Genetics of Bacteria, Mutations, and the Human Microbiome: Study Notes

Genetics of Bacteria

Horizontal Gene Transfer in Bacteria

Bacteria can exchange genetic material through several mechanisms, allowing rapid adaptation and evolution. This process is called horizontal gene transfer (HGT), which differs from vertical gene transfer (inheritance from parent to offspring).

• Transformation: Uptake of free (naked) DNA from the environment by a competent bacterial cell.

• Transduction: Transfer of DNA from one bacterium to another via a bacteriophage (virus that infects bacteria).

• Conjugation: Direct transfer of DNA between two bacterial cells in contact, typically mediated by a plasmid (e.g., F plasmid).

Requirements for Each Method:

• Transformation: Recipient must be competent (naturally or artificially induced).

• Transduction: Requires bacteriophage infection; can be generalized (any gene) or specialized (specific genes).

• Conjugation: Requires cell-to-cell contact and a conjugative plasmid in the donor.

Example: Antibiotic resistance genes can spread rapidly in bacterial populations via these mechanisms.

Genetic Recombination

Genetic recombination is the physical exchange of DNA between genetic elements, often resulting in new genotypes.

• Homologous recombination: Exchange between similar or identical DNA sequences, mediated by the RecA protein.

• Occurs after DNA is introduced by transformation, transduction, or conjugation.

• Possible fates of incoming DNA: degradation, autonomous replication, or recombination with host genome.

Complementation: Occurs when a functional gene copy is supplied (e.g., on a plasmid), restoring wild-type phenotype in a mutant.

Mechanisms of DNA Transfer

• Transformation: Free DNA is incorporated into a recipient cell, leading to genetic change. Only a small portion of the genome is typically transferred.

• Competence: The ability of a cell to take up DNA. In some bacteria, this is regulated by quorum sensing or induced by laboratory methods (e.g., electroporation).

• Transduction: DNA is transferred by a bacteriophage. In generalized transduction, any gene can be transferred; in specialized, only certain genes.

• Conjugation: Involves plasmid-encoded transfer functions (e.g., tra genes on the F plasmid). DNA is transferred via a pilus and replicated by rolling circle replication.

Example: The F (fertility) plasmid in Escherichia coli encodes genes for pilus formation and DNA transfer.

Mutations and Mutagenesis

Types and Causes of Mutations

A mutation is a heritable change in the genome. Mutations can arise spontaneously or be induced by external factors.

• Spontaneous mutations: Occur naturally, often due to errors in DNA replication.

• Induced mutations: Result from exposure to mutagens (chemical, physical, or biological agents).

Types of Mutations:

• Point mutations: Change a single base pair. Can be:

  • Missense mutation: Alters amino acid sequence of a protein.

  • Nonsense mutation: Introduces a premature stop codon, truncating the protein.

  • Silent mutation: No change in amino acid sequence (often at the third codon position).

• Frameshift mutations: Insertions or deletions that shift the reading frame, altering downstream amino acid sequence. Usually severe.

Example: A single base insertion in a gene can render the encoded protein nonfunctional.

Mutagenesis: Mechanisms and Agents

• Base analogs: Chemicals resembling DNA bases, causing mispairing during replication (e.g., 5-bromouracil).

• Alkylating agents: Add alkyl groups to bases, altering base pairing (e.g., nitrosoguanidine).

• Intercalating agents: Insert between DNA bases, causing insertions or deletions (e.g., acridines, ethidium bromide).

• Radiation:

  • Non-ionizing (UV): Causes pyrimidine dimers, leading to replication errors.

  • Ionizing (X-rays, gamma rays): Generates free radicals, causing DNA breaks and large deletions.

Detection and Selection of Mutants

• Replica plating: Technique to identify mutants (e.g., auxotrophs) by transferring colonies to selective media.

• Selection: Growth conditions favoring mutants with desired traits (e.g., antibiotic resistance).

• Screening: Identifying mutants by phenotype, often requiring examination of many colonies.

Example: Replica plating can identify bacteria unable to synthesize a specific nutrient.

Regulation of Gene Expression in Prokaryotes

Operons and Their Regulation

An operon is a cluster of genes under the control of a single promoter and operator, allowing coordinated regulation.

• Inducible operons: Activated by inducers; genes are transcribed only when needed (e.g., lac operon for lactose metabolism).

• Repressible operons: Transcribed continuously unless repressed by a corepressor (e.g., trp operon for tryptophan synthesis).

lac Operon: Classic example of an inducible operon. When lactose is present, the operon is activated, allowing the cell to metabolize lactose. Regulation involves:

• CAP protein: Positive regulation in response to glucose levels.

• Repressor protein (LacI): Binds operator to block transcription in absence of lactose.

trp Operon: Example of a repressible operon. When tryptophan is abundant, it acts as a corepressor, binding the repressor and shutting off operon expression.

The Human Microbiome

Introduction to the Microbiome

The microbiome refers to the collection of microorganisms (bacteria, archaea, viruses, fungi) that inhabit the human body. Each person’s microbiome is unique and influences health, metabolism, and disease susceptibility.

• Microbial cells outnumber human cells by about 10:1.

• Dysbiosis: An imbalance in the microbiome, often associated with disease.

Development and Diversity of the Microbiome

• Acquisition: Begins at birth (vaginal vs. C-section delivery), influenced by breastfeeding, environment, and genetics.

• Enterotypes: Three main gut microbiome types in humans, dominated by Bacteroides, Prevotella, or Ruminococcus.

• Factors affecting diversity: diet, age, geography, lifestyle, health status, and gender.

Example: Breastfed infants have a more diverse gut microbiome than formula-fed infants.

Microbiome and Health

• Gut microbiome influences metabolism, immune function, and even brain development and behavior.

• Associated with conditions such as obesity, type 2 diabetes, irritable bowel disease (IBD), and cholesterol metabolism disorders.

• Microbiome can affect blood pressure and inflammation.

Example: Children with autism have been shown to have different gut microbiomes compared to neurotypical children.

Immune System and the Hygiene Hypothesis

• T-helper (Th) cells: Subtypes Th1 and Th2 help regulate immune responses.

• Th1: Promotes destruction of infected or abnormal cells (cell-mediated immunity).

• Th2: Promotes responses against extracellular parasites and allergens (humoral immunity).

• Balance between Th1 and Th2 is crucial for immune health.

• Hygiene hypothesis: Suggests that reduced exposure to microbes in early life may lead to immune imbalances and increased risk of allergies and autoimmune diseases.

Example: Newborns have a Th2-biased immune system, which should balance with Th1 over time; lack of early microbial exposure may prevent this balance.

Microbiome Interventions

• Fecal microbiota transplant (FMT): Transfer of stool from a healthy donor to a patient to restore microbiome balance, used in treating recurrent Clostridioides difficile infections.

• Probiotics: Consumption of beneficial microbes to support gut health and potentially reduce anxiety and depression.

• Global microbiome conservancy: Efforts to preserve microbial diversity worldwide.

Glossary of Key Terms

Term	Definition
Auxotroph	Mutant organism requiring an additional nutrient not needed by the wild-type
Bacteriophage	Virus that infects bacteria
Conjugation	Direct transfer of DNA between bacteria via cell-to-cell contact
Dysbiosis	Imbalance in the microbiome associated with disease
Frameshift mutation	Insertion or deletion altering the reading frame of a gene
Horizontal gene transfer	Gene transfer between organisms not by descent
Inducible operon	Operon activated by an inducer (e.g., lac operon)
Mutagen	Agent that increases mutation rate
Mutant	Organism with a heritable genetic change
Operon	Cluster of genes under control of a single promoter/operator
Plasmid	Small, circular DNA molecule independent of the chromosome
Recombination	Exchange of genetic material between DNA molecules
Repressible operon	Operon transcribed until repressed (e.g., trp operon)
Transformation	Uptake of free DNA by a bacterial cell
Transduction	DNA transfer via bacteriophage
Transposon	Mobile genetic element that can move within the genome
Wild-type strain	Organism with the typical genotype found in nature

Key Equations and Concepts

• Mutation Rate: The frequency of mutations per gene per generation.

• Rolling Circle Replication: Mechanism for DNA replication in some plasmids and viruses.

$\text{Mutation Rate} = \frac{\text{Number of mutations}}{\text{Total number of cells or generations}}$

$\text{Rolling Circle Replication:}$

$\text{A nick is made in one strand of the circular DNA, the 3' end is extended, displacing the 5' end, which is then replicated.}$

Summary Table: Horizontal Gene Transfer Mechanisms

Mechanism	Agent	Requirements	Example
Transformation	Free DNA	Competent recipient	Uptake of plasmid DNA by Bacillus subtilis
Transduction	Bacteriophage	Phage infection	Transfer of toxin genes in Vibrio cholerae
Conjugation	Plasmid	Cell contact, conjugative plasmid	F plasmid transfer in E. coli

Additional info: Some details, such as the molecular mechanisms of RecA-mediated recombination and the specifics of quorum sensing in competence regulation, have been expanded for academic completeness.