PowerPoint 5 - Microbial Growth

Introduction to Microbial Growth

• Source of Content: Modified PowerPoint for BIO 275 (CVCC)

• Textbook and resources available at OpenStax

Microbial Growth Basics

• Definition: Microbial growth refers to the increase in the number of bacterial cells, not the size of individual microorganisms.

• Colony Formation:

  • Starts from a single mother cell.

  • Cell division occurs via binary fission, creating offspring that form visible colonies in favorable conditions.

Binary Fission Process

• Mechanism of Cell Division:

  • Common in bacteria, binary fission is a form of asexual reproduction.

  • The cell grows and duplicates its cellular components before dividing.

• DNA Replication:

  • Begins at the origin of replication on the circular chromosome, attached to the inner cell membrane.

  • Continues until reaching the terminus, leading to the formation of new daughter cells through cytokinesis.

• Visual Representation:

  • Electron micrographs depict the process and stages of binary fission in bacteria like Salmonella typhimurium.

Cytokinesis and FtsZ Action

• Role of FtsZ Protein:

  • A key protein (similar to tubulin in eukaryotic cells) directs cytokinesis.

  • It assembles a Z ring on the cytoplasmic membrane, defining the division plane between daughter cells.

• Divisome Formation:

  • Additional proteins join the Z ring to form the divisome, initiating new cell wall synthesis and septum formation.

Doubling Time and Generation Time

• Definition: Time for a bacterial population to double through one round of binary fission.

• Escherichia coli: Optimal doubling time of about 20 minutes, which may extend to several days under harsh conditions.

• Generation Figures:

  • A single cell increases to over 1 million in 20 generations (~7 hours).

  • Example: Mycobacterium leprae doubling time is 14 days.

Microbial Growth Curve

• Growth Curve Phases:

  • Lag Phase: Initial phase, characterized by cell adjustment and preparation for division.

  • Log Phase: Rapid exponential growth and division; cells are most metabolically active and susceptible to antibiotics.

  • Stationary Phase: Balance between cell division and death, reaching carrying capacity.

  • Death Phase: Increased waste and nutrient depletion lead to cell death; persister cells may survive.

Measuring Microbial Growth

• Methods:

  • Plate Count: Viable cells counted based on visible colonies, expressed in CFUs.

  • Serial Dilution: Helps achieve manageable counts within accurate statistical ranges for microbial estimates.

  • Filtration Method: Used for low bacterial counts in samples like water to detect coliforms.

  • Coulter Counter: Counts cells as they pass through a detector in a liquid sample.

Biofilms and Their Importance

• Characteristics:

  • Structured microbial communities that thrive on surfaces, secreting extracellular polymeric substances (EPS) to form a protective matrix.

• Quorum Sensing:

  • Communication between cells in biofilms using signaling molecules to coordinate responses based on population density.

• Clinical Relevance:

  • Biofilms can enhance resistance to antibiotics and contribute to chronic infections.

Environmental Conditions Affecting Microbial Growth

• Anaerobic Environments: Locations like bogs or the rumen provide oxygen-free conditions favorable for specific bacteria.

• pH and Temperature Tolerance:

  • Microbes have specific ranges required for optimal growth (e.g. neutrophiles near pH 7, thermophiles thrive at high temperatures).

• Salt Concentration: Halophiles adapt to high salinity environments, such as those found in salt lakes.

Culture Media in Microbiology

• Types of Media:

  • Agar: The most common solidifying agent, solidifies at temperatures below 40°C.

  • Selective and Differential Media: Used to isolate specific microbes or distinguish between different species based on color change.

• Examples:

  • MacConkey agar differentiates lactose fermenters (e.g., E. coli) from non-fermenters.

  • Blood agar identifies hemolytic bacteria like Listeria.