Microbial World and You

Introduction to Microbiology

Microbiology is the study of microorganisms, which are organisms too small to be seen with the naked eye. This field encompasses the study of bacteria, archaea, fungi, protozoa, algae, viruses, and animal parasites, as well as their roles in health, disease, and the environment.

• Normal microbiota: Microorganisms that reside on or within the human body without causing disease under normal conditions.

• Transient microbiota: Microbes that are present temporarily; they may be removed by the host's defenses or by hygiene.

• Pathogenic species: Microorganisms capable of causing disease.

Naming and Classifying Microorganisms

• Nomenclature: The scientific naming of organisms uses a two-part system: Genus and specific epithet (species).

• Types of Microorganisms:

  • Bacteria: Prokaryotic, unicellular organisms with peptidoglycan cell walls.

  • Archaea: Prokaryotic, often found in extreme environments; cell walls lack peptidoglycan.

  • Fungi: Eukaryotic organisms, including yeasts and molds; cell walls contain chitin.

  • Protozoa: Unicellular eukaryotes, often motile.

  • Algae: Photosynthetic eukaryotes, important for oxygen production.

  • Viruses: Acellular entities, require a host cell to replicate.

  • Animal parasites: Multicellular eukaryotes, such as helminths.

Historical Foundations

• Pasteur: Developed the theory of biogenesis, fermentation, and pasteurization.

• Koch: Discovered the cause of anthrax; formulated Koch’s postulates for linking microbes to disease.

• Jenner: Developed the first vaccine (smallpox).

Microbes and Human Disease

• Biofilms: Complex communities of microorganisms adhering to surfaces; can contribute to disease and antibiotic resistance.

• Infectious diseases: Disorders caused by pathogenic microorganisms.

• Emerging Infectious Diseases (EIDs): New or changing diseases, e.g., COVID-19, Influenza A viruses.

Chemical Principles in Microbiology

Organic Compounds

Organic compounds are molecules containing carbon and hydrogen, essential for life. Major classes include proteins, nucleic acids, and ATP.

• Proteins:

  • Building blocks: Amino acids.

  • Formation: Amino acids are linked by peptide bonds to form polypeptide chains.

  • Levels of structure:

    1. Primary: Sequence of amino acids.

    2. Secondary: Alpha helices and beta sheets.

    3. Tertiary: 3D folding of a single polypeptide.

    4. Quaternary: Association of multiple polypeptides.

• Nucleic Acids:

  • Two types: DNA and RNA.

  • Building blocks: Nucleotides (composed of a sugar, phosphate group, and nitrogenous base).

  • Differences: DNA contains deoxyribose and is double-stranded; RNA contains ribose and is single-stranded.

  • Master molecule: DNA is the genetic material of the cell.

• ATP (Adenosine Triphosphate):

  • Structure: Adenine, ribose, and three phosphate groups.

  • Function: Main energy currency of the cell.

Observing Microorganisms Through a Microscope

Light Microscopy

Light microscopy is essential for visualizing microorganisms. It involves the use of lenses and visible light to magnify specimens.

• Basic parts: Ocular lens, objective lens, stage, condenser, light source, and focusing knobs.

Staining Techniques

• Staining: Coloring microorganisms with dyes to increase contrast.

• Smear: Thin film of a specimen on a slide.

• Chromophore: The colored ion in a dye.

• Acidic dye: Negatively charged; stains background (negative staining).

• Basic dye: Positively charged; stains cells.

• Negative staining: Stains the background, leaving cells colorless.

Types of Stains

• Simple stain: Uses a single dye; may require a mordant to enhance binding.

• Differential stains:

  • Gram stain: Differentiates bacteria into Gram-positive and Gram-negative based on cell wall structure.

    • Steps: Crystal violet, iodine (mordant), alcohol (decolorizer), safranin (counterstain).

  • Acid-fast stain: Identifies Mycobacterium species with waxy cell walls.

• Special stains:

  • Negative staining: For capsules.

  • Endospore staining: Highlights bacterial endospores.

  • Flagellar staining: Visualizes bacterial flagella.

Functional Anatomy of Prokaryotic and Eukaryotic Cells

Shapes and Arrangements of Bacterial Cells

• Cocci: Spherical bacteria; may form chains (streptococci), clusters (staphylococci), or pairs (diplococci).

• Bacilli: Rod-shaped bacteria; may be single, in chains, or palisades.

• Spirilla: Spiral-shaped bacteria.

Structures External to the Cell Wall

• Glycocalyx (capsule): Gelatinous outer layer; protects against phagocytosis and aids in attachment.

• Flagella: Long, whip-like structures for motility.

• Axial filament (endoflagella): Found in spirochetes; enables corkscrew movement.

• Fimbriae and Pili: Hair-like appendages for attachment (fimbriae) and DNA transfer (pili).

Cell Wall Composition

• Main molecule: Peptidoglycan in bacteria.

• Other molecules: Teichoic acids (Gram-positive), lipopolysaccharides (Gram-negative).

Gram-Positive vs. Gram-Negative Cell Walls

Atypical Cell Walls

• Some bacteria (e.g., Mycoplasma) lack cell walls; others (e.g., Mycobacterium) have waxy cell walls.

Effect of Lysozymes and Antibiotics

• Lysozyme: Enzyme that breaks down peptidoglycan.

• Antibiotics (e.g., penicillin): Inhibit cell wall synthesis.

Structures Internal to the Cell Wall

• Plasma membrane: Phospholipid bilayer; regulates transport of molecules.

• Cytoplasm: Gel-like substance inside the cell.

• Nucleoid: Region containing the bacterial chromosome (DNA).

• Ribosomes: Sites of protein synthesis.

Endospores

• Definition: Highly resistant, dormant structures formed by some bacteria.

• Sporogenesis: Process of endospore formation.

• Clinical significance: Endospores are resistant to heat, desiccation, and disinfectants.

Prokaryotic vs. Eukaryotic Cells

Microbial Metabolism

Definition and Overview

Metabolism is the sum of all chemical reactions within a cell, including catabolism (breakdown of molecules) and anabolism (synthesis of molecules).

Enzymes in Metabolism

• Enzymes: Biological catalysts that speed up chemical reactions without being consumed.

• Structure: Most are proteins; may require cofactors (inorganic ions) or coenzymes (organic molecules).

• Cofactor vs. Coenzyme:

  • Cofactor: Inorganic ion (e.g., Mg2+, Fe2+).

  • Coenzyme: Organic molecule (e.g., NAD+, FAD).

• Factors influencing enzyme activity: Temperature, pH, substrate concentration, inhibitors.

Oxidation-Reduction (Redox) Reactions

• Oxidation: Loss of electrons.

• Reduction: Gain of electrons.

• Redox reactions: Essential for energy production in cells.

Phosphorylation

• Definition: Addition of a phosphate group to a molecule.

• Types:

  • Substrate-level phosphorylation

  • Oxidative phosphorylation

  • Photophosphorylation

Glucose Metabolism

• Glycolysis: Breakdown of glucose to pyruvate; produces ATP and NADH.

• Krebs cycle: Oxidizes acetyl-CoA to CO2; generates NADH, FADH2, and ATP.

• Electron Transport Chain (ETC): Uses electrons from NADH and FADH2 to produce ATP via oxidative phosphorylation.

• Role of redox reactions: Transfer of electrons releases energy for ATP synthesis.

• Role of coenzymes: NAD+ and FAD act as electron carriers.

• ATP yield from one glucose:

  • Total: Up to 38 ATP (prokaryotes), 36 ATP (eukaryotes).

Energy Production Methods

Microbial Growth

Physical Factors Affecting Growth

• Temperature: Microbes have optimal, minimum, and maximum growth temperatures.

• pH: Most bacteria grow best near neutral pH (6.5–7.5).

• Osmotic pressure: High salt or sugar concentrations can inhibit growth (plasmolysis).

Chemical Factors Affecting Growth

• Oxygen: Required by some microbes, toxic to others.

• Toxic forms of oxygen: Superoxide radicals, hydrogen peroxide, and hydroxyl radicals can damage cells.

Growth for Diagnostic Purposes

• Microbial growth patterns can help identify species in clinical diagnostics.

Growth Phases and Binary Fission

• Binary fission: Main method of prokaryotic cell division.

• Growth phases:

  1. Lag phase: Adaptation, no division.

  2. Log phase: Rapid cell division.

  3. Stationary phase: Nutrient depletion slows growth.

  4. Death phase: Cells die off.