MCB 2010 Midterm Study Guide: Genetics, Microbial Growth, Metabolism, Epidemiology, and Host-Microbe Interactions

Chapter 5: Genetics

Structure and Function of DNA and RNA

Genetic information in microorganisms is stored and expressed through DNA and RNA. Understanding their structure and function is fundamental to microbiology.

• DNA (Deoxyribonucleic Acid): Composed of nucleotides, each containing a deoxyribose sugar, a phosphate group, and a nitrogenous base (adenine [A], thymine [T], cytosine [C], guanine [G]). DNA is double-stranded and antiparallel (one strand runs 5′ to 3′, the other 3′ to 5′), stabilized by hydrogen bonds between complementary bases (A-T, C-G).

• RNA (Ribonucleic Acid): Single-stranded, contains ribose sugar, and uracil (U) replaces thymine. Types include:

  • mRNA (Messenger RNA): Carries genetic code from DNA to ribosomes.

  • tRNA (Transfer RNA): Contains hairpin loops; brings amino acids to ribosomes during translation.

  • rRNA (Ribosomal RNA): Structural and catalytic component of ribosomes.

• Antiparallel Structure: Ensures accurate replication and transcription by aligning complementary bases in opposite directions.

Example: The sequence 5'-ATCG-3' pairs with 3'-TAGC-5'.

Genetic Processes: Replication, Transcription, and Translation

Genetic information flows from DNA to RNA to protein, a concept known as the central dogma of molecular biology.

• Replication: Semiconservative process where each new DNA molecule contains one original and one new strand. Key enzymes:

  • DNA Polymerase: Synthesizes new DNA in the 5′ to 3′ direction.

  • Helicase: Unwinds the DNA helix.

  • Ligase: Seals gaps between DNA fragments.

  • Single-Stranded Binding Proteins: Stabilize unwound DNA.

• Transcription: Synthesis of mRNA from DNA template, initiated at a promoter and terminated at a terminator sequence by RNA polymerase.

• Translation: Ribosomes read mRNA codons, tRNA brings amino acids, and proteins are synthesized. Begins at start codon (AUG) and ends at stop codon (UAA, UAG, UGA).

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

Mutations and Genetic Variation

Mutations are changes in the DNA sequence that can affect protein synthesis and phenotype.

• Types of Mutations:

  • Silent: No change in amino acid sequence.

  • Missense: Changes one amino acid.

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

  • Frameshift: Insertion or deletion shifts the reading frame.

• Spontaneous Mutations: Occur naturally (e.g., DNA polymerase errors).

• Induced Mutations: Caused by external factors (e.g., radiation, chemicals).

• Genotype vs. Phenotype: Genotype is the genetic makeup; phenotype is the observable traits.

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

Horizontal Gene Transfer in Bacteria

Bacteria can exchange genetic material, increasing genetic diversity.

• Transformation: Uptake of free DNA from the environment.

• Conjugation: Direct transfer of plasmids via a pilus.

• Transduction: Transfer of DNA by bacteriophages (viruses that infect bacteria).

Example: Streptococcus pneumoniae can acquire antibiotic resistance genes via transformation.

Chapter 7: Microbial Growth and Control

Microbial Growth Conditions and Preferences

Microbes require specific environmental conditions for growth, including temperature, oxygen, pH, and pressure.

• Temperature Preferences:

  • Psychrophiles: Grow best at 0–20°C.

  • Mesophiles: Prefer 20–45°C (human pathogens).

  • Thermophiles: Thrive at 45–80°C.

• Oxygen Requirements:

  • Obligate Aerobes: Require oxygen.

  • Facultative Anaerobes: Grow with or without oxygen.

  • Obligate Anaerobes: Cannot tolerate oxygen.

• Other Preferences:

  • Halophiles: Salt-loving.

  • Barophiles: Pressure-loving.

Control of Microbial Growth

Microbial control is essential in healthcare, laboratory, and food settings. Methods are classified as physical or chemical.

• Sterilization: Complete destruction of all microbes, including spores (e.g., autoclaving at 121°C, 15 psi, 15 min).

• Disinfection: Destroys most microbes on inanimate objects (e.g., bleach, alcohol).

• Antisepsis: Reduces microbes on living tissue (e.g., iodine, alcohol-based sanitizers).

• Sanitization: Reduces microbial populations to safe levels.

• Physical Methods: Heat (moist/dry), filtration (HEPA filters), radiation (UV, ionizing), desiccation.

• Chemical Methods: Halogens, alcohols, oxidizing agents (e.g., peroxygens), aldehydes, ethylene oxide gas.

• Microbial Resistance: Endospores are highly resistant; enveloped viruses are less resistant.

Microbial Growth Curve and Estimation

Bacterial populations grow in predictable phases, and several methods estimate cell numbers.

• Growth Curve Phases: Lag, log (exponential), stationary, death.

• Estimation Methods: Plate counts, turbidity, direct microscopic count.

Equation (Exponential Growth): $N = N_0 \times 2^n$ Where $N$ = final cell number, $N_0$ = initial cell number, $n$ = number of generations.

Comparison of Microbial Control Methods

Method	Purpose	Example
Sterilization	Destroy all microbes	Autoclave
Disinfection	Destroy most microbes (inanimate)	Bleach
Antisepsis	Reduce microbes (living tissue)	Iodine
Sanitization	Reduce to safe levels	Dishwashing

Chapter 8: Microbial Metabolism

Overview of Metabolism

Microbial metabolism includes all chemical reactions in a cell, divided into catabolism (energy-releasing) and anabolism (energy-consuming).

• Enzymes: Biological catalysts with optimal temperature and pH; may require cofactors (metal ions) or coenzymes (vitamins).

• Redox Reactions: Oxidation (loss of electrons), reduction (gain of electrons); central to energy production.

• ATP (Adenosine Triphosphate): Main energy currency; composed of adenine, ribose, and three phosphates.

Equation (ATP Hydrolysis): $ATP + H_2O \rightarrow ADP + P_i + \text{energy}$

Energy Pathways

• Glycolysis: Occurs in cytoplasm; glucose → 2 pyruvate + 2 ATP + 2 NADH.

• Krebs Cycle: Further oxidizes pyruvate; generates NADH, FADH2, ATP, CO2.

• Electron Transport Chain (ETC): Uses NADH/FADH2 to generate ATP via oxidative phosphorylation.

• Fermentation: Anaerobic; regenerates NAD+, produces less ATP.

• Aerobic Respiration: Oxygen is final electron acceptor.

• Anaerobic Respiration: Other inorganic molecules (e.g., nitrate) are final electron acceptors.

Equation (Aerobic Respiration): $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy (ATP)}$

Enzyme Regulation and Biochemical Tests

• Feedback Inhibition: End product inhibits enzyme activity.

• Biochemical Tests: Used to identify microbes (e.g., catalase test, fermentation tests).

Chapter 9: Epidemiology and Infectious Disease

Principles of Epidemiology

Epidemiology studies the distribution and determinants of diseases in populations.

• Incidence: Number of new cases in a specific time period.

• Prevalence: Total number of cases at a given time.

• Morbidity Rate: Proportion of individuals affected.

• Mortality Rate: Proportion of deaths due to disease.

Stages and Transmission of Infectious Disease

• Stages: Incubation, prodromal, acute, decline, convalescent.

• Modes of Transmission: Direct, indirect, droplet, airborne, vehicle (e.g., food, water), vector (e.g., insects).

• Reservoirs: Living (humans, animals) or non-living (soil, water) sources of infection.

• Types of Infections: Communicable, noncommunicable, zoonotic, latent, opportunistic.

Healthcare-Associated Infections (HAIs) and Control

• HAIs: Infections acquired in healthcare settings; often caused by bacteria.

• Aseptic Techniques: Prevent contamination and infection.

• Epidemiological Triangle: Host, agent, environment.

Chapter 10: Host-Microbe Interactions

Pathogenesis and Virulence Factors

Pathogens must overcome host defenses to cause disease, using various virulence factors.

• Adhesins: Allow attachment to host tissues without puncturing cells.

• Toxins: Exotoxins (secreted proteins) and endotoxins (part of Gram-negative cell wall).

• Immune Evasion: Capsules, antigenic variation, mimicry of host molecules.

• Portals of Entry/Exit: Skin, mucous membranes, parenteral route.

• Infectious Dose (ID50): Number of organisms required to cause infection in 50% of hosts.

• Virulence: Degree of pathogenicity; measured by LD50 (lethal dose for 50% of hosts).

Infection Control and Biosafety

• Biosafety Levels (BSL): BSL-1 (minimal risk) to BSL-4 (high risk, dangerous pathogens).

• Standard Precautions: Basic infection control practices in healthcare.

• Transmission-Based Precautions: Additional measures for specific pathogens.

Summary Table: Virulence Factors and Their Functions

Virulence Factor	Function	Example
Capsule	Evades phagocytosis	Streptococcus pneumoniae
Adhesins	Attachment to host cells	Pili in E. coli
Exotoxins	Damage host tissues	Diphtheria toxin
Endotoxins	Trigger inflammation	Lipopolysaccharide (LPS)

Additional info: These notes expand on the review sheet by providing definitions, examples, and context for each topic, ensuring a comprehensive understanding suitable for exam preparation.