Microbiology and Molecular Biology

Microbial Growth

Binary Fission and Growth Phases

  • Bacterial Division (Binary Fission):

    • Definition: Bacterial growth refers to the increase in cell numbers (reproduction).

    • Reproduction Type: Asexual, through binary fission.

    • Example: Under ideal conditions, Escherichia coli (E. coli) divides every 20 minutes (generation time of 20 minutes).

    • Note: Other bacteria may divide much slower or under natural conditions.

Exponential Growth

  • Definition: Exponential growth is when the number of cells doubles during each constant time interval, leading to a characteristic logarithmic growth curve, also known as logarithmic growth.

Mutation During DNA Replication

  • Mutation Explanation:

    • DNA replication is not 100% error-free. Mistakes occur when wrong bases are added, leading to mutations.

    • Result of Mutations: New genetic traits (new genes or new functions) resulting from mutations.

    • Some mutations may confer antibiotic resistance.

Mutation Rates in Bacteria

  • Frequency of Errors: Occur at a rate of $10^{-8}$ to $10^{-11}$ errors per base pair during DNA replication.

    • Example Calculation 1: In Staphylococcus aureus

    • After 20 generations: $2^{20} = 1,048,576$ cells

    • Expected mutations: Nearly 300 mutations within approximately 10 hours.

    • Example Calculation 2: In E. coli with a liquid culture of $10^7$ cells/mL:

    • E. coli genome has about $4 imes 10^6$ base pairs.

    • Total base pairs = $(4 imes 10^6 ext{ bp}) imes (10^7 ext{ cells/mL}) = 4 imes 10^{13} ext{ bp}$.

    • Result: Approximately 1,000 mutations after one round of replication.

    • Conclusion: Although mutation rates are low, the vast number of bacteria in 1 mL results in hundreds or thousands with one or more mutations.

Example of Bacterial Evolution

  • Mega-Plate Experiment: A video illustrates bacteria evolving on a mega-plate Petri dish, demonstrating adaptation and survival as they encounter progressively higher concentrations of antibiotics (up to 1000x the recommended dose).

Measuring Microbial Growth

Methods

  • Microscopy and Viable Counts:

    • Measurement through microscopy or viable counts is very labor-intensive and time-consuming.

    • Direct total counts involve microscopy, while viable counts require serial dilutions.

Turbidity Measurements

  • Definition of Turbidity: The cloudiness of a liquid culture, used to measure microbial growth.

  • Measurement Approach:

    • Turbidity is measured at $600 ext{ nm}$ using a spectrophotometer, which assesses the amount of unscattered light in a cell suspension.

    • Useful for constructing growth curves and comparatively easier than direct counting methods.

Growth Curve Phases

  1. Lag Phase:

    • Initial period after inoculation into fresh medium where growth begins after a delay due to depletion of essential constituents.

    • If cells from an exponentially growing culture are transferred to the same medium, growth begins immediately without a lag phase.

  2. Exponential Phase (Logarithmic Phase):

    • Bacteria grow exponentially until a limiting factor is encountered.

    • Hypothetical Growth Scenario: If a single bacterium with a 20-minute generation time continued to grow for 48 hours, its mass would exceed that of Earth.

  3. Stationary Phase:

    • Growth rate drops to zero due to the depletion of essential nutrients or accumulation of inhibitory wastes.

    • There is no net increase or decrease in the cell number; energy metabolism and biosynthetic processes can still proceed.

  4. Death Phase:

    • Characterized by the death of cells, often at an exponential rate, although this rate is generally much slower than the earlier exponential growth phase.

Environmental Factors Affecting Microbial Growth

  • Factors include:

    • Temperature

    • Oxygen availability

    • Water availability

    • pH

    • Salinity

    • Pressure

    • Radiation

Temperature Effects on Growth

  • Variation: Minimum and maximum temperatures vary widely among microorganisms.

    • Optimum Temperature: The temperature at which growth is fastest, reflecting the natural habitat's range.

Temperature Requirements for Various Microorganisms

  • Psychrophiles: Optimal growth between -5°C and 15°C.

  • Mesophiles: Optimal growth between 15°C and 45°C.

  • Thermophiles: Optimal growth between 45°C and 70°C.

  • Hyperthermophiles: Optimal growth above 70°C.

Adaptations of Psychrophiles (Cold Temperatures)

  • Possess specialized proteins and membranes active in low temperatures.

  • Produce cryoprotectants (e.g., antifreeze proteins, glycerol) to prevent ice crystal formation.

  • Growth can occur under sub-0°C conditions through concentrated solutes in small pockets of liquid water.

Adaptations of Hyperthermophiles (High Temperatures)

  • Enzymes remain stable at high temperatures and support growth in extreme conditions, making them valuable in industrial microbiology (e.g., Taq polymerase from Thermus aquaticus for PCR processes).

pH Requirements

  • Most organisms thrive at pH 5.5 to 7.9 (circumneutral) with exceptions:

    • Acidophiles: Prefer below pH 5.5.

    • Alkaliphiles: Prefer above pH 8.

Salinity Tolerance

  • Halotolerant Microorganisms: Grow in environments with high salt concentrations, such as salt lakes (3% NaCl).

  • Extreme Extremophiles: Some can tolerate dehydration, pressure, and radiation, such as Deinococcus radiodurans.

Oxygen Requirements

  • Microorganisms can thrive in anaerobic environments, including sediments, bogs, and animal intestinal tracts.

  • Types include:

    • Obligate Aerobes: Require oxygen for growth.

    • Facultative Anaerobes: Grow in both aerobic and anaerobic environments, with better growth in the presence of oxygen.

    • Obligate Anaerobes: Cannot grow in oxygen-rich environments.

    • Aerotolerant Anaerobes: Growth occurs without oxygen but can tolerate low concentrations.

    • Microaerophiles: Require low concentrations of oxygen for growth.

Toxicity of Oxygen

  • Oxygen can be reduced by organisms, resulting in toxic derivatives (e.g., hydrogen peroxide).

  • Aerobic Microorganisms: Have enzymes (e.g., catalase and superoxide dismutase) that neutralize oxygen's toxic effects.

  • Obligate Anaerobes: Lack these protective enzymes and are harmed by oxygen.

Nutritional Requirements and Culturing

Nutritional Elements for Microbial Growth

  • Macronutrients: C, O, H, N, P, S are essential for cellular structure and function.

  • Micronutrients: Trace elements, such as Iron (Fe), are required in smaller amounts for various cellular processes.

Growth Media

  • Can be liquid or solidified (using agar).

  • Two main types:

    • Defined Media: Exact composition known and measured.

    • Complex Media: Nutrient-rich but composition unknown; used to promote high growth rates.

Types of Culture Media

  1. General Purpose Media: Supports growth of a wide range of bacteria (e.g., Nutrient Agar, LB media).

  2. Selective Media: Target specific microorganisms by inhibiting others (e.g., bile acids for enteric bacteria).

  3. Differential Media: Contains indicators to reveal specific biochemical reactions (e.g., mannitol fermentation).

Examples of Selective and Differential Media

  • Mannitol Salt Agar (MSA):

    • High salt concentration selects for Staphylococcus. Fermentation of mannitol changes the pH indicator.

  • Blood Agar: Enriched for bacteria with varying hemolytic capabilities.

  • MacConkey Agar: Inhibits Gram-positive bacteria and differentially indicates lactose fermentation.

Diagnostic Microbiology

Overview of Methods Used in Diagnostics
Sampling Techniques

  • From humans: Nose, throat, mouth, urethra, vagina, wounds, blood.

  • From non-human sources: Food, water, air, soils.

Identification Techniques

  • Antibody assays (search for antibodies against pathogens).

  • Enrichment methods (using selective media), followed by isolation and pure culture techniques.

  • Molecular Biology Techniques: PCR for pathogen detection, looking for gene sequences or hybridization.

Important Tests for Pathogen Identification

  • Includes carbohydrate fermentation, catalase, citrate utilization, coagulase test, and more.

  • Immunoassays for antibody detection and nucleic acid-based methods for genetic analysis approved for diagnostic use.

Biosafety in the Laboratory

Biosafety Levels and Practices

  • Biosafety Level 1: Basic containment; low-risk organisms.

  • Biosafety Level 2: Moderate risk, requires biosafety cabinets for aerosol-generating procedures.

  • Biosafety Level 3: High risk for serious infections; specialized containment.

  • Biosafety Level 4: Maximum containment for dangerous pathogens; strict protocols required.