Chapter 1: Introduction to Microbiology

Pathogens & Disease Causation

Microbiology studies microorganisms, including those that cause disease (pathogens). Understanding how pathogens interact with hosts is fundamental to disease prevention and treatment.

• Pathogen: A microorganism that causes disease.

• Disease occurs when:

  • The pathogen enters the host

  • Multiplies

  • Damages tissues or produces toxins

• Not all microbes cause disease; many are harmless or beneficial.

Example: Streptococcus pyogenes causes strep throat, while Lactobacillus species aid digestion.

Naming Organisms (Genus & Species)

Organisms are named using binomial nomenclature, which provides a universal system for identification.

• Genus: General category (capitalized, italicized)

• Species: Specific organism (lowercase, italicized)

• Genus is broader; species is specific.

Example: Escherichia coli (Genus: Escherichia, Species: coli)

What Is Fermentation?

Fermentation is a metabolic process used by certain microorganisms to generate energy in the absence of oxygen.

• Occurs without oxygen

• Uses organic molecules as the final electron acceptor

• Produces products like lactic acid or alcohol

Example: Yeast fermentation produces ethanol in brewing.

What Is Biogenesis?

Biogenesis is the principle that life arises from pre-existing life, disproving spontaneous generation.

• Supported by experiments (e.g., Pasteur's swan-neck flask)

Occurrence of Infectious Disease

The spread and emergence of infectious diseases are influenced by multiple factors.

• Human behavior

• Population density

• Global travel

• Climate change

• Antibiotic resistance

• Diseases can be emerging or re-emerging

Normal Microbiota (Normal Flora)

Normal microbiota are microorganisms that reside in and on humans, providing health benefits.

• Prevent pathogen colonization

• Aid digestion

• Produce vitamins (e.g., vitamin K)

• Train the immune system

Environmental Roles of Microorganisms

Microorganisms play essential roles in ecosystems.

• Decomposers

• Nutrient cycling (carbon, nitrogen, sulfur)

• Oxygen production (photosynthetic microbes)

• Bioremediation (removing pollutants)

Chapter 3: Microscopy

Microscopic Images & Oil Immersion

Microscopy is vital for observing microorganisms. Oil immersion increases resolution for high-magnification objectives.

• Oil immersion uses 100x objective

• Oil matches the refractive index of glass, reducing light loss

• Commonly used for bacteria

Simple Staining

Simple staining uses a single dye to visualize cell shape, size, and arrangement.

• Does not differentiate cell types

Length Conversions (nm → km)

Metric conversions are essential for understanding microbial sizes.

• 1 km = m

• 1 m = μm

• 1 μm = nm

Bacterial Smears

Bacterial smears are prepared to fix bacteria to slides for staining.

• Procedure:

  1. Place bacteria on slide

  2. Air dry

  3. Heat fix

• Purpose: Kills bacteria, attaches cells to slide, preserves shape

Total Magnification

Total magnification is calculated by multiplying the eyepiece and objective lens powers.

• Example: 10x (eyepiece) × 40x (objective) = 400x

Gram Staining – Stages & Appearance

Gram staining differentiates bacteria based on cell wall structure.

1. Crystal violet – stains all cells purple

2. Iodine – mordant (locks dye in)

3. Alcohol – decolorizer

4. Safranin – counterstain

• Gram-positive: purple

• Gram-negative: pink/red

Path of Light Through Microscope

Light passes through several components to form an image.

• Light source → condenser → specimen → objective lens → eyepiece → eye

Endospore Staining

Endospore staining identifies bacterial endospores.

• Uses heat + malachite green

• Endospores = green, vegetative cells = red/pink

Chapter 4: Prokaryotic Cells

DNA in Prokaryotes

Prokaryotes have unique genetic organization.

• Single circular chromosome

• Located in the nucleoid

• No nuclear membrane

• May have plasmids

Prokaryotes vs Eukaryotes

Key differences exist between prokaryotic and eukaryotic cells.

Cell Wall–Targeting Antibiotics

Some antibiotics target bacterial cell walls, exploiting differences from human cells.

• Target peptidoglycan

• Humans do not have cell walls → selective toxicity

• Examples:

  • Penicillins

  • Cephalosporins

Lysozyme & Cell Walls

Lysozyme is an enzyme that breaks bonds in peptidoglycan, especially effective against Gram-positive bacteria.

• Found in tears, saliva

Salt (NaCl) & Osmosis

Salt can pull water out of cells, causing plasmolysis and is used in food preservation.

Tonicity

Tonicity describes the movement of water across cell membranes.

• Isotonic: equal water movement

• Hypotonic: water enters cell – swelling

• Hypertonic: water leaves cell – shrinking

Organisms with Cell Walls

• Bacteria

• Plants

• Fungi

• Algae

• Note: Animal cells do NOT have cell walls

ATP & Active Transport

Active transport moves substances against their concentration gradient using energy (ATP).

• Uses membrane proteins

Passive Transport

Passive transport moves substances down their concentration gradient without energy input.

• Includes diffusion & facilitated diffusion

Chromatophores

Internal membranes in some bacteria serve as sites of photosynthesis.

Role of Endospores

Endospores are survival structures formed by some bacteria, resistant to heat, radiation, and chemicals. They are not for reproduction.

Organelles & Prokaryotes

Mitochondria and chloroplasts resemble bacteria and cyanobacteria, supporting the endosymbiotic theory.

Run and Tumble & Taxis

Bacterial movement involves runs (straight movement) and tumbles (random direction change). Taxis is movement toward or away from stimuli (e.g., chemicals, phototaxis).

Gas Diffusion Across Membranes

O2 and CO2 cross membranes by simple diffusion.

Gram Staining & Cell Wall (BIG EXAM TOPIC)

Gram-Positive

• Thick peptidoglycan

• No outer membrane

• Retains crystal violet → purple

Gram-Negative

• Thin peptidoglycan

• Outer membrane with LPS

• Loses crystal violet → pink

Summary Table:

Additional info: Gram staining is a key diagnostic tool in microbiology, guiding antibiotic selection and identification of bacterial species.