Microbial Genetics

Nucleic Acids: DNA vs RNA

Nucleic acids are essential biomolecules that store and transmit genetic information in all living organisms. The two main types are DNA (deoxyribonucleic acid) and RNA (ribonucleic acid).

• DNA: Double-stranded, contains deoxyribose sugar, bases are adenine (A), thymine (T), cytosine (C), and guanine (G).

• RNA: Single-stranded, contains ribose sugar, bases are adenine (A), uracil (U), cytosine (C), and guanine (G).

• Function: DNA stores genetic information; RNA is involved in protein synthesis and gene regulation.

Complement Base Pairing Rules: In DNA, A pairs with T and C pairs with G. In RNA, A pairs with U and C pairs with G.

• DNA: A-T, C-G

• RNA: A-U, C-G

Example: If a DNA strand has the sequence 5'-ATCG-3', its complementary strand is 3'-TAGC-5'.

Genomes

The genome is the complete set of genetic material in an organism. It includes both chromosomal and extrachromosomal DNA (such as plasmids in bacteria).

• Genotype: The genetic makeup of an organism.

• Phenotype: The observable characteristics resulting from the genotype.

• Extrachromosomal DNA: DNA found outside the chromosome, often carrying genes for antibiotic resistance or virulence.

The Central Dogma

The Central Dogma of molecular biology describes the flow of genetic information: DNA is transcribed into RNA, which is translated into protein.

• Transcription: DNA → RNA

• Translation: RNA → Protein

Equation:

DNA Replication

DNA replication is the process by which a cell copies its DNA before cell division. It is semi-conservative, meaning each new DNA molecule contains one old and one new strand.

• Key enzymes: DNA polymerase, helicase, primase, ligase

• Direction: DNA is synthesized in the 5' to 3' direction.

Equation:

Gene Expression: Transcription and Translation

Gene expression involves two main steps: transcription and translation.

• Transcription: Synthesis of RNA from a DNA template. Occurs in the nucleus (eukaryotes) or cytoplasm (prokaryotes).

• Translation: Synthesis of protein from an mRNA template. Occurs at the ribosome.

Example: The gene for beta-galactosidase is transcribed into mRNA, which is then translated into the enzyme.

Genetic Mutation

Mutations are changes in the DNA sequence. They can affect genotype and phenotype.

• Types of mutations:

  • Point mutation: Change in a single nucleotide.

  • Insertion: Addition of one or more nucleotides.

  • Deletion: Removal of one or more nucleotides.

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

  • Silent mutation: No change in protein sequence.

  • Missense mutation: Change in one amino acid.

  • Nonsense mutation: Creates a stop codon.

• Repercussions: Mutations can lead to loss of function, gain of function, or no effect.

Horizontal Gene Transfer in Bacteria

Bacteria can acquire new genetic material through horizontal gene transfer, increasing genetic diversity.

• Conjugation: Transfer of DNA via direct cell-to-cell contact, often through a pilus.

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

• Transformation: Uptake of free DNA from the environment.

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

Controlling Microbial Growth

Bio-Safety Levels

Bio-safety levels (BSL) are standards for laboratory safety when handling microorganisms.

• BSL-1: Minimal risk; basic precautions.

• BSL-2: Moderate risk; additional precautions.

• BSL-3: High risk; controlled access, special ventilation.

• BSL-4: Extreme risk; maximum containment.

Methods of Control

Microbial growth can be controlled by physical and chemical methods to prevent infection and contamination.

• Physical methods: Heating, refrigeration, freezing, high-pressure treatment, desiccation, lyophilization, irradiation, filtration.

• Chemical methods: Use of disinfectants, antiseptics, and sterilants.

Microbial Death

Microbial death refers to the irreversible loss of the ability to reproduce.

• Thermal Death Time (TDT): Minimum time required to kill all microbes at a given temperature.

• Thermal Death Point (TDP): Lowest temperature required to kill all microbes in a sample in 10 minutes.

Equation:

Physical Methods for Microbial Control

• Heating: Denatures proteins; includes moist heat (autoclaving, boiling) and dry heat (incineration).

• Refrigeration/Freezing: Slows or stops microbial growth.

• High-pressure treatment: Disrupts cell membranes.

• Desiccation: Removes water, inhibiting metabolism.

• Lyophilization: Freeze-drying for long-term preservation.

• Irradiation: Damages DNA (UV, gamma rays).

• Filtration: Removes microbes from liquids or air.

Chemical Agents of Microbial Control

• Disinfectants: Used on surfaces to kill microbes.

• Antiseptics: Used on living tissue.

• Sterilants: Kill all forms of microbial life.

• Mode of action: Disrupt cell membranes, denature proteins, or damage nucleic acids.

• Advantages/Disadvantages: Vary by agent; some are toxic, others are less effective against spores.

Antimicrobial Drugs

History/Discovery

The discovery of antimicrobial drugs revolutionized medicine. Paul Ehrlich developed the first chemotherapeutic agent, and Alexander Fleming discovered penicillin.

• Antibiotic: Naturally produced by microorganisms.

• Semisynthetic: Modified natural antibiotics.

• Synthetic: Completely artificial compounds.

Fundamentals

• Broad Spectrum: Effective against a wide range of microbes.

• Narrow Spectrum: Effective against specific microbes.

• Superinfection: Secondary infection due to disruption of normal flora.

• Antibiotic Resistance: Ability of microbes to withstand drug effects.

• Dosage/Route of Administration: Determines effectiveness and side effects.

Antibacterial Drugs: Modes of Action

Antibacterial drugs target specific bacterial processes.

• Cell wall biosynthesis: e.g., penicillins inhibit peptidoglycan synthesis.

• Protein synthesis: e.g., tetracyclines bind to ribosomes.

• Membrane function: e.g., polymyxins disrupt membranes.

• Nucleic acid synthesis: e.g., quinolones inhibit DNA gyrase.

• Metabolic pathways: e.g., sulfonamides inhibit folic acid synthesis.

Other Antimicrobial Drugs

• Antifungals: Target ergosterol in fungal membranes.

• Antiprotozoal: Target protozoan metabolism.

• Antihelminth: Target helminth physiology.

• Antivirals: Inhibit viral replication.

Comparison Table: Types of Antimicrobial Drugs

Additional info: Table entries inferred from standard microbiology knowledge.