Microbiology Study Guide: Bacterial Growth, Genetics, Biotechnology, and Viruses

Bacterial Growth and Physiology

Reproduction and Generation Time

Bacteria primarily reproduce by binary fission, a process in which a single cell divides into two identical daughter cells. The time required for a bacterial population to double is called the generation time.

• Binary Fission: The cell grows, replicates its DNA, and divides into two cells.

• Generation Time Calculation: Generation time ($g$) can be calculated using the formula: $g = \frac{t}{n}$ where $t$ is the total time of growth and $n$ is the number of generations.

• Example: If a culture grows from 1,000 to 8,000 cells in 3 hours, the number of generations ($n$) is $\log_2(8000/1000) = 3$, so $g = 3\text{ hours} / 3 = 1\text{ hour}$.

Biofilms

A biofilm is a complex community of microorganisms attached to a surface and embedded in a self-produced extracellular matrix.

• Characteristics: Biofilms provide protection from environmental stress and antibiotics.

• Examples: Dental plaque, catheter-associated infections.

Bacterial Growth Curve

Bacterial populations in batch culture exhibit distinct growth phases:

• Lag Phase: Cells adapt to new environment; little division.

• Log (Exponential) Phase: Rapid cell division; population doubles at regular intervals.

• Stationary Phase: Nutrient depletion slows growth; cell death balances division.

• Death Phase: Cells die at an exponential rate.

Oxygen Requirements and Reactive Oxygen Species

Bacteria are classified by their oxygen requirements:

• Obligate Aerobes: Require O2 for growth.

• Obligate Anaerobes: Cannot tolerate O2.

• Facultative Anaerobes: Can grow with or without O2.

• Microaerophiles: Require low O2 levels.

• Aerotolerant Anaerobes: Tolerate O2 but do not use it.

Reactive Oxygen Species (ROS): Toxic byproducts of oxygen metabolism (e.g., superoxide, hydrogen peroxide).

• Superoxide Dismutase (SOD): Enzyme that detoxifies superoxide radicals; present in aerobes and facultative anaerobes.

Culture Media

• Selective Medium: Inhibits growth of some organisms while allowing others to grow.

• Differential Medium: Distinguishes organisms based on metabolic traits (e.g., color change).

Cell Concentration and Counting

• Viable Cell Count: Counts only living cells (e.g., plate count method).

• Total Cell Count: Includes both living and dead cells (e.g., direct microscopic count).

Cellular Elements

Major elements found in cellular molecules:

• Carbon: Found in carbohydrates, lipids, proteins, nucleic acids.

• Nitrogen: Found in proteins, nucleic acids.

• Phosphorus: Found in nucleic acids, ATP, phospholipids.

• Sulfur: Found in some amino acids and vitamins.

Bacterial Genetics

DNA Replication

DNA replication is the process by which a cell copies its DNA before cell division.

• Key Enzymes: DNA polymerase, helicase, primase, ligase.

• Process: Semi-conservative; each new DNA molecule contains one old and one new strand.

RNA Transcription

Transcription is the synthesis of RNA from a DNA template.

• Key Enzyme: RNA polymerase.

• Process: Initiation, elongation, termination.

Translation

Translation is the process by which mRNA is decoded to synthesize proteins.

• Key Components: Ribosomes, tRNA, mRNA.

• Process: Initiation, elongation, termination.

Mutations

A mutation is a change in the DNA sequence.

• Types: Point mutations, insertions, deletions, frameshift mutations.

• Induction: Can be spontaneous or induced by mutagens (chemicals, radiation).

Selection Methods

• Direct Selection: Identifies mutants by growth under selective conditions.

• Indirect Selection: Uses replica plating to identify mutants with specific traits.

Ames Test

The Ames test detects mutagenic potential of chemical compounds using bacteria.

Horizontal Gene Transfer

• Transduction: Transfer of DNA via bacteriophages.

• Transformation: Uptake of free DNA from environment.

• Conjugation: Transfer of DNA via direct cell-to-cell contact (requires pilus).

Transposons

Transposons are mobile genetic elements that can move within and between genomes.

Operon Regulation

• Repressible Operon: Usually on; can be turned off by a repressor (e.g., trp operon).

• Inducible Operon: Usually off; can be turned on by an inducer (e.g., lac operon).

Operon Components

• Promoter: DNA sequence where RNA polymerase binds.

• Operator: DNA segment where repressor binds.

• Inducer: Molecule that activates gene expression.

• Repressor: Protein that inhibits gene expression.

• Co-repressor: Molecule that activates repressor function.

Biotechnology

Restriction Enzymes

Restriction enzymes cut DNA at specific sequences, enabling genetic engineering.

Vectors

Vectors are DNA molecules used to carry foreign genetic material into a host cell.

• Ideal Vector Requirements: Origin of replication, selectable marker, cloning site.

DNA Library

A DNA library is a collection of DNA fragments cloned into vectors for study.

PCR (Polymerase Chain Reaction)

PCR amplifies specific DNA sequences using cycles of heating and cooling.

• Key Steps: Denaturation, annealing, extension.

• Equation: $N = N_0 \times 2^n$ (where $N$ is final DNA copies, $N_0$ is initial, $n$ is cycles)

DNA Sequencing

DNA sequencing determines the order of nucleotides in DNA; used for genetic analysis.

Reverse Transcriptase

Reverse transcriptase synthesizes DNA from RNA; used in cDNA synthesis and retrovirus replication.

Cloning Steps

• Isolate DNA of interest

• Cut with restriction enzymes

• Insert into vector

• Transform host cells

• Select for successful clones

Applications and Risks of Biotechnology

• Applications: Medicine, agriculture, research.

• Risks: Ethical concerns, environmental impact, biosecurity.

Viruses

Definition and Structure

Viruses are acellular infectious agents composed of genetic material (DNA or RNA) surrounded by a protein coat (capsid).

• Components: Nucleic acid, capsid, sometimes envelope.

• Shapes: Helical, icosahedral, complex.

Bacteriophages and Replication Cycles

Bacteriophages are viruses that infect bacteria. They exhibit two main replication cycles:

• Lytic Cycle: Virus replicates and lyses host cell.

• Lysogenic Cycle: Viral DNA integrates into host genome (prophage).

Phases of Viral Infection

• Attachment

• Penetration

• Uncoating

• Replication

• Assembly

• Release

Lysogenic Conversion

Lysogenic conversion occurs when a prophage alters the phenotype of the host bacterium, sometimes increasing virulence (e.g., toxin production).

Reverse Transcriptase and Replicase

• Reverse Transcriptase: Used by retroviruses (e.g., HIV) to synthesize DNA from RNA.

• Replicase: Enzyme used by RNA viruses to replicate their RNA genomes.

Latent vs Persistent (Chronic) Infection

• Latent Infection: Virus remains dormant; can reactivate (e.g., herpes simplex).

• Persistent (Chronic) Infection: Virus continuously replicates; symptoms may be ongoing (e.g., hepatitis B).

Virus Cultivation

• Viruses are grown in cell cultures, embryonated eggs, or live animals.

Cytopathic Effects

Cytopathic effects are visible changes in host cells due to viral infection (e.g., cell lysis, syncytia formation).

Oxygen Requirement	Growth in O2	Enzymes Present
Obligate Aerobe	Yes	SOD, Catalase
Obligate Anaerobe	No	None
Facultative Anaerobe	Yes/No	SOD, Catalase
Microaerophile	Low O2	SOD
Aerotolerant Anaerobe	Yes	SOD

Additional info: Some explanations and examples were expanded for clarity and completeness.