Foundations of Microbiology: Cell Structure, Diversity, and Pathogenicity

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Introduction to Microbiology

Microbiology is the study of microscopic organisms, including bacteria, archaea, viruses, fungi, protozoa, and multicellular parasites. This field explores their structure, function, classification, and roles in health, disease, and the environment.

Microscopy and Cell Size

Measuring Microorganisms

Viruses are typically measured in nanometers (nm); most are about 100 nm in size.
Bacteria are measured in micrometers (μm); typical bacterial cells are about 1 μm.
Objects larger than 100 μm can be seen without a microscope.

Electron Microscope vs. Light Microscope

Electron microscopes use electron beams and are suitable for visualizing very small organisms (1 nm–1 μm).
Light microscopes use visible light and are suitable for larger microorganisms (1 μm–1 mm).

Cell Types: Prokaryotes vs. Eukaryotes

Prokaryotes

Single circular chromosome, not enclosed in a membrane.
No histones or membrane-bound organelles.
70S ribosomes for protein synthesis.
Cell wall contains peptidoglycan (in bacteria) or pseudomurein (in archaea).
Divide by binary fission.

Eukaryotes

Paired chromosomes within a nuclear membrane.
Contain histones and membrane-bound organelles.
80S ribosomes for protein synthesis.
Divide by mitosis.

Prokaryotic vs. Eukaryotic Cells

Prokaryotic cells: Simple inside, complex outside.
Eukaryotic cells: Complex inside, often with compartmentalized organelles.

Bacteria: Structure and Diversity

General Features

Single-celled prokaryotes.
Peptidoglycan cell walls (except in archaea).
Classified by shape: cocci (spherical), bacilli (rod-shaped), spirilla (spiral), spirochetes (flexible spirals), filamentous (thread-like).

Arrangement of Bacterial Cells

Diplococci: Pairs of cocci.
Streptococci: Chains of cocci.
Tetrads: Groups of four cocci.
Sarcinae: Cubic configuration of eight cocci.
Staphylococci: Irregular clusters.

Pathogenic vs. Non-Pathogenic Bacteria

Pathogenic: Cause disease in hosts.
Non-pathogenic: Do not cause disease; may be beneficial (e.g., decomposers, vitamin producers).

Opportunistic Bacteria

Cause disease only in special cases, such as weakened immune systems.

Properties of All Cells

Structure: Cytoplasmic membrane, cytoplasm, DNA, ribosomes.
Metabolism: Use genetic information to make RNA and proteins, take up nutrients, conserve energy, expel wastes.
Growth: Convert nutrients into new cell material.
Evolution: DNA mutations allow adaptation and evolution.

Properties of Some Cells

Differentiation: Formation of new cell structures (e.g., spores).
Communication: Cells interact via chemical signals.
Genetic exchange: Horizontal gene transfer.
Motility: Movement via flagella or other structures.

Archaea

Prokaryotes with cell walls made from pseudomurein.
Live in extreme environments (e.g., hot springs, salt lakes).
Lack known pathogens of plants and animals.

Eukarya

Large domain including plants, animals, fungi, and protists.
First eukaryotes were unicellular; multicellularity evolved later.
Vary in size and shape; complex internal structure.

How Eukaryotic Cells Overcome Surface Area to Volume Problem

Organized internal structure (organelles) allows efficient biochemical reactions.

Fungi, Protozoa, and Multicellular Parasites

Fungi

Eukaryotes with chitin cell walls.
Absorb organic chemicals for energy.
Yeasts are unicellular; molds and mushrooms are multicellular.

Protozoa

Unicellular eukaryotes.
Absorb or ingest organic chemicals.

Multicellular Animal Parasites

Eukaryotes; include helminths (parasitic flatworms and roundworms).

Viruses and Prions

Viruses

Acellular; consist of DNA or RNA core surrounded by a protein coat (capsid).
Some have an outer envelope derived from host cell membrane.
Replicate only when inside a living host cell.

Virion Morphology and Hosts

Viruses infect a wide range of hosts (plants, animals, bacteria).
Examples: Zika virus (human), Pepino mosaic virus (tomato), Mimivirus (Acanthamoeba), Bacteriophage (Lactococcus lactis).

Capsid and Nucleocapsid

Capsid: Protein shell surrounding the viral genome.
Nucleocapsid: Capsid plus the enclosed nucleic acid.

Naked vs. Enveloped Viruses

Naked virus: Only nucleocapsid, no envelope.
Enveloped virus: Nucleocapsid surrounded by a lipid bilayer envelope with viral proteins.

Viron Surface Proteins

Important for host cell attachment and entry; may include enzymes for infection/replication.

Prions

Infectious proteins causing transmissible spongiform encephalopathies (e.g., mad cow disease).
Cause misfolding of normal proteins, leading to brain damage.

Selected Diseases: Lyme Disease and Bovine Spongiform Encephalopathy

Bovine Spongiform Encephalopathy (BSE)

Prion disease in cows, also known as mad cow disease.
First identified in England in the 1980s.

Lyme Disease

Caused by Borrelia burgdorferi.
Most common tick-borne disease in the US.
Ticks feed on deer but are not infected; field mice are the main reservoir.
Symptoms: Joint pain, heart inflammation, muscle fatigue, encephalopathy.
Early form can be treated with antibiotics.

Life Cycle of a Tick

Larva hatches from egg.
Larva feeds on small animals.
Larva is dormant.
Larva develops into eight-legged nymph.
Nymph feeds on animals.
Nymph develops into adult tick.
Adult feeds on deer and mates.
Female lays eggs.

Transmission of Lyme Disease

Tick bite introduces B. burgdorferi into blood vessels and endothelium.
Bacteria enter the extracellular matrix of tissue.

How Lyme Disease Avoids the Immune System

Can change form to hide.
Survive intracellularly.
Produce outer membrane vesicles to evade destruction.
Regulate cholesterol in membrane for protection.
Can reprogram and cause cellular changes.

Cell Membranes and Transport

The Cytoplasmic Membrane

Phospholipid bilayer enclosing the cytoplasm.
Contains integral and peripheral proteins.
Semipermeable, allowing selective transport.

Functions of the Cytoplasmic Membrane

Regulates movement of molecules.
Defines barriers for internal chemistry.
Cell-to-cell communication and environmental sensing.

Movement of Materials Across Membranes

Active transport: Requires energy (e.g., ATP).
Passive transport: No energy required; movement from high to low concentration.

Facilitated Diffusion

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

Classes of Transport Systems

Simple transport: Driven by proton motive force.
ABC transporters: ATP-binding cassette transporters; use ATP for substrate uptake.
Group translocation: Substance is chemically modified during transport (e.g., phosphorylation).

Proton Motive Force

Generated by electron transport chain; drives transport and ATP synthesis.

Osmosis and Solutions

Isotonic solution: No net movement of water; cell membrane attached to cell wall.
Hypertonic solution: Water moves out; cell shrinks.
Hypotonic solution: Water moves in; cell swells and may lyse.

Bacterial Cell Wall Structure

Function of the Cell Wall

Protection against osmotic pressure changes.
Maintains cell shape and structural integrity.
Contributes to pathogenicity.

Chemical Structure of Bacterial Cell Walls

Composed of peptidoglycan: repeating subunits of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM).

Gram-Positive vs. Gram-Negative Bacteria

Feature	Gram-Positive	Gram-Negative
Peptidoglycan	Thick, multi-layered	Thin, single-layered
Teichoic acids	Present	Absent
Outer membrane	Absent	Present
Stain color	Purple	Pink/red

Lysozyme

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

Peptidoglycan Synthesis

Occurs in the cytoplasm and at the cell membrane.
Uses NAG and NAM as building blocks.
Penicillin-binding proteins (PBPs) help assemble the cell wall.

Key Equations and Concepts

Surface Area to Volume Ratio (SA:V):
Osmosis: