Microbial Genetics, Growth, and Pathogenesis: Study Guide for Exam 2

Microbial Genetics

Structure and Function of the Genetic Material

This section covers the fundamental concepts of microbial genetics, including the structure and function of genetic material, DNA replication, protein synthesis, and the central dogma of molecular biology.

• Genetics: The study of heredity and variation in organisms.

• Genome: The complete set of genetic material in an organism.

• Chromosome: A DNA molecule containing part or all of the genetic material of an organism.

• Gene: A segment of DNA that encodes a functional product, usually a protein.

• Genetic Code: The set of rules by which information encoded in genetic material is translated into proteins.

• Genotype: The genetic makeup of an organism.

• Phenotype: The observable characteristics of an organism resulting from the interaction of its genotype with the environment.

• Genomics: The study of genomes, including their structure, function, evolution, and mapping.

DNA as Genetic Information

• Function: DNA stores genetic information and serves as a template for replication and transcription.

• Structure: DNA is a double helix with antiparallel strands held together by hydrogen bonds between complementary bases (A-T, G-C).

DNA Replication

• Process: DNA replication is semi-conservative, meaning each new DNA molecule consists of one old and one new strand.

• Enzymes: Key enzymes include DNA polymerase (synthesizes new DNA), helicase (unwinds DNA), and topoisomerase (relieves supercoiling).

• Leading vs. Lagging Strands: The leading strand is synthesized continuously, while the lagging strand is synthesized in Okazaki fragments.

Protein Synthesis

• Transcription: The process of copying a gene's DNA sequence into mRNA.

• RNA Processing: In eukaryotes, pre-mRNA undergoes splicing to remove introns and join exons.

• Translation: The process by which ribosomes synthesize proteins using mRNA as a template.

• Prokaryotes vs. Eukaryotes: In prokaryotes, transcription and translation can occur simultaneously; in eukaryotes, they are separated by the nuclear envelope.

Central Dogma

• Definition: The central dogma describes the flow of genetic information: DNA → RNA → Protein.

• Equation: $\text{DNA} \xrightarrow{\text{transcription}} \text{RNA} \xrightarrow{\text{translation}} \text{Protein}$

RNA and Protein Synthesis

RNA Types and Functions

• mRNA (messenger RNA): Carries genetic information from DNA to the ribosome.

• tRNA (transfer RNA): Brings amino acids to the ribosome during translation.

• rRNA (ribosomal RNA): Forms the core of the ribosome's structure and catalyzes protein synthesis.

Transcription

• Process: RNA polymerase binds to the promoter region and synthesizes RNA from the DNA template until it reaches a terminator sequence.

• Prokaryotes vs. Eukaryotes: In prokaryotes, transcription occurs in the cytoplasm; in eukaryotes, it occurs in the nucleus and involves RNA processing (splicing).

Translation

• Codons: Triplets of nucleotides in mRNA that specify amino acids.

• Anticodons: Triplets in tRNA that are complementary to mRNA codons.

• Peptide Bond Formation: Ribosomes catalyze the formation of peptide bonds between amino acids.

Simultaneous Transcription and Translation

• In prokaryotes, translation can begin before transcription is complete.

Regulation of Gene Expression and Mutations

Operons

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

• Components: Promoter, operator, genes, and terminator.

• Types: Inducible operons (e.g., lac operon) are usually off but can be turned on; repressible operons (e.g., trp operon) are usually on but can be turned off.

Mutations

• Types:

  • Base Substitution: Replacement of one nucleotide with another.

  • Missense Mutation: Changes one amino acid in a protein.

  • Nonsense Mutation: Creates a premature stop codon.

  • Frameshift Mutation: Insertion or deletion of nucleotides that shifts the reading frame.

• Effects: Mutations can be beneficial, neutral, or harmful.

• Repair Mechanisms: Cells have mechanisms to repair DNA damage, such as excision repair and mismatch repair.

• Mutagens: Agents that increase the mutation rate (e.g., chemicals, radiation).

Microbial Growth

Requirements for Microbial Growth

• Temperature Range: Microbes are classified by their optimal temperature range:

  • Psychrophiles (cold-loving)

  • Mesophiles (moderate temperature)

  • Thermophiles (heat-loving)

  • Psychrotrophs (grow at low temperatures but prefer moderate)

  • Hyperthermophiles (extreme heat)

• pH Control: Microbial growth is affected by pH; culture media are buffered to maintain optimal pH.

• Chemical Requirements: Carbon, nitrogen, sulfur, phosphorus, trace elements, oxygen, and organic growth factors are essential for microbial growth.

• Biofilms: Communities of microorganisms attached to surfaces; biofilms can cause persistent infections and are resistant to antibiotics.

Culture Media and Growth Conditions

• Chemically Defined vs. Complex Media: Chemically defined media have known compositions; complex media contain extracts and digests of natural products.

• Anaerobic Techniques: Methods such as anaerobic jars and chambers are used to culture anaerobic bacteria.

• Biosafety Levels: Laboratory safety is categorized into four levels (BSL-1 to BSL-4) based on the risk of the organisms handled.

• Pure Cultures: Obtained by isolating single colonies using techniques like the streak plate method.

• Bacterial Growth: Bacteria reproduce by binary fission, leading to exponential population growth.

• Measuring Growth:

  • Direct methods: Plate counts, microscopic counts.

  • Indirect methods: Turbidity, metabolic activity.

Clinically Significant Bacteria

Pathology, Infection, and Disease

• Pathology: The study of disease, including its cause (etiology), development, and effects on the body.

• Etiology: The cause of a disease.

• Infection: The invasion and multiplication of pathogens in the body.

• Disease: An abnormal state in which the body is not functioning normally.

Normal Microbiota and Host Relationships

• Normal Microbiota: Microorganisms that colonize the body without causing disease.

• Transient Microbiota: Microbes present for a short time; may become opportunistic pathogens.

• Opportunistic Microorganisms: Cause disease when the host's defenses are compromised.

• Symbiosis: Includes commensalism, mutualism, and parasitism.

• Microbial Antagonism: Normal microbiota inhibit the growth of pathogens.

Koch's Postulates

• Set of criteria used to establish a causative relationship between a microbe and a disease.

• Exceptions exist, such as for viruses that cannot be cultured on artificial media.

Classifying Infectious Diseases

• Communicable Diseases: Can be spread from one host to another.

• Noncommunicable Diseases: Do not spread between hosts.

• Symptoms: Subjective changes felt by the patient.

• Signs: Objective changes observed by others.

• Syndrome: A group of symptoms and signs that characterize a disease.

• Herd Immunity: Resistance of a population to infection due to immunity of a high proportion of individuals.

• Extent of Host Involvement: Local, systemic, and focal infections.

• Primary vs. Secondary Infections: Primary infection is the initial illness; secondary infection is caused by an opportunistic pathogen after the primary infection.

Patterns and Spread of Disease

Predisposing Factors

• Factors that make an individual more susceptible to disease (e.g., age, genetics, lifestyle).

Development of Disease

• Stages: Incubation period, prodromal period, period of illness, period of decline, period of convalescence.

Reservoirs and Transmission

• Reservoirs: Sources of infection (human, animal, nonliving).

• Transmission: Direct contact, indirect contact (fomites), droplet, vehicle, vector.

Healthcare-Associated Infections (HAIs)

• Infections acquired in healthcare settings.

• Compromised hosts are at higher risk.

• Prevention includes hand hygiene, sterilization, and isolation procedures.

Summary Tables

Classification of Microbes by Temperature Range

Group	Temperature Range (°C)	Example
Psychrophiles	0–15	Pseudomonas fluorescens
Psychrotrophs	0–30	Listeria monocytogenes
Mesophiles	10–50	Escherichia coli
Thermophiles	40–70	Bacillus stearothermophilus
Hyperthermophiles	65–110	Pyrolobus fumarii

Biosafety Levels

Level	Description	Example Organisms
BSL-1	Minimal risk; basic precautions	Non-pathogenic E. coli
BSL-2	Moderate risk; lab coat, gloves	Staphylococcus aureus
BSL-3	High risk; biosafety cabinets, controlled access	Mycobacterium tuberculosis
BSL-4	Extreme risk; full-body suits, isolated facilities	Ebola virus

Types of Mutations

Type	Description	Effect
Base Substitution	One base replaced by another	May be silent, missense, or nonsense
Missense	Change in one amino acid	Protein function may change
Nonsense	Creates stop codon	Premature termination of protein
Frameshift	Insertion/deletion shifts reading frame	Usually nonfunctional protein

Additional info:

• Some context and examples were expanded for clarity and completeness.

• Tables were inferred and formatted for study purposes.