental hazards. They are significantly more resistant to microbicides, up to 1000 times more resistant than their planktonic (free-floating) counterparts.

  • Prevalence: Biofilms are implicated in 70% of infections including those found on catheters, heart valves, and dental caries.

Environmental Requirements for Growth

Physical Requirements

  • Temperature: Different bacteria thrive at different temperature ranges:

    • Minimum Growth Temperature: Lowest temperature for growth.

    • Optimum Growth Temperature: Temperature range where growth is maximized.

    • Maximum Growth Temperature: Highest temperature for growth.

    • Categories of Bacteria by Temperature:

      • Psychrophiles

      • Psychrotrophs

      • Mesophiles

      • Thermophiles

      • Hyperthermophiles

pH Requirements

  • Most Bacteria: Grow best at a near-neutral pH (typically between 6.5 to 7.5).

  • Molds and Yeasts: Prefer a more acidic environment, generally between pH 4.0 and 6.0.

  • Categories:

    • Acidophiles: Prefer low pH.

    • Neutrophiles: Prefer neutral pH.

    • Alkaliphiles: Prefer high pH.

  • Preservation: The acidity in foods can inhibit spoilage microbes, as most thrive at near-neutral pH levels.

Osmotic Pressure Requirements

  • Plasmolysis: Occurs when microbial cells are in a hypertonic solution, leading to water leaving the cell and inhibiting growth. Exceptions are extreme halophiles and facultative halophiles.

Chemical Requirements for Bacterial Growth

  • Key Nutrients:

    • Carbon: Source of structure and energy (chemoheterotrophs use organic forms; autotrophs use CO₂).

    • Nitrogen: Component of amino acids, used in pathways to build cell structures (bacteria can utilize various inorganic forms).

    • Sulfur & Phosphorus: Important in protein structures and nucleic acids, bacteria obtain it from decomposing proteins and phosphate reserves.

    • Trace Elements: Inorganic elements required in small amounts (e.g., iron, copper) typically act as cofactors for enzymes.

    • Oxygen: Required for ATP generation through cellular respiration. Bacteria categorized by their oxygen requirement:

    1. Obligate aerobes

    2. Facultative anaerobes

    3. Obligate anaerobes

    4. Aerotolerant anaerobes

    5. Microaerophiles

Interpretation of Oxygen Requirements in Bacteria (Table)

  • Obligate Aerobes: Grow only in oxygen-rich environments.

  • Facultative Anaerobes: Prefer oxygen, but can grow without it.

  • Obligate Anaerobes: Cannot grow in the presence of oxygen.

  • Aerotolerant Anaerobes: Can tolerate oxygen but do not use it.

  • Microaerophiles: Require low concentrations of oxygen.

Culture Media for Growing Bacteria

  • Culture Medium: Nutrient sources prepared for microbial growth; must be sterile, containing no living organisms.

  • Definitions:

    • Inoculum: Introduction of microbes into culture medium.

    • Culture: The microbes that are growing in or on the culture medium.

  • Types of Media:

    • Agar: A complex polysaccharide used as a solidifying agent, liquefies at 100°C and solidifies at ~40°C.

    • Chemically Defined Media: Contains known quantities of pure chemicals.

    • Complex Media: Composition varies, derived from natural sources.

Pure Cultures

  • Definition: Contains only one species or strain, often referred to as a colony-forming unit (CFU).

  • Isolation Method: The streak plate method is commonly used to obtain a pure culture from a mixture of microorganisms.

Normal Microbiota

  • Transient Microbiota: Present for short durations.

  • Normal (Resident) Microbiota: Colonize the host permanently without causing disease under normal conditions.

  • Human Microbiome Project: Studies the relationships between microbial communities and human health.

Relationships Between Normal Microbiota and the Host

  • Microbial Antagonism: Competition between normal microbiota and pathogens that helps protect the host.

  • Symbiosis Types:

    • Commensalism: One organism benefits while the other is unaffected.

    • Mutualism: Both organisms benefit.

    • Parasitism: One benefits at the expense of the other.

    • Opportunistic pathogens may arise from normal microbiota under certain conditions.

Summary Tables of Normal Microbiota by Body Region

  • Skin, Mouth, Large Intestine, and Urinary/Reproductive Systems each have diverse components and populations, shaped by factors like moisture, nutrient availability, and antimicrobial properties of secretions.