Comprehensive Review Notes for Microbiology Final Exam

Bacteria: Shapes and Classification

Major Bacterial Shapes

Bacteria are classified by their morphology, which is essential for identification and understanding pathogenicity.

• Bacillus: Rod-shaped bacteria.

• Coccus: Spherical or circular bacteria.

• Spirillum: Spiral-shaped bacteria.

The Germ Theory of Disease

Koch's Postulates and Disease Causation

The Germ Theory of Disease established that specific microbes cause specific diseases, a foundational concept in microbiology.

• Robert Koch proved that Bacillus anthracis causes anthrax.

• Koch's postulates are experimental steps to link a microbe to a disease:

  • Microbe found in diseased organisms.

  • Microbe isolated and grown in pure culture.

  • Microbe causes disease when introduced to a healthy organism.

  • Same microbe re-isolated from the newly diseased host.

Biofilms

Formation and Significance

Biofilms are communities of microbes attached to surfaces, impacting health and industry.

• Grow on rocks, pipes, teeth, and medical implants.

• Protect mucous membranes from harmful microbes.

• Can cause infections on catheters and implants.

• Form barriers against antibiotics, complicating treatment.

Gram Stain Technique

Differentiation of Bacterial Cell Walls

The Gram stain is a differential staining method used to classify bacteria based on cell wall properties.

	Color of Gram-Positive Cells	Color of Gram-Negative Cells
Primary Stain: Crystal Violet	Purple	Purple
Mordant: Iodine	Purple	Purple
Decolorizing Agent: Alcohol-Acetone	Purple	Colorless
Counterstain: Safranin	Purple	Red

Microbial Identification Methods

Techniques for Identifying Microorganisms

Accurate identification is crucial for diagnosis and treatment.

• Macroscopic and microscopic appearance.

• Biochemical tests (e.g., pH indicator color change).

• Genetic characteristics (DNA analysis).

• Immunological testing (antibody reactions).

Staining Techniques

Types of Stains and Their Purposes

Staining enhances visualization and differentiation of microbial structures.

• Simple stains: One dye; reveals shape, size, arrangement.

• Differential stains: Use primary and counterstain to distinguish cell types.

  • Gram stain: Differentiates Gram-positive and Gram-negative.

  • Acid-fast stain: Identifies cells with waxy cell walls (e.g., Mycobacterium).

  • Endospore stain: Distinguishes endospores from vegetative cells.

• Structural stains: Reveal capsules, flagella, etc.

Prokaryotes vs. Eukaryotes

Cellular Differences

Understanding cell structure is fundamental to microbiology.

• Prokaryotes:

  • One circular chromosome, no nuclear membrane.

  • No histones or organelles.

  • Cell wall: peptidoglycan (bacteria), pseudomurein (archaea).

  • Binary fission for cell division.

• Eukaryotes:

  • Multiple paired chromosomes in nucleus.

  • Histones and organelles present.

  • Polysaccharide cell walls (if present).

  • Mitotic spindle for cell division.

Glycocalyx

Structure and Function

The glycocalyx is a protective, sticky layer outside the cell wall.

• Composed of polysaccharides and/or polypeptides.

• Allows attachment to surfaces (teeth, intestines, implants).

• Prevents desiccation and phagocytosis (increases virulence).

• Forms: Capsule (organized, attached) and slime layer (unorganized, loose).

Gram-Positive vs. Gram-Negative Cell Walls

Structural Differences

Cell wall composition affects staining, pathogenicity, and antibiotic susceptibility.

• Gram-Positive:

  • Thick peptidoglycan layer.

  • Contains teichoic acids.

• Gram-Negative:

  • Thin peptidoglycan layer bonded to lipoproteins.

  • Outer membrane with lipopolysaccharides, lipoproteins, phospholipids.

  • Periplasmic space between outer membrane and plasma membrane.

Plasmids

Role in Bacterial Genetics

Plasmids are small, circular DNA molecules that confer advantages to bacteria.

• Contain 5-100 genes; replicate independently.

• Not essential for survival but provide antibiotic resistance, toxin production, etc.

• Can be transferred via pilus (conjugation).

Endospores

Survival Structures

Endospores enable bacteria to withstand harsh conditions.

• Resistant to desiccation, heat, chemicals.

• Produced by Bacillus and Clostridium.

• Sporulation: Formation of endospore.

• Germination: Return to vegetative state when conditions improve.

The Plasma Membrane and Transport

Mechanisms of Molecular Movement

The plasma membrane controls the movement of substances in and out of the cell.

• Simple diffusion: Movement from high to low concentration.

• Facilitative diffusion: Movement via membrane proteins.

• Osmosis: Diffusion of water across a membrane.

• Active transport: Movement against concentration gradient using energy.

• Endocytosis (in eukaryotes):

  • Phagocytosis: Engulfing particles.

  • Pinocytosis: Engulfing fluids.

  • Receptor-mediated: Specific uptake via receptors.

Organelles and Their Functions

Key Eukaryotic Structures

Organelles perform specialized functions within eukaryotic cells.

• Nucleus: Contains DNA, site of RNA synthesis.

• Endoplasmic reticulum (ER): Transport network.

• Golgi complex: Membrane formation and secretion.

• Lysosome: Contains digestive enzymes.

• Vacuole: Stores food and provides support.

• Mitochondrion: Site of cellular respiration, ATP production.

• Chloroplast: Site of photosynthesis (in plants/algae).

• Peroxisome: Oxidation of fatty acids, destroys H2O2.

• Centrosome: Contains centrioles, organizes microtubules.

Enzymes and Metabolism

Role of Enzymes in Biochemical Reactions

Enzymes are biological catalysts essential for metabolic processes.

• Lower activation energy, increasing reaction rates.

• Not consumed in reactions; reusable.

• Bind substrates at active sites to facilitate reactions.

General reaction:

$\text{Substrates (Reactants)} \xrightarrow{\text{Enzyme}} \text{Products}$

Cellular Respiration

Anaerobic and Aerobic Pathways

Cells generate energy through the breakdown of glucose via different metabolic pathways.

• Glycolysis: Splits glucose into two pyruvate molecules; occurs in cytosol; anaerobic; net gain of 2 ATP.

• Aerobic respiration: Requires oxygen; includes citric acid cycle (Krebs/TCA) and electron transport chain (ETC); produces up to 36 ATP, CO2, and H2O.

• Anaerobic respiration: Uses inorganic ions (nitrate, sulfate) as final electron acceptors; less ATP produced.

• Fermentation: Incomplete oxidation; organic compounds as electron acceptors; produces alcohol or lactic acid; yields small ATP.

Photosynthesis equation:

$6\text{CO}_2 + 6\text{H}_2\text{O} \xrightarrow{\text{light}} \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2$

Growth Factors and Nutritional Requirements

Essential Organic Nutrients

Growth factors are organic compounds required for growth but not synthesized by the organism.

• Examples: Essential amino acids, vitamins.

• Distinction: Growth factors (organic only) vs. essential nutrients (organic and inorganic).

Temperature Adaptation Groups

Microbial Growth and Temperature

Microorganisms are classified by their optimal growth temperatures.

• Psychrophiles: Optimum below 15°C; can grow at 0°C; rarely pathogenic.

• Mesophiles: Optimum 20–40°C; most human pathogens.

• Thermophiles: Optimum above 45°C; some grow above 60°C.

Heterotrophs and Energy Sources

Types of Chemoheterotrophic Microbes

Heterotrophs obtain energy by consuming organic compounds.

• Saprobes: Feed on dead organic matter; some are opportunistic pathogens or facultative parasites.

• Parasites: Derive nutrients from living hosts; all are pathogens, some are obligate.

Oxygen Requirements

Microbial Classification by Oxygen Use

• Aerobe: Utilizes and detoxifies oxygen.

• Obligate aerobe: Requires oxygen.

• Facultative anaerobe: Uses oxygen if available, can grow without it.

• Microaerophilic: Requires low oxygen levels.

Effects of pH

Microbial Growth and pH

• Most microbes grow at pH 6–8.

• Acidophiles: Grow at low pH.

• Alkalinophiles: Grow at high pH.

Population Growth Curve

Phases of Microbial Growth

Microbial populations exhibit predictable growth patterns in laboratory cultures.

• Lag phase: Adjustment, little growth.

• Exponential (log) phase: Rapid growth.

• Stationary phase: Growth rate equals death rate.

• Death phase: Decline due to depleted resources.

Culturing Microbes

Techniques and Media

Microbes are cultured using specific techniques and media for observation and study.

• Inoculation: Introduction of sample into media.

• Medium: Nutrient source for growth.

• Culture: Accumulation of microbes in/on medium.

• Isolation techniques: Streak plate, pour plate, spread plate; allow growth of colonies from single cells.

• Colony: Mound of cells from one species.