Microbial Genetics (Bauman Ch. 7)

Mutations and Their Types

Mutations are changes in the nucleotide sequence of DNA. They can affect gene function and lead to genetic diversity or disease.

• Substitution mutations: One base is replaced by another, potentially altering the amino acid sequence.

• Frameshift mutations: Insertions or deletions of bases shift the reading frame, often resulting in nonfunctional proteins.

• Silent mutations: Do not change the amino acid sequence due to redundancy in the genetic code.

• Missense mutations: Change one amino acid in the protein, possibly affecting function.

• Nonsense mutations: Convert a codon to a stop codon, truncating the protein.

Mutagens are agents that cause mutations. Examples include radiation (UV light, X-rays) and chemical mutagens (nitrous acid, base analogs).

Gene Transfer in Bacteria

Bacteria can exchange genetic material through several mechanisms, contributing to genetic diversity and adaptation.

• Transformation: Uptake of naked DNA from the environment. The Griffith experiment demonstrated transformation in Streptococcus pneumoniae.

• Conjugation: Direct transfer of DNA between bacteria via cell-to-cell contact, often mediated by plasmids (F plasmid).

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

F plasmids carry genes for conjugation. R plasmids carry antibiotic resistance genes.

Biotechnology (Bauman Chapter 8)

Genetic Engineering and Recombinant DNA Technology

Biotechnology uses living organisms or their systems to develop products. Genetic engineering involves manipulating DNA to create recombinant organisms.

• Restriction enzymes: Cut DNA at specific sequences.

• DNA ligase: Joins DNA fragments together.

• Vectors: DNA molecules (often plasmids) used to carry foreign genes into host cells.

• Recombinant DNA technology: Used to introduce new genes into bacteria, producing proteins or other products.

Applications include production of insulin, growth hormones, and genetically modified organisms (GMOs).

CRISPR-Cas System

The CRISPR-Cas system is a bacterial defense mechanism adapted for gene editing. It allows precise modification of DNA sequences in various organisms.

• CRISPR-Cas9: Most commonly used for genome editing.

• Applications include gene therapy, agriculture, and research.

Control of Microbial Growth in the Environment (Bauman Chapter 9)

Antiseptics, Disinfectants, and Sterilization

Controlling microbial growth is essential in healthcare, food safety, and research.

• Antiseptics: Chemicals used on living tissue to reduce infection risk.

• Disinfectants: Chemicals used on inanimate objects to destroy microbes.

• Sterilization: Complete destruction of all microbial life, including spores.

Factors Affecting Microbial Control

• Microbial load, environmental conditions, and exposure time affect the efficacy of control methods.

• Disk diffusion (Kirby-Bauer) assay: Measures the effectiveness of antimicrobial agents by the size of inhibition zones.

• Physical methods include heat (moist and dry), filtration, and radiation.

• Chemical methods include alcohols, aldehydes, halogens, heavy metals, oxidizing agents, phenolics, and surfactants.

Table: Classes of Germicidal Chemicals and Examples

Control of Microbial Growth in the Body (Bauman Chapter 10)

Antimicrobial Agents and Antibiotics

Antimicrobial agents are chemicals that kill or inhibit the growth of microorganisms. Antibiotics are a subset produced by microbes to inhibit others.

• Common sources: Soil bacteria (e.g., Streptomyces), fungi (Penicillium).

• Selective toxicity: Ability to target microbes without harming the host.

• Mechanisms of action:

  • Inhibition of cell wall synthesis (e.g., penicillins, vancomycin)

  • Inhibition of protein synthesis (e.g., aminoglycosides, tetracyclines, macrolides)

  • Disruption of cell membrane integrity (e.g., polymyxins, polyenes)

  • Inhibition of nucleic acid synthesis

  • Inhibition of metabolic pathways

Antibiotic Resistance

Resistance occurs when bacteria evolve mechanisms to survive exposure to antibiotics.

• Intrinsic resistance: Natural, due to inherent structural or functional traits.

• Acquired resistance: Due to mutation or gene transfer (e.g., plasmids).

• Mechanisms:

  • Drug inactivating enzymes (e.g., beta-lactamases)

  • Altered target sites

  • Efflux pumps

  • Reduced permeability

Table: Mechanisms of Antibiotic Resistance

Adverse Effects of Antimicrobial Drugs

• Allergic reactions: Immune response to drug components.

• Toxicity: Damage to host tissues or organs.

• Disruption of normal flora: Can lead to secondary infections (e.g., C. difficile).

Equations and Calculations

• Microbial death rate: , where is the number of surviving microbes at time , is the initial number, and is the death rate constant.

• Dilution factor calculation:

Additional info:

• Examples and applications have been expanded for clarity.

• Equations and tables have been inferred and formatted for academic completeness.