MBIO 3400 Study Guide: Foundations of Microbiology (L2–L5)

A Brief History of Microbiology

Microbial Groups

Microbiology studies a diverse array of microscopic organisms, each with unique structural and functional characteristics.

• Bacteria: Unicellular prokaryotes, typically possessing peptidoglycan in their cell walls.

• Archaea: Unicellular prokaryotes with cell walls lacking peptidoglycan; often found in extreme environments.

• Fungi: Eukaryotic organisms; molds are multicellular filaments, while yeasts are unicellular.

• Protozoa: Single-celled eukaryotes, many of which move via pseudopods, cilia, or flagella.

• Algae: Photosynthetic organisms; can be unicellular or multicellular.

• Parasites: Includes multicellular organisms such as parasitic worms (helminths) studied in microbiology.

• Viruses: Acellular entities that infect cellular hosts; important in disease and molecular biology.

Spontaneous Generation: Experimental Logic

The concept of spontaneous generation was challenged through a series of experiments:

• Redi: Demonstrated that maggots do not arise from meat protected from flies, refuting spontaneous generation in larger organisms.

• Needham: Observed microbial growth after heating nutrient broth, interpreted as support for spontaneous generation.

• Spallanzani: Showed that more thorough heating and sealing prevented microbial growth, suggesting contamination as the source.

• Pasteur: Used swan-neck flasks to allow air in but prevent dust; broth remained sterile unless exposed to contaminants, disproving spontaneous generation.

High-yield connection: Pasteur's design allowed air (and any 'life force') in, but prevented contamination, directly addressing critics of earlier experiments.

Fermentation, Disease, and Prevention: Key Scientists

Several scientists made foundational contributions to microbiology:

Scientist	Contribution
Leeuwenhoek	First microscopic observations of microorganisms
Linnaeus	Developed taxonomic naming and classification
Pasteur	Fermentation, pasteurization, germ theory, swan-neck flask experiments
Buchner	Cell-free fermentation; enzymes drive chemical reactions
Koch	Etiology, bacterial culture methods, Koch's postulates
Semmelweis	Handwashing to prevent disease
Lister	Antiseptic technique in surgery
Nightingale	Modern nursing practices
Snow	Infection control, epidemiology
Jenner	Vaccination and immunology
Ehrlich	Chemotherapy ('magic bullets')
Fleming	Discovery of penicillin
Domagk	Development of sulfa drugs
Woese & Fox	Three-domain classification (Bacteria, Archaea, Eukarya)

Koch's Postulates

Koch's postulates are a set of criteria to establish a causative relationship between a microbe and a disease:

1. The suspected agent is found in every case of the disease and absent from healthy hosts.

2. The agent is isolated and grown in pure culture outside the host.

3. The cultured agent causes the disease when introduced into a healthy, susceptible host.

4. The same agent is re-isolated from the experimentally infected host.

The Chemistry of Microbiology

Chemical Reactions in Cells

Cells carry out various chemical reactions essential for life, classified as follows:

Type	Description	Energy	Example
Synthesis	Combining smaller components into larger molecules	Requires energy (anabolism)	Dehydration synthesis
Decomposition	Breaking down large molecules into smaller components	Releases energy (catabolism)	Hydrolysis
Exchange	Atoms are moved between molecules; bonds broken and formed	Both endothermic and exothermic steps	Phosphorylation reactions

Metabolism is the sum of all chemical reactions in an organism, encompassing both anabolism and catabolism.

Properties of Water

• Most abundant substance in living organisms

• Excellent solvent for polar and ionic substances

• Remains liquid over a wide temperature range

• High heat capacity (absorbs heat with minimal temperature change)

• Participates directly in many chemical reactions (e.g., hydrolysis, dehydration synthesis)

Macromolecules

Cells are built from four major classes of macromolecules:

Macromolecule	Key Features
Lipids	Hydrophobic; includes fats, phospholipids (membrane structure), waxes, steroids
Carbohydrates	Monosaccharides, disaccharides, polysaccharides; energy, nucleic acid backbone, cell wall, cell interactions
Proteins	Polymers of amino acids joined by peptide bonds; functions: structure, catalysis (enzymes), regulation, transport, defense, offense
Nucleic acids	DNA and RNA; built from nucleotides; DNA stores genetic information, RNA involved in protein synthesis

Nucleotides and Nucleic Acids

• Nucleotide: Composed of a phosphate group, a pentose sugar, and a nitrogenous base.

• Nucleoside: A nucleotide lacking the phosphate group.

• Base pairing: C–G pairs form three hydrogen bonds; A–T (DNA) and A–U (RNA) pairs form two hydrogen bonds.

• DNA: Usually double-stranded, with complementary and antiparallel strands.

Cell Structure and Function

Characteristics of Life

All living organisms share several fundamental characteristics:

• Growth: Increase in size

• Reproduction: Production of new organisms

• Responsiveness: Ability to respond to environmental stimuli

• Metabolism: Chemical processes for energy and building cellular components

• Note: Movement is not required for life

Prokaryotic vs. Eukaryotic Cells

Feature	Prokaryotes	Eukaryotes
Nucleus	Absent	Present
Membrane-bound organelles	Absent	Present
Typical size	~1 µm or smaller	~10–100 µm
Groups	Bacteria, Archaea	Algae, protozoa, fungi, animals, plants
DNA/protein timing	Transcription and translation can occur simultaneously	Separated by nuclear compartment

Bacterial External Structures

Structure	Main Function / Distinction
Capsule	Organized glycocalyx, firmly attached; helps evade host immune system
Slime layer	Loosely attached, sticky glycocalyx; aids in surface attachment
Flagellum	Motility; composed of filament, hook, and basal body; rotates
Fimbriae	Attachment to surfaces or cells; important in biofilm formation
Pilus	Specialized fimbria for DNA transfer during conjugation

• Run, tumble, and taxis: Bacterial movement involves runs (straight movement), tumbles (random changes in direction), and taxis (movement toward/away from stimuli).

• Flagellar arrangements: Monotrichous (single flagellum), amphitrichous (flagella at both ends), lophotrichous (tuft at one end), peritrichous (flagella all over).

Bacterial Cell Walls

Feature	Gram-positive	Gram-negative
Peptidoglycan	Thick layer	Thin layer
Unique components	Teichoic and lipoteichoic acids	Outer membrane with phospholipids, proteins, LPS
Gram-stain result	Purple	Pink
Clinical connection	Cell wall is antimicrobial target	Lipid A (LPS) can cause fever, inflammation, shock

• Mycoplasma: Lacks a cell wall but retains other prokaryotic features (e.g., ribosomes).

• Antimicrobial drugs: Drugs targeting peptidoglycan are ineffective against wall-less bacteria.

Cytoplasmic Membrane and Transport

• Phospholipid bilayer: Described by the fluid mosaic model; selectively permeable.

• Transport: Proteins facilitate movement of substances across the membrane.

• Gradients: Maintains concentration and electrical gradients; involved in energy production.

• Passive transport: No direct energy input (e.g., diffusion, osmosis).

• Active transport: Requires cellular energy (e.g., ATP).

• Group translocation: Substance is chemically modified during transport.

Environment	Net Water Movement
Isotonic	No net movement
Hypertonic	Water leaves the cell
Hypotonic	Water enters the cell

Cytoplasm, Endospores, and Organelles

• Nucleoid: Region containing bacterial DNA.

• Ribosomes: Sites of protein synthesis; prokaryotic ribosomes are 70S.

• Inclusions: Reserve deposits of chemicals.

• Endospores: Survival structures formed under adverse conditions; highly resistant to heat, radiation, and chemicals.

• Archaea: Lack peptidoglycan; share genetic similarities with eukaryotes.

Eukaryotic Cell Structures

Structure	Function
Nucleus	Contains most DNA; surrounded by nuclear envelope with pores
Rough ER	Protein synthesis and transport; studded with ribosomes
Smooth ER	Lipid synthesis and detoxification; lacks ribosomes
Golgi body	Processes and packages molecules for export
Lysosome	Contains digestive enzymes
Peroxisome	Breaks down toxic substances
Mitochondrion	ATP production; contains 70S ribosomes and circular DNA
Chloroplast	Photosynthesis in plants/algae; contains DNA and 70S ribosomes
Cytoskeleton	Cell shape, movement, and structural support
Cilia	Movement of cells or substances along surfaces

Endosymbiotic theory: Mitochondria and chloroplasts likely evolved from free-living prokaryotes, as evidenced by their own DNA, 70S ribosomes, and double membranes.

Microscopy, Staining, and Classification

Microscopy Fundamentals

• Magnification: The degree to which an image is enlarged compared to the actual specimen.

• Resolution: The ability to distinguish two points as separate entities.

• Contrast: The difference in intensity between an object and its background.

Compound microscope total magnification:

$\text{Total magnification} = \text{ocular magnification} \times \text{objective magnification}$

• Oil immersion: Increases resolution by reducing light refraction.

• Higher magnification does not guarantee better resolution.

Types of Microscopes

Microscope	Best Use
Bright-field	General viewing; stained specimens
Dark-field	Viewing pale/colorless objects against a dark background
Phase	Observing living, unstained specimens
Fluorescence	Detecting fluorescent dyes, proteins, or pathogens
Confocal	3D imaging of fluorescently labeled specimens
TEM	Detailed internal ultrastructure
SEM	Surface structure imaging
Probe microscopy	Extremely high magnification (atomic level)

Staining Techniques

Stain Type	Purpose / Examples
Simple stain	Single dye; reveals size, shape, arrangement
Differential stain	Multiple dyes; distinguishes cell types/structures (e.g., Gram, acid-fast, endospore stains)
Special stain	Highlights specific structures (e.g., capsule, flagella, fluorescent stains)

• Chromophore: The colored part of a dye molecule.

• Basic dyes: Positively charged; bind well to negatively charged cell surfaces.

• Gram stain: Gram-positive cells remain purple; Gram-negative cells become pink after counterstaining.

Classification and Identification

• Taxonomy: The science of classification, nomenclature, and identification of organisms.

• Linnaeus: Developed binomial nomenclature (genus and species names).

• Woese: Used rRNA sequences to define three domains: Bacteria, Archaea, Eukarya.

• Modern taxonomy: Emphasizes evolutionary relationships.

Identification Method	What It Evaluates
Physical characteristics	Cell/colony morphology
Biochemical tests	Ability to use/produce specific chemicals
Serological tests	Antigen–antibody reactions
Phage typing	Susceptibility to specific bacteriophages
Nucleic-acid analysis	Genetic sequence comparison
Dichotomous key	Stepwise identification using paired statements

High-Yield Connections Across L2–L5

• Gram stain & cell wall: Structural differences in cell walls explain Gram stain results.

• Phospholipids & membranes: Chemical properties of phospholipids underlie membrane structure and function.

• Peptidoglycan & antimicrobials: Cell wall synthesis is a target for antibiotics like penicillin.

• Proteins & enzymes: Proteins function as enzymes, as shown in Buchner's experiments.

• Structure & identification: Features like cell wall, capsule, and morphology aid in microbial identification.

• Microscopy & technique choice: The choice of microscopy depends on the specimen and required information.

Example Self-Check Questions

• Why did Pasteur's swan-neck flask design provide stronger evidence against spontaneous generation than simply boiling broth?

• How does hydrolysis differ from dehydration synthesis?

• How is a nucleotide different from a nucleoside?

• Why do amino-acid side groups matter for proteins?

• Which bacterial structure is most directly associated with attachment? Movement? DNA transfer?

• Why do Gram-positive and Gram-negative cells produce different staining outcomes?

• Why is Lipid A clinically important?

• What happens to water when a bacterial cell enters a hypertonic environment?

• Why is an endospore considered a survival structure rather than a reproductive structure?

• What evidence supports the endosymbiotic origin of mitochondria?

• When would phase microscopy be preferable to bright-field microscopy?

• Why does immersion oil improve a microscope image?

• When would TEM be more useful than SEM?

• How does a biochemical identification test differ from a serological test?

• How does a dichotomous key guide identification?