Lecture 14: Microbial Gene Regulation

Overview of Regulation in Microbial Cells

Microbial cells regulate gene expression to adapt to environmental changes and efficiently utilize resources. Regulation occurs at multiple levels, including transcription, translation, and post-translational modification.

• Transcriptional Regulation: Control of gene expression by influencing the initiation or rate of transcription.

• Translational Regulation: Control of protein synthesis by regulating mRNA translation.

• Post-Translational Regulation: Modification of proteins after synthesis, affecting their activity or stability.

• Enzyme Regulation: Not all enzymes are regulated; some are constitutively expressed, while others are inducible or repressible.

Example: The lac operon in Escherichia coli is regulated at the transcriptional level in response to lactose availability.

DNA Binding Proteins and Regulation of Transcription

DNA binding proteins play a crucial role in regulating transcription by interacting with specific DNA sequences.

• Structure: DNA binding proteins have domains that recognize and bind to operator or promoter regions.

• Function: They can act as repressors, activators, or both, influencing gene expression.

• Binding Sites: Proteins bind to major or minor grooves of DNA, affecting RNA polymerase access.

Example: The lac repressor binds to the operator region, preventing transcription of the lac operon.

Key Regulatory Elements and Their Functions

• Operon: A cluster of genes under the control of a single promoter, transcribed together.

• Promoter: DNA sequence where RNA polymerase binds to initiate transcription.

• Operator: DNA segment where regulatory proteins bind to control transcription.

• Repressor: Protein that binds to the operator to block transcription.

• Activator: Protein that increases transcription by assisting RNA polymerase binding.

• Quorum Sensing: Cell-to-cell communication mechanism regulating gene expression in response to population density.

• Catabolite Repression: Regulatory mechanism where the presence of a preferred carbon source (e.g., glucose) inhibits the use of other sources.

Negative and Positive Control of Transcription

• Negative Control (Repression): Repressor proteins block transcription. Example: Tryptophan operon is repressed when tryptophan is abundant.

• Positive Control (Induction): Activator proteins enhance transcription. Example: Maltose operon is activated in the presence of maltose.

Equation:

Global Control Systems

• Global Regulation: Simultaneous regulation of multiple operons in response to environmental signals.

• Catabolite Repression: Involves cyclic AMP (cAMP) and catabolite activator protein (CAP/CRP).

• Example: Glucose effect—when glucose is present, cAMP levels drop, reducing activation of alternative sugar operons.

Sensing and Signal Transduction in Microbes

• Phosphorylation: Addition of phosphate groups to proteins, often involved in two-component regulatory systems.

• Quorum Sensing: Regulation of gene expression in response to cell density via signaling molecules (autoinducers).

Regulation by Small RNA (sRNA)

• sRNA: Small non-coding RNAs that regulate translation by binding to mRNA, affecting its stability or translation efficiency.

• Binding Sites: sRNAs typically bind near the ribosome binding site on mRNA.

Lecture 15: Mechanism of Microbial Evolution and Genomic Diversity

Types of Mutations and Genetic Variation

Microbial evolution is driven by genetic changes, including mutations and recombination events.

• Substitution: Replacement of one nucleotide with another.

• Deletion: Loss of a DNA segment.

• Insertion: Addition of extra nucleotides.

• Duplication: Copying of a DNA segment.

• Frameshift Mutation: Insertion or deletion that alters the reading frame.

• Silent Mutation: Change in DNA that does not affect the protein sequence.

• Missense Mutation: Change resulting in a different amino acid.

• Nonsense Mutation: Change resulting in a premature stop codon.

• Homologous/Non-homologous Recombination: Exchange of genetic material between similar or dissimilar DNA sequences.

• Genetic Drift: Random changes in allele frequencies in a population.

• Gene Families: Groups of related genes with similar functions.

• Orthologs: Genes in different species that evolved from a common ancestor.

• Paralogs: Genes related by duplication within a genome.

• Core Genome: Genes shared by all strains of a species.

• Pan Genome: All genes present in all strains of a species.

DNA Polymerase Fidelity

• High-Fidelity DNA Polymerases: Enzymes with proofreading activity, reducing mutation rates during replication.

• Role: Maintain genetic stability in microbes.

Genetic Drift vs. Selection

• Genetic Drift: Random changes, significant in small populations.

• Selection: Non-random changes favoring beneficial mutations.

• Example: Antibiotic resistance genes spreading due to selection pressure.

Horizontal Gene Transfer (HGT)

• Transformation: Uptake of free DNA from the environment.

• Transduction: Transfer of DNA via bacteriophages.

• Conjugation: Direct transfer of DNA between cells via pili.

Lecture 16: Microbial Taxonomy and Systematics

Microbial Classification Concepts

• Phenetic Species Concept: Classification based on observable traits.

• Phylogenetic Species Concept: Classification based on evolutionary relationships.

Kingdoms and Domains

• Whittaker's Classification: Five kingdoms: Monera, Protista, Fungi, Plantae, Animalia.

• Carl Woese's Domains: Three domains: Bacteria, Archaea, Eukarya, based on rRNA sequences.

Microbial Properties Used in Classification

• Morphological Characters: Cell shape, size, arrangement.

• Differential Staining: Gram stain, acid-fast stain.

• Biochemical Tests: Metabolic capabilities, enzyme activities.

Dichotomous Keys

• Definition: Stepwise identification tool using paired contrasting statements.

• Application: Used to identify unknown bacteria from clinical samples.

Phylogenetic Trees and Molecular Methods

• Phylogenetic Tree: Diagram showing evolutionary relationships.

• Nodes: Branch points representing common ancestors.

• Branches: Distance reflects genetic divergence.

• 16S rRNA: Widely used for prokaryotic phylogeny.

• 18S rRNA: Used for eukaryotic phylogeny.

Comparative Genomics

• Comparative Genomics: Analysis of genome sequences to determine evolutionary relationships and functional similarities.

• Difference from Phenotypic Analysis: Genomics provides more precise and comprehensive data.

Lecture 17: Microbial Ecology and Diversity

Microbial Ecology Vocabulary and Species Interactions

• Ecosystem: Community of organisms and their environment.

• Habitat: Physical location where microbes live.

• Guild: Group of species exploiting the same resources.

• Niche: Role and position of a microbe in its environment.

• Microenvironments: Small, specialized habitats within larger environments.

• Mutualism: Both partners benefit.

• Commensalism: One benefits, other unaffected.

• Parasitism: One benefits at the expense of the other.

• Amensalism: One is inhibited, other unaffected.

• Syntrophy: Cooperative interaction for metabolic benefit.

Species Richness vs. Abundance

• Species Richness: Number of different species present.

• Species Abundance: Proportion of each species in the community.

Microbial Mats and Biofilms

• Phototrophic Mats: Layers of photosynthetic microbes.

• Chemolithotrophic Mats: Layers of microbes using inorganic compounds for energy.

• Biofilms: Structured microbial communities attached to surfaces, protected by extracellular matrix.

Microbial Enumeration Methods

• Culture-Dependent Methods: Isolation and growth on media.

• Culture-Independent Methods: Molecular techniques (e.g., DNA sequencing) to assess diversity.

Lecture 18: Microbiology of Built Environment

Microbial Contaminants in Drinking Water

• Sources: Soil, sewage, animal waste, biofilms in pipes.

• Water Treatment Steps: Settling, coagulation, sedimentation, filtration, disinfection (chlorine/UV).

• Disinfection: Chlorine kills most pathogens; UV damages microbial DNA.

• Fecal Contamination Test: Fecal coliforms (e.g., Escherichia coli) indicate contamination.

Wastewater Treatment Microbiology

• Goals: Reduce organic/inorganic material, eliminate pathogens.

• BOD (Biochemical Oxygen Demand): Measure of organic matter; lower BOD indicates cleaner water.

• Treatment Levels: Primary (physical), secondary (biological), tertiary (chemical/nutrient removal).

Microbial Activities in Homes and Public Spaces

• Microbiota of Built Environment: Microbes colonize surfaces, appliances, and water systems.

• Biodegradation: Microbes break down materials like concrete and hydrocarbons.

• Metal Extraction: Microbes facilitate recovery of metals from ores.

Lecture 19: The Human Microbiome

Microbiota vs. Microbiome

• Microbiota: Community of microorganisms living in a specific environment.

• Microbiome: The collective genomes of the microbiota.

• Multicellular Organisms with Microbiota: Legumes, lichens, bees, ruminants.

Acquisition and Diversity of Human Microbiota

• Acquisition: Microbiota acquired during birth and early life; influenced by environment and diet.

• Diversity: Varies between individuals and anatomical sites.

Microbiota of Skin, Respiratory Tract, Oral Cavity, and Gut

• Skin: Microbial composition depends on site, age, and environment; Staphylococcus epidermidis is common.

• Respiratory Tract: Upper tract has more microbes; lower tract is mostly sterile.

• Oral Cavity: Complex community; saliva and dental plaque support diverse microbes.

• Gut: High surface area and nutrient availability support dense microbial populations; composition varies by location and pH.

Microbiome and Human Health

• Functions: Digestion, vitamin synthesis, immune modulation, protection against pathogens.

• Diseases: Dysbiosis linked to obesity, inflammatory bowel disease, dental caries, periodontitis.

• Modulation: Microbiome can be influenced by diet, antibiotics, and probiotics.

Table: Comparison of Microbiota at Different Human Body Sites

Additional info: These notes expand on the provided lecture outlines with definitions, examples, and context to support exam preparation for college-level microbiology.