Introduction to Microbiology

Definition and Scope

• Microbiology is the study of organisms too small to be seen with the naked eye, known as microbes.

• Major groups of microbes include bacteria, viruses, algae, fungi, protozoa, and helminths (parasitic worms).

• Microbes are named using a two-part Latin-based system: Genus species (e.g., Escherichia coli).

• Classification is based on cellular characteristics and rRNA sequence analysis.

Historical Perspectives

• Debate between spontaneous generation (life arises from non-living matter) and biogenesis (life arises from pre-existing life).

• Key discoveries: Microscope, cell theory, germ theory of disease, immunology, vaccination, aseptic technique, and DNA structure.

Branches of Microbiology

• Bacteriology: Study of bacteria

• Virology: Study of viruses

• Mycology: Study of fungi

• Parasitology: Study of parasites

• Immunology: Study of the immune system

Microbes and Humans

• Most microbes are non-pathogenic and beneficial, forming the normal microbiota and contributing to food, medicine, and chemical production.

Observing Microorganisms Through a Microscope

Microscopy Basics

• Common units: nanometer (nm), micrometer (μm), millimeter (mm); 1,000 nm = 1 μm = 0.001 mm.

• Stains are used to increase contrast for visualization under light microscopes.

Types of Microscopes

• Light Microscope: Uses visible light; typical magnification up to ~1,000x.

• Dark Field Microscope: Provides a black background for observing live, unstained specimens.

• Fluorescence Microscope: Uses fluorescent antibodies to tag specific structures.

• Electron Microscope: Uses electrons and heavy metals; achieves magnification up to millions-fold.

Staining Techniques

• Gram Stain: Differentiates bacteria into Gram-positive and Gram-negative.

• Endospore Stain: Identifies bacterial endospores.

• Capsule Stain: Visualizes microbial capsules.

• Acid-Fast Stain: Detects Mycobacterium and Nocardia.

• Flagella Stain: Reveals bacterial flagella.

Functional Anatomy of Prokaryotic and Eukaryotic Cells

Prokaryotic Cells

• Single, circular chromosome; no nucleus or membrane-bound organelles.

• Key structures:

  • Pilus: Conjugation (DNA transfer via plasmids)

  • Fimbriae: Attachment to surfaces

  • Flagella: Motility

  • Cell Wall: Protection and shape

• Gram-positive: Thick peptidoglycan wall with teichoic acids.

• Gram-negative: Thin peptidoglycan wall, outer membrane with lipopolysaccharide (LPS).

Plasma Membrane

• Composed of a lipid bilayer; semi-permeable barrier.

• Transport mechanisms:

  • Passive transport: Down concentration gradient (no energy required).

  • Active transport: Against concentration gradient (requires ATP).

  • Transport proteins facilitate movement of molecules.

Bacterial Endospores

• Produced by Bacillus and Clostridium during harsh conditions (sporulation).

• Dormant, highly resistant structures; can germinate into active cells.

Eukaryotic Cells

• Multiple, linear chromosomes within a nucleus; contain membrane-bound organelles.

• Key organelles:

  • Nucleus: Contains DNA

  • Rough ER: Protein synthesis

  • Smooth ER: Steroid synthesis

  • Golgi Apparatus: Modifies and sorts molecules

  • Mitochondria: ATP production

  • Lysosome: Digestive enzymes

  • Peroxisome: Detoxification

• Endosymbiotic Theory: Eukaryotic organelles (mitochondria, chloroplasts) originated from engulfed prokaryotes.

Microbial Metabolism

Overview of Metabolism

• Metabolism: All chemical reactions in a cell, including catabolism (breakdown) and anabolism (synthesis).

Enzymes and Inhibition

• Enzymes: Biological catalysts that speed up reactions by lowering activation energy.

• Competitive inhibitors: Resemble substrate and compete for the active site.

• Non-competitive inhibitors: Bind to allosteric site, altering enzyme shape and function.

ATP Generation Mechanisms

• Substrate-level phosphorylation: Direct transfer of phosphate to ADP.

• Oxidative phosphorylation: Electron transport chain and chemiosmosis.

• Photophosphorylation: Light-driven ATP synthesis (photosynthesis).

Major Metabolic Pathways

• Glycolysis: Glucose (6C) → 2 pyruvate (3C each), producing ATP and NADH.

• Transition Step: Pyruvate → Acetyl-CoA (2C), generating NADH and CO2.

• Krebs Cycle: Acetyl-CoA + oxaloacetate → citric acid; cycle produces NADH, FADH2, ATP, and CO2.

• Electron Transport Chain (ETC): NADH and FADH2 donate electrons, generating a proton gradient and ATP via ATP synthase.

  • Equation for aerobic respiration:

• Fermentation: NADH reduces pyruvate to end products (e.g., alcohol, lactic acid), regenerating NAD+ for glycolysis.

• Photosynthesis: Light energy generates ATP and NADPH; Calvin-Benson cycle synthesizes organic molecules.

Catabolism of Other Molecules

• Proteins: Broken into amino acids, deaminated, and enter glycolysis or Krebs cycle.

• Lipids: Glycerol enters glycolysis; fatty acids undergo beta-oxidation to acetyl-CoA.

Microbial Classification by Energy and Carbon Source

Microbial Growth

Physical and Chemical Requirements

• Physical: Temperature, osmotic pressure, pH.

• Chemical: Nutrient availability.

Temperature Classification

Biofilms

• Microbial communities encased in a self-produced matrix; protective and often resistant to treatments.

Culture Media

• Chemically defined media: Exact composition known.

• Complex media: Contains extracts; composition varies.

• Differential media: Distinguishes microbes by color change or reaction.

• Selectivemedia: Favors growth of certain microbes.

Pure Culture Techniques

• Streak plate and pour plate methods isolate single colonies.

Bacterial Growth and Measurement

• Bacteria divide by binary fission; generation time is typically 20–30 minutes.

• Direct measurement: Plate count, microscopic count, filtration, Most Probable Number (MPN).

• Indirect measurement: Turbidity, dry weight, metabolic activity, waste production.

The Control of Microbial Growth

Physical and Chemical Control Methods

• Physical: Heat, osmotic pressure, pH adjustments.

• Chemical: Disinfectants, antiseptics.

Measuring Effectiveness

• Disk-diffusion test: Assesses antimicrobial activity by measuring zones of inhibition.

• Use-dilution test: Determines effectiveness of disinfectants against microbes.

Microbial Resistance

• Structures such as capsules and endospores increase resistance to control methods.