Fundamental Concepts in Microbiology: Cell Structure, Classification, and Pathogenicity

Study Guide - Smart Notes

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Microscopy and Measurement in Microbiology

Electron Microscope vs Light Microscope

Microscopes are essential tools in microbiology for visualizing organisms too small to be seen with the naked eye. The type of microscope used depends on the size of the specimen.

Electron Microscope: Used for smaller organisms (1 nm–1 μm). Provides higher resolution and magnification.
Light Microscope: Used for larger specimens (1 μm–1 mm). Suitable for bacteria and some eukaryotic cells.

Key Fact: Objects larger than 100 μm can be seen without a microscope.

Cell Types: Prokaryotes vs Eukaryotes

Prokaryotic Cells

Prokaryotes are unicellular organisms lacking a nucleus and membrane-bound organelles.

One circular chromosome, not enclosed in a membrane
Plasmids (extrachromosomal DNA)
No histones
No membrane-bound organelles
70S ribosomes
Bacteria: peptidoglycan cell wall
Archaea: pseudomurein cell wall
Divide by binary fission

Eukaryotic Cells

Eukaryotes are more complex cells with internal compartmentalization.

Paired chromosomes within a nuclear membrane
No plasmids
Histones present
Membrane-bound organelles (e.g., mitochondria, endoplasmic reticulum)
80S ribosomes (site of protein synthesis)

Bacteria: Structure, Classification, and Morphology

Bacterial Cell Structure

Bacteria are single-celled prokaryotes with diverse shapes and cell wall structures.

Cell membrane, cytoplasm, chromosomal DNA (nucleoid region)
Peptidoglycan cell wall (Gram-positive: thick; Gram-negative: thin with extra membrane)
Divide by binary fission

Bacterial Shapes and Arrangements

Bacteria exhibit a variety of shapes and arrangements, which can be important for identification and pathogenicity.

Coccus: Spherical
Rod (Bacillus): Cylindrical
Spirillum: Spiral-shaped
Spirochete: Flexible spiral-shaped
Stalk: Stalk-shaped
Hypha and Stalk: Budding and appendaged bacteria
Filamentous: Thread-like

Bacterial Arrangements

Arrangement	Description
Diplococci	Pairs of cocci
Streptococci	Chains of cocci
Tetrad	Four cocci in a square
Sarcinae	Eight cocci in a cube
Staphylococci	Clusters resembling grapes

Colony Morphology

Physical characteristics of bacterial colonies are used for identification.

Shape
Margin
Elevation
Size
Texture
Appearance
Pigmentation
Optical property

Archaea

Characteristics of Archaea

Archaea are prokaryotes distinct from bacteria, often found in extreme environments.

Cell walls made from pseudomurein
Live in extreme environments (e.g., boiling water)
Lack known parasites or pathogens of plants and animals

Other Microbial Groups

Fungi

Fungi are eukaryotes with chitin cell walls, absorbing organic chemicals for energy.

Yeasts: unicellular
Molds and mushrooms: multicellular

Protozoa

Protozoa are unicellular eukaryotes that absorb or ingest organic chemicals and may be motile.

Algae

Algae are eukaryotes with cellulose cell walls, using photosynthesis for energy.

Produce oxygen and carbohydrates

Multicellular Animal Parasites

These are eukaryotic multicellular organisms, including parasitic flatworms and roundworms (helminths).

Viruses

Viruses are acellular entities consisting of a DNA or RNA core surrounded by a protein coat.

Cannot metabolize or reproduce independently
Obligate intracellular parasites

Viral Structure and Classification

Capsid

The protein shell that surrounds the genome of a virus.

Naked vs Enveloped Viruses

Naked Virus: Contains only nucleocapsid (protein shell and nucleic acid); no outer envelope.
Enveloped Virus: Has an outer lipid bilayer (from host) and viral proteins in addition to nucleocapsid.

Nucleocapsid

Complex of nucleic acid and protein in enveloped viruses.

Virion Surface Protein

Proteins important for host cell attachment and may include enzymes involved in infection and replication.

Prions

Infectious misfolded proteins causing transmissible, untreatable, and fatal brain diseases in mammals.

Cause diseases such as mad cow disease (bovine spongiform encephalopathy)

Lyme Disease

Etiology and Transmission

Lyme disease is caused by Borrelia burgdorferi, transmitted by ticks.

Most common tickborne disease in the US
Field mice are the main reservoir
Ticks feed on deer but deer are not infected

Life Cycle of a Tick

Egg hatches into larva
Larva feeds on small animals
Larva is dormant
Larva develops into eight-legged nymph
Nymph feeds on animals
Nymph develops into adult tick
Adult ticks feed on deer and mate
Female ticks lay eggs

Symptoms of Lyme Disease

Joint pain
Heart inflammation
Muscle fatigue
Encephalopathy

Immune Evasion by Lyme Disease

Can change form to hide
Can survive intracellularly
Produces outer membrane vesicle to destroy flagellum and hide from immune system
Incorporates cholesterol into its membrane

Cellular Structure and Function

Structure of All Cells

Cytoplasmic membrane
Cytoplasm
Genome made of DNA
Ribosomes

Metabolism

Cells use information encoded in DNA to make RNA and protein, transform nutrients, conserve energy, and expel wastes.

Catabolism: Transforming molecules to produce energy
Anabolism: Synthesizing macromolecules

Growth

DNA information is converted into proteins, which do work. Proteins are used to convert nutrients from the environment into new cells.

Evolution

Mutations in DNA cause new traits, promoting evolution. Phylogenetic trees built from DNA sequence data capture evolutionary relationships.

Properties of Some Cells

Differentiation (e.g., forming spores)
Communication (chemical messengers)
Motility (self-propulsion)
Horizontal gene transfer

Prokaryotic DNA

Prokaryotic DNA is typically a single circular chromosome in the nucleoid region. Plasmids may confer special properties such as antibiotic resistance.

Small and compact (0.5–10 million base pairs)

Activities of Microbial Cells

Microbial cells typically live in communities and perform various activities:

Metabolism: chemical transformation of nutrients
Enzymes: protein catalysts
Transcription: DNA info converted to RNA
Translation: RNA used by ribosome to synthesize protein
DNA replication: copying genome

Cytoplasmic Membrane and Transport

Cytoplasmic Membrane Structure

The cytoplasmic membrane is a phospholipid bilayer enclosing the cytoplasm.

Peripheral proteins on membrane surface
Integral and transmembrane proteins penetrate the membrane
Semipermeable membrane
Hydrophilic head (water-loving), hydrophobic fatty acid tail

Functions of the Cytoplasmic Membrane

Regulate movement of molecules
Define barriers
Allow for internal chemistry
Cell-to-cell communication
Environmental sensing

Movement of Materials Across Membranes

Active transport: Movement from low to high concentration; energy required
Passive transport: Movement from high to low concentration; no energy required

Facilitative Diffusion

Movement of solute from high concentration to low concentration with the aid of a protein.

Simple Transport Systems

Driven by proton motive force
Symport: Solute and H+ move in one direction
Antiport: Solute and H+ move in opposite directions

Proton Motive Force

Generated through energy source, specifically by the electron transport chain.

ABC Transporters

ATP-binding cassette transporters are involved in the uptake of organic and inorganic compounds.

Over 200 different systems
Substrate binding protein outside of cells with high substrate affinity
ATP-driven uptake
Implicated in multidrug resistance
Iron acquisition

Group Translocation

Substance transported is chemically modified; energy-rich organic compound (not proton motive force) drives transport.

Osmotic Environments

Bacteria are always changing due to osmotic pressure of environments
Isotonic solution: No net movement of water; cell membrane attached to cell wall

Chemical Structure of Bacterial Cell Walls

Gram-Positive vs Gram-Negative

Type	Structure
Gram-Positive	Thick peptidoglycan layer, no outer membrane
Gram-Negative	Thin peptidoglycan layer, outer membrane present

Lysozyme

Enzyme that breaks β-1,4 bonds in peptidoglycan, leading to cell wall degradation.

Peptidoglycan Synthesis

Process by which bacterial cell walls are constructed, essential for cell shape and integrity.

Key Equations and Concepts

Surface Area to Volume Ratio:
Facilitated Diffusion:

Additional info: Some explanations and definitions have been expanded for clarity and completeness based on standard microbiology curriculum.