Microbiology Exam 1 Review: History, Cell Structure, Microscopy, 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 reduce puerperal fever in hospitals.

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

• Robert Koch: Developed techniques for isolating bacteria; formulated Koch’s Postulates to link microbes to diseases.

• Hans Christian Gram: Developed the Gram stain for bacterial classification.

• Carl Woese: Proposed the three-domain system based on ribosomal RNA.

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

• John Needham: Supported spontaneous generation with flawed experiments.

• Joseph Lister: Introduced antiseptic surgery.

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

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

• Carl Linnaeus: Developed binomial nomenclature for classification.

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

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

Koch’s Postulates

• Set of criteria to establish a causative relationship between a microbe and a disease:

  1. The microorganism must be found in all organisms suffering from the disease, but not in healthy organisms.

  2. The microorganism must be isolated and grown in pure culture.

  3. The cultured microorganism should cause disease when introduced into a healthy organism.

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

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

Pasteur’s Experiments

• Fermentation: Demonstrated that yeast ferments grape juice to alcohol; bacteria produce acids.

• 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-substantiated 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 or eukaryotic based on structural differences.

• Prokaryotes: Include Bacteria and Archaea; lack a nucleus and membrane-bound organelles.

• Eukaryotes: Include Fungi, Algae, Protozoa, and Helminthes; have a nucleus and organelles.

• Similarities: Both have cell membranes, cytoplasm, and genetic material.

• Differences: Eukaryotes are generally larger, have complex internal structures.

Bacterial Cell Walls

• Gram Positive: Thick peptidoglycan layer; stains purple.

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

• Function: Provides structural support and protection.

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

Surface Structures

• Glycocalyx: External layer; capsule (organized) or slime sheath (loose).

• Flagellum: Used for motility; structure includes filament, hook, and basal body.

• Fimbriae: Short, numerous; used for attachment.

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

Endosymbiotic Theory

• Explains origin of mitochondria and chloroplasts in eukaryotes as formerly free-living prokaryotes.

Biofilms

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

• Provide protection and enhanced survival.

Characterizing and Classifying Prokaryotes

Bacterial Morphologies

• Coccus: Spherical

• Bacillus: Rod-shaped

• Coccobacillus: Short rod

• Spirochete: Flexible spiral

• Vibrio: Curved rod

• Spirillum: Rigid spiral

Bacterial Endospores

• Resistant structures formed by Bacillus and Clostridium species.

• Function: Survival under harsh conditions.

• Formation: Sporulation process.

Bacterial Reproduction

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

• Snapping Division: Variation of binary fission.

• Budding: New cell grows from parent.

Bacterial Arrangements

• Determined by plane of division and whether cells remain attached.

• Cocci: Single, diplo (pairs), strepto (chains), staphylo (clusters), tetrads, sarcinae.

• Bacilli: Single, diplo, strepto, palisades.

Bergey’s Manual

• Reference for bacterial classification and identification.

• Contains descriptions, taxonomy, and identification keys.

• Taxonomy based on morphology, physiology, genetics, and molecular data.

Microscopy, Staining, and Classification

Measurements of Microbes

• Bacteria: Measured in micrometers (μm).

• Viruses: Measured in nanometers (nm).

• Eukaryotes are generally larger than prokaryotes.

Microscope Terminology

• Magnification: Enlargement of image.

• Refraction: Bending of light as it passes through media.

• Contrast: Difference in intensity between object and background.

• Numerical Aperture: Measure of lens ability to gather light.

• Wavelength: Distance between peaks of light waves; shorter wavelengths yield higher resolution.

• Blue Filters: Used to decrease wavelength and increase resolution.

• Ocular Lenses: Eyepiece lenses; further magnify image.

Types of Microscopes

Different microscopes are used for specific purposes in microbiology.

Microscope Type	Function	Image Type
Bright-field	General observation of stained specimens	Colored, flat
Dark-field	Observation of live, unstained specimens	Bright objects on dark background
Phase-contrast	Enhances contrast in transparent specimens	Detailed internal structures
Fluorescent	Uses fluorescent dyes; detects specific structures	Glowing structures on dark background
Scanning Electron (SEM)	Surface details of specimens	3D, high-resolution
Transmission Electron (TEM)	Internal structures	2D, high-resolution
Atomic Force	Surface topography at atomic level	3D surface map

Bright-field Microscope

• Uses visible light; requires staining for contrast.

• Parts: Ocular lens, objective lens, stage, condenser, diaphragm, light source.

• Oil immersion increases resolution by reducing refraction.

Staining Techniques

• Simple Stains: Use basic dyes; color all cells.

• Negative Stains: Use acidic dyes; stain background.

• Differential Stains: Distinguish cell types; examples: Gram stain, acid-fast stain.

Gram Stain

• Distinguishes Gram-positive (purple) from Gram-negative (pink) bacteria.

• Steps: Crystal violet, iodine, alcohol decolorization, safranin counterstain.

Acid-fast Bacteria

• Resist decolorization due to mycolic acid in cell wall.

• Detected with acid-fast stain (e.g., Ziehl-Neelsen).

Bacterial Endospores (Microscopy)

• Detected with endospore stain (e.g., malachite green).

• Appear as green structures within pink cells.

Dichotomous Key

• Tool for identification based on sequential choices.

Binomial Nomenclature

• Genus and species names; genus capitalized, species lowercase, both italicized (e.g., Escherichia coli).

Phylogenetic Groupings

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

Characterizing and Classifying Eukaryotes

General Characteristics of Fungi and Algae

• Fungi: Eukaryotic, non-photosynthetic, cell walls of chitin, reproduce by spores.

• Algae: Eukaryotic, photosynthetic, cell walls of cellulose, aquatic habitats.

Helminthes and Protozoa

• Helminthes: Parasitic worms; include nematodes (roundworms), cestodes (tapeworms), trematodes (flukes).

• Protozoa: Unicellular, eukaryotic, diverse morphologies, often motile.

Diseases Caused by Fungal and Protozoan Pathogens

• Examples: Histoplasma (fungal), Plasmodium (protozoan, malaria).

Characterizing and Classifying Viruses, Viroids, and Prions

General Characteristics of Viruses

• Non-living infectious agents; require host cells for replication.

• Structure: Nucleic acid (DNA or RNA), protein capsid, sometimes envelope.

• Classification: Based on genome type, capsid shape, presence of envelope.

Viral Genomes

• Types: dsDNA, ssDNA, dsRNA, +ssRNA, -ssRNA.

• +ssRNA: Can be directly translated by host.

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

Animal Virus Cycles

• Attachment, entry (fusion or endocytosis), uncoating, replication, assembly, exit (lysis or budding).

Bacteriophage Cycles

• Lytic Cycle: Virus replicates and lyses host cell.

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

Viral Envelope

• Derived from host cell membranes; contains viral proteins.

• Function: Aids in attachment and evasion of immune response.

Viral Latency

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

Viruses and Cancer

• Some viruses disrupt cell cycle control, leading to oncogenesis (e.g., HPV).

Viroids

• Small, circular RNA molecules; infect plants.

Prions

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

• Normal PrP: Non-infectious, cellular form.

• Prion PrP: Misfolded, infectious form.

Comparison of Infectious Agents

Agent	Cellular Structure	Genome	Replication	Diseases
Prion	None (protein only)	None	Induces misfolding	Neurodegenerative
Virus	Non-cellular	DNA or RNA	Host cell machinery	Varied (e.g., influenza)
Bacterium	Prokaryotic	DNA	Binary fission	Varied (e.g., tuberculosis)
Fungus	Eukaryotic	DNA	Sexual/asexual spores	Varied (e.g., candidiasis)
Protozoan	Eukaryotic	DNA	Asexual/sexual	Varied (e.g., malaria)
Helminth	Eukaryotic	DNA	Complex life cycles	Varied (e.g., schistosomiasis)

Additional info: Academic context was added to expand brief review points into full explanations, and tables were constructed to clarify comparisons and microscope functions.