Microbiology Exam 1 Review: History, Cell Structure, Growth, Classification, and Infectious Agents

History of Microbiology

Contributions of Key Scientists

The development of microbiology was shaped by numerous scientists whose discoveries laid the foundation for the field.

• Antonie van Leeuwenhoek: First to observe and describe microorganisms using a simple microscope.

• Ignaz Semmelweis: Introduced handwashing to prevent puerperal fever in hospitals.

• John Snow: Father of epidemiology; traced cholera outbreak to contaminated water.

• Robert Koch: Developed Koch's postulates; identified causative agents of tuberculosis and anthrax.

• Hans Christian Gram: Developed Gram staining technique to differentiate bacterial cell walls.

• Carl Woese: Classified life into three domains based on rRNA sequences (Bacteria, Archaea, Eukarya).

• Louis Pasteur: Disproved spontaneous generation; developed pasteurization; studied fermentation.

• John Needham: Supported spontaneous generation with flawed experiments.

• Joseph Lister: Introduced antiseptic surgery using carbolic acid.

• Florence Nightingale: Applied statistical analysis to improve hospital sanitation.

• Edward Jenner: Developed the first vaccine (smallpox).

• Carl Linnaeus: Developed binomial nomenclature and classification system.

• Lazzaro Spallanzani: Disproved spontaneous generation with sealed flask experiments.

• Francesco Redi: Disproved spontaneous generation with meat and maggot experiments.

Koch's Contributions and Postulates

• Koch's Postulates: Criteria to establish a causal relationship between a microbe and a disease:

  1. The microorganism must be found in all cases of the disease.

  2. It must be isolated and grown in pure culture.

  3. The cultured microbe must cause disease when introduced into a healthy host.

  4. The same microbe must be re-isolated from the experimentally infected host.

• Application: Used to identify pathogens responsible for infectious diseases.

Pasteur's Experiments

• Fermentation: Demonstrated that yeast ferments grape juice to alcohol; bacteria cause spoilage.

• Spontaneous Generation: Used swan-neck flasks to show that microbes do not arise spontaneously.

• Contributions: Developed vaccines, pasteurization, and disproved spontaneous generation.

Scientific Method

• Hypothesis: Testable explanation for an observation.

• Theory: Well-supported explanation based on evidence.

• Control Group: Group in an experiment that does not receive the treatment; used for comparison.

Cell Structure and Function

Prokaryotic vs. Eukaryotic Cells

Cells are classified as prokaryotic (Bacteria and Archaea) or eukaryotic (Eukarya), each with distinct structures and functions.

• Prokaryotic Cells: Lack membrane-bound organelles; DNA in nucleoid; cell wall present; smaller size.

• Eukaryotic Cells: Have membrane-bound organelles (nucleus, mitochondria, etc.); larger size; cell wall present in some (plants, fungi).

• Similarities: Both have plasma membranes, ribosomes, and cytoplasm.

• Differences: Presence of nucleus, organelles, cell wall composition.

Bacterial Cell Wall

• Gram Positive: Thick peptidoglycan layer; stains purple; contains teichoic acids.

• Gram Negative: Thin peptidoglycan; outer membrane with lipopolysaccharide (LPS); stains pink.

• Function: Provides structural support, protection, and shape.

• Lipid A: Component of LPS; acts as endotoxin in Gram-negative bacteria.

Other Cell Structures

• Glycocalyx: Sticky layer outside cell wall; capsule (organized) or slime layer (loose); aids in protection and attachment.

• Flagellum: Used for motility; composed of filament, hook, and basal body.

• Fimbriae: Short, hair-like structures for attachment.

• Pili: Longer than fimbriae; used for conjugation (DNA transfer).

Endosymbiotic Theory

• Explains origin of mitochondria and chloroplasts as formerly free-living prokaryotes engulfed by ancestral eukaryotes.

Biofilms

• Communities of microbes attached to surfaces and embedded in extracellular matrix.

• Clinically important due to increased resistance to antibiotics and immune responses.

• Quorum Sensing: Cell-to-cell communication regulating gene expression in response to population density.

Binomial Nomenclature

• Genus and species names; Escherichia coli.

Phylogenetic Groupings

• Hierarchy: Domain > Kingdom > Phylum > Class > Order > Family > Genus > Species.

Microbial Nutrition and Growth

Oxygen Requirements and Energy Metabolism

• Obligate Aerobe: Requires oxygen; aerobic respiration.

• Obligate Anaerobe: Cannot tolerate oxygen; anaerobic respiration or fermentation.

• Facultative Anaerobe: Can grow with or without oxygen; prefers aerobic respiration.

• Aerotolerant Anaerobe: Tolerates oxygen; uses fermentation.

• Microaerophile: Requires low oxygen concentration.

• FTM (Fluid Thioglycollate Medium): Used to determine oxygen requirements.

Toxic Forms of Oxygen and Protective Enzymes

• Singlet Oxygen: Protected by carotenoids.

• Superoxide Radical (O2-): Protected by superoxide dismutase.

• Peroxide Anion (O22-): Protected by catalase and peroxidase.

• Hydroxyl Radical (OH·): No specific enzyme; highly reactive.

Binary Fission vs. Mitosis

• Binary Fission: Asexual reproduction in prokaryotes; cell divides into two identical cells.

• Mitosis: Eukaryotic cell division; involves mitotic spindle and multiple steps.

• Result: Binary fission produces genetically identical cells.

Phases of Microbial Growth

• Lag Phase: Cells adapt to environment; no division.

• Log (Exponential) Phase: Rapid cell division; population doubles at regular intervals.

• Stationary Phase: Nutrient depletion; growth rate equals death rate.

• Death Phase: Cells die faster than they divide.

Bacterial Population Growth Calculation

• Population doubles each generation: $N = N_0 \times 2^n$

• N: Final cell number; N0: Initial cell number; n: Number of generations.

Nutritional Requirements

• Microbes require carbon, nitrogen, phosphorus, sulfur, trace elements, and energy sources.

• Used for biosynthesis, energy production, and cellular maintenance.

Trophic Classification of Microbes

• Phototrophs: Use light as energy source.

• Chemotrophs: Use chemicals as energy source.

• Autotrophs: Use CO2 as carbon source.

• Heterotrophs: Use organic compounds as carbon source.

Environmental Effects on Growth

• Temperature: Human pathogens grow best at 37°C.

• pH: Most pathogens prefer neutral pH (6.5–7.5).

• Osmotic Pressure: Isotonic or slightly hypotonic environments are optimal.

Culturing Bacteria

• Defined Media: Exact chemical composition known.

• Complex Media: Contains extracts; composition varies.

• Differential Media: Distinguishes between organisms based on biochemical reactions.

• Selective Media: Favors growth of specific microbes.

• Pure Culture: Isolated population of one species; obtained by streak plate or pour plate methods.

• CFU (Colony Forming Unit): Estimate of viable cells.

• Axenic Culture: Pure culture without contaminants.

• Aseptic Technique: Procedures to prevent contamination.

Cell Count Principles

• CFU: Number of colonies on plate reflects viable cells.

• Turbidity Measurement: Optical density measured by spectrophotometer; correlates with cell concentration.

• Viable Count: Number of living cells determined by plating.

Characterizing and Classifying Prokaryotes

Bacterial Morphologies and Arrangements

• Coccus: Spherical.

• Bacillus: Rod-shaped.

• Coccobacillus: Short rod.

• Spirochete: Spiral, flexible.

• Vibrio: Curved rod.

• Spirillum: Spiral, rigid.

• Arrangements: Single, diplo (pairs), strepto (chains), staphylo (clusters).

• Plane of Division: Determines arrangement (e.g., cocci dividing in one plane form chains).

Bacterial Endospores

• Formation: Produced by Bacillus and Clostridium; survival structure under harsh conditions.

• Function: Resistant to heat, desiccation, chemicals.

Methods of Bacterial Reproduction

• Binary Fission: Most common; cell divides into two.

• Snapping Division: Variation of binary fission.

• Budding: New cell grows from parent.

Bergey's Manual of Systematic Bacteriology

• Purpose: Reference for bacterial classification and identification.

• Information: Morphology, physiology, genetics, taxonomy.

Taxonomy of Prokaryotes

• Based on morphology, biochemical tests, genetic analysis, and phylogenetic relationships.

General Characteristics of Fungi, Algae, Protozoa, and Helminths

Fungi

• Characteristics: Eukaryotic, cell wall of chitin, heterotrophic, reproduce by spores.

• Pathogenic Fungi: Cause diseases like candidiasis, aspergillosis.

Algae

• Characteristics: Eukaryotic, photosynthetic, cell wall of cellulose, aquatic.

Protozoa

• Characteristics: Unicellular, eukaryotic, lack cell wall, motile.

• Categories: Based on movement: flagellates, ciliates, amoebae, sporozoans.

• Pathogenic Protozoa: Cause diseases like malaria, giardiasis.

Helminths

• Characteristics: Multicellular, eukaryotic, parasitic worms (nematodes, cestodes, trematodes).

Opportunistic Pathogen

• Microbe that causes disease only when host defenses are compromised.

Characterizing and Classifying Viruses, Viroids, and Prions

General Characteristics of Viruses

• Non-living, acellular, obligate intracellular parasites.

• Composed of nucleic acid (DNA or RNA) and protein coat (capsid).

• Some have envelopes derived from host membranes.

Viral Classification

• Based on genome type (DNA, RNA; single or double stranded), capsid shape, presence of envelope.

Types of Viral Genomes

• +ssRNA: Can be directly translated by host ribosomes.

• -ssRNA: Must be converted to +ssRNA before translation.

Animal Virus Life Cycles

• Attachment: Virus binds to host cell.

• Entry: Virus enters cell by fusion or endocytosis.

• Replication: Viral genome replicated.

• Assembly: New virions assembled.

• Exit: Virions released by lysis or budding.

Bacteriophage Life Cycles

• Lytic Cycle: Virus replicates and lyses host cell.

• Lysogenic Cycle: Viral genome integrates into host DNA; can later enter lytic cycle.

Viral Envelope

• Made of lipid bilayer from host cell membrane; contains viral proteins.

• Functions in attachment and protection.

Viral Latency

• Virus remains dormant in host cell; can reactivate later.

Viruses and Cancer

• Some viruses disrupt cell cycle control, leading to uncontrolled cell division (oncogenesis).

Viroids

• Small, circular RNA molecules; infect plants.

Prions

• Infectious proteins; cause neurodegenerative diseases (e.g., Creutzfeldt-Jakob disease).

• Normal PrP: Non-infectious, alpha-helical structure.

• Prion PrP: Infectious, beta-sheet structure; induces misfolding of normal PrP.

Comparison of Infectious Agents

Agent	Cellular Structure	Genome	Diseases
Prion	Protein only	None	Spongiform encephalopathies
Virus	Acellular	DNA or RNA	Influenza, HIV, etc.
Bacterium	Prokaryotic	DNA	Tuberculosis, strep throat
Fungus	Eukaryotic	DNA	Candidiasis, ringworm
Protozoan	Eukaryotic	DNA	Malaria, giardiasis
Helminth	Eukaryotic	DNA	Ascariasis, schistosomiasis

Additional info: Table entries inferred for clarity and completeness.