Biol 20: Chapter 9

Introduction to Microbial Growth

  • Biofilms: Collections of different types of bacteria living together, producing a sticky substance.

  • Formation of biofilm will be discussed in detail later.

Bacterial Growth Mechanism

  • Binary Fission: Most bacterial cells divide by binary fission, where one cell divides to form two identical cells.

  • Generation Time: Defined as the doubling time of the bacterial population.

  • Cell Growth Process:

    • DNA Replication: The process begins with the replication of DNA in the mother cell.

    • Elongation: The cell elongates after DNA replication.

    • Division Septum: A septum forms at the center of the cell to facilitate division.

    • Cytokinesis: Directed by the FITC protein, which assembles into a Z ring.

    • Divisome Formation:

      • The Z ring facilitates additional proteins' assembly to form the divisome.

      • The divisome produces a peptidoglycan cell wall, ultimately dividing the mother cell.

Generation Times of Bacteria

  • E. Coli (Escherichia coli): 20 minutes under optimal lab conditions.

  • Mycobacterium tuberculosis: 15 to 20 hours.

  • Mycobacterium leprae: 14 days, indicating slow growth rates for certain bacteria.

Growth Patterns in Closed Culture

  • Closed Culture (Batch Culture): No addition of nutrients and limited waste removal, leading to a reproducible growth pattern represented as a Growth Curve.

  • Growth Curve Phases:

    • Lag Phase:

      • No change in cell number, cells increase in size while metabolically active.

    • Log Phase (Exponential Phase):

      • Active cell division occurs, leading to exponential growth, with the number of cells doubling under optimal conditions.

    • Stationary Phase:

      • Plateau where nutrient depletion and waste accumulation cause growth stalling.

      • New cells equal dying cells during this phase.

      • Virulence factors expression, quantum sensing, and toxic waste accumulation occur here.

    • Death Phase:

      • More cells die than divide; higher toxic waste levels lead to extensive cell death.

      • Surviving cells may produce endospores and persister cells - medically significant as they can contribute to chronic infections like tuberculosis.

Summary of Key Phases

  • Lag Phase: Cells adapt to the culture medium without increasing in number.

  • Log Phase: Cells divide rapidly; number increases exponentially.

  • Stationary Phase: Cell number stabilizes due to resource depletion, producing virulence factors and responding through quantum sensing.

  • Death Phase: Cells die at greater rates; presence of endospores and persister cells highlights survival strategies during hostile conditions.

Conclusion

  • Important to understand the implications of each phase of growth to improve knowledge of microbial behavior in various environments.

Video 2

Growth Curve Phases

  • Four Phases of Growth Curve: Key to understanding bacterial growth. Review all phases to grasp concepts fully.

  • Graph of Log Phase: Can be plotted on an arithmetic scale, illustrating the growth phase.

    • Arithmetic Scale: Shows a clear increase in bacterial numbers.

    • Logarithmic Scale: When plotted on semi-log paper, it appears linear, showcasing different growth patterns.

Continuous Culture: Chemostat

  • Chemostat Definition: A device designed to maintain a continuous bacterial culture, contrasting with closed cultures where growth halts due to nutrient depletion and waste accumulation.

  • Operation:

    • Nutrients added at a steady rate via an opening.

    • Waste materials and excess bacterial suspension are continually removed.

  • Industry Application: Used for producing or extracting microbial products while keeping bacteria in the log phase to maximize yield.

  • Importance of Log Phase: This is when bacteria are most active in producing chemicals and are sensitive to adverse conditions (e.g., antibiotics).

Measuring Bacterial Growth

  • Direct vs. Indirect Methods:

    • Direct Method: Counting bacterial cells individually; considered strenuous.

      • Examples of Direct Counting Methods:

        • Hemocytometer: Used to count blood cells by diluting and calculating total cell numbers.

        • Fluorescence Microscope: Distinguishes live (green) from dead (red) cells.

        • Colter Counter: Automated counting tool for bacteria (not available in all laboratories).

    • Indirect Method: Measures cell presence or activity without directly counting cells; easier approach.

Serial Dilution and Colony Counting

  • Serial Dilution Process:

    • Start with concentrated bacterial solution and dilute it step-wise (1 mL to 9 mL diluent):

      • 1:10 dilution → 1:100 → 1:1000, etc. (expressed as exponents: 10^-1, 10^-2, 10^-3)

  • Plating Samples:

    • After dilution, take samples (e.g., 0.1 mL = 100 µL) for plating on growth medium.

    • Bacterial colonies visible after incubation; individual colonies facilitate counts and determinations of bacterial concentration.

  • Methods of Plating:

    • Pour Plate Method: Mix diluted sample with melted agar, pour into a plate, and swirl to mix.

    • Spread Plate Method: Spread a small amount of diluted sample atop pre-prepared agar medium.

  • Countable Colonies Guidelines:

    • Aim for plates with 30-300 colonies for accurate results.

    • Plates outside this range (<30 or >300 colonies) are not usable.

  • Calculating Colony Forming Units (CFU):

    • Formula: CFU/mL = (number of colonies) / (volume plated in mL) x (dilution factor).

Summary

  • Understanding the growth curve phases, the function of chemostats, and methods of measuring bacterial growth (direct and indirect counting) is essential in microbiology, especially in lab settings involving bacterial culture and analysis.

Video 3:

Indirect Cell Counts

  • Definition: Measurement of bacterial activity instead of direct counting of bacteria.

  • Importance: Easier than direct methods, used widely in labs to assess bacteria growth.

Spectrophotometer

  • Introduction: Instrument used to measure turbidity in bacterial suspensions.

  • Turbidity: Refers to the cloudiness of a liquid; higher turbidity indicates increased bacterial growth.

  • Usage: A beam of light is transmitted through a bacterial suspension.

    • Measurement: Amount of light passing through is detected and can be expressed as either:

      • Percent Transmission: Higher in clear cultures.

      • Absorbance: Increases with turbidity; higher bacterial counts result in higher absorbance due to more light being absorbed.

  • Importance in Lab: Understanding growth dynamics in bacterial cultures through turbidity measurements.

Biofilms

  • Definition: Structured communities of bacteria that provide advantages to their microorganisms.

  • Formation: Started by a few bacteria that send signals to invite others, leading to a complex community.

  • Extracellular Polymeric Substances (EPS):

    • Composition: A hydrated gel of polysaccharides, proteins, nucleic acids, and lipids; secreted by organisms to form the biofilm matrix.

    • Functions:

      • Nutrient and Waste Movement: Channels in EPS facilitate nutrient transportation and waste removal.

      • Hydration: Maintains moisture and protects against desiccation.

      • Defense Mechanism: Shields microorganisms from predation and makes it difficult for antibiotics to penetrate.

Quorum Sensing

  • Definition: Mechanism by which bacteria coordinate collective behaviors based on cell density.

  • Process: Cells release signaling molecules known as autoinducers.

  • Types of Autoinducers:

    • Gram-Negative Bacteria: Produce N-acetyl homoserine lactones.

    • Gram-Positive Bacteria: Utilize small peptides as signals for communication.

  • Importance: Enables bacteria to respond to environmental stimuli and synchronize actions within a biofilm.

Bacterial Growth Requirements

  • General Overview: Bacteria categorized based on oxygen requirements and environmental conditions.

  • Key Terminology:

    • Obligate Aerobes: Require oxygen to survive; grow only at the top of thioglycolate media (oxygen-rich zone).

    • Obligate Anaerobes: Cannot tolerate oxygen; grow only at the bottom of anaerobic zones (oxygen-free).

    • Facultative Anaerobes: Prefer oxygen but can grow without it; tend to be present mostly in the oxygen-rich area, but some can be found throughout.

    • Aerotolerant Anaerobes: Can endure oxygen but do not utilize it; distribution is uniform throughout the culture.

    • Microaerophiles: Require low levels of oxygen; found just below the oxygen-rich zone.

Conclusion

  • Future Labs: Laboratory sessions to reinforce learning about indirect counting, biofilms, and the various bacterial growth requirements.

Video 4:

Microbial Terminology Overview

Oxygen Requirements

  • Obligate Aerobes: Organisms that require oxygen for survival and growth. They thrive in environments rich in oxygen.

  • Obligate Anaerobes: These can only survive in environments devoid of oxygen. They are often found in deep tissues or sediments.

  • Facultative Anaerobes: These organisms can grow in both the presence and absence of oxygen, but tend to grow better with oxygen.

  • Aerotolerant Anaerobes: They can tolerate oxygen but do not utilize it for growth.

  • Microaerophiles: These require low levels of oxygen for growth, typically around 2-10% O2, which is less than atmospheric levels.

Anaerobic Culturing

  • Brewer's Jar: A special jar used to culture obligate anaerobes by creating an oxygen-free environment. It uses a gas package that releases carbon dioxide to displace oxygen.

  • Conditions for Obligate Anaerobes: Anaerobic environments (like infected tissue, as seen in diabetic ulcers) provide suitable conditions for obligate anaerobes like Clostridium perfringens, which thrive in dead or necrotic tissue.

pH Tolerance of Organisms

  • Acidophiles: Prefer acidic environments (pH less than 7). Optimal growth usually around pH 3-3.5.

  • Neutrophiles: Thrive in neutral pH conditions, optimal growth at pH around 7.

  • Alkaliphiles: Prefer basic environments (pH greater than 7). Optimal growth typically around pH 9.5.

  • Halophiles: These organisms can tolerate high salt concentrations and are often found in extreme saline environments.

Temperature Preference of Organisms

  • Psychrophiles: Prefer low temperatures; thrive between -10°C to 20°C. Can be found in refrigerated conditions.

  • Mesophiles: Optimal growth at around 37°C, which is the normal body temperature for humans. Most human pathogens fall within this category.

  • Thermophiles: Thrive at higher temperatures, specifically between 42°C and 80°C, with an optimum around 65°C.

  • Hyperthermophiles: Require extremely high temperatures, usually found in environments exceeding 65°C, such as hot springs. Optimal growth around 95°C.

Summary of Terminology

  • Oxygen Requirements: Differentiates organisms based on their need or tolerance for oxygen.

  • pH Requirements: Organisms are categorized based on their preferred pH levels for optimal growth.

  • Temperature Requirements: Refers to the preferred temperature ranges for various microbial categories.

Visual Aids and Examples

  • Images and illustrations have been used throughout to provide visual context to the discussed organisms, including examples of extreme environments such as saline lakes and thermal vents, which are home to specialized microorganisms.

Conclusion

  • Understand and familiarize yourself with the terminology related to microbial oxygen, pH, and temperature preferences, as these are crucial for understanding microbial ecology and pathogenicity. Prepare for an interactive discussion session to reinforce these concepts.