Introduction to Microbiology: Microorganisms, Cell Structure, and Biological Macromolecules

Introduction to Microbiology

Definition and Scope

Microbiology is the study of microorganisms and microbes, including their structure, function, and roles in the environment and human health. Microbes are organisms too small to be seen with the naked eye and include bacteria, archaea, fungi, protozoa, algae, and viruses.

• Microorganisms: Bacteria, archaea, fungi, protozoa, algae, viruses

• Microbes: General term for microscopic organisms

• Importance: Found everywhere, impact health, environment, and industry

Domains of Life and Microbial Diversity

Classification of Microorganisms

Microorganisms are classified into three domains based on cellular structure and genetics:

• Bacteria: Prokaryotic, unicellular, peptidoglycan cell walls

• Archaea: Prokaryotic, unicellular, unique membrane lipids, often extremophiles

• Eukarya: Eukaryotic, includes fungi, protozoa, algae, plants, and animals

Viruses are acellular entities that require host cells for replication.

Major Groups of Microbes

• Bacteria: Unicellular prokaryotes

• Archaea: Unicellular prokaryotes, distinct from bacteria

• Fungi: Unicellular (yeasts) or multicellular (molds, mushrooms) eukaryotes

• Protozoa: Unicellular eukaryotes, often motile

• Algae: Unicellular or multicellular photosynthetic eukaryotes

• Viruses: Acellular, DNA or RNA genome, require host cells

Cell Structure and Function

Summary Table: Prokaryotes vs. Eukaryotes

Cell Structure	Bacteria	Archaea	Eukaryotes
Size	~1-5 μm	~1-5 μm	~10-100 μm
Nucleus	No	No	Yes
Genome characteristics	Single chromosome, circular, no histones	Single chromosome, circular, histone-like proteins	Multiple chromosomes, linear, histones
Cell division	Binary fission	Binary fission	Mitosis, meiosis
Membrane lipid composition	Ester-linked phospholipids, unbranched fatty acids	Ether-linked phospholipids, branched hydrocarbons	Ester-linked phospholipids, unbranched fatty acids
Cell wall composition	Peptidoglycan	Varied (no peptidoglycan)	Cellulose, chitin, or none
Motility structures	Rigid, rotary flagella composed of flagellin	Rigid, rotary flagella composed of archaellin	Flexible flagella and cilia composed of tubulin
Ribosome size	70S	70S	80S

Additional info: Table summarizes key differences among domains relevant to microbiology.

Chemistry of Microbiology

Water and Its Properties

• Water: Universal solvent, polar covalent bonds

• Hydrogen bonding enables cohesion, adhesion, and temperature stability

• Acids and bases: Affect pH, important for cellular processes

Biological Macromolecules

Cells are composed of four major classes of macromolecules:

• Proteins: Polymers of amino acids, perform structural and enzymatic functions

• Nucleic acids: DNA and RNA, store and transmit genetic information

• Carbohydrates: Sugars and polysaccharides, energy storage and structural roles

• Lipids: Hydrophobic molecules, form membranes, energy storage

Macromolecule Synthesis and Breakdown

• Dehydration (Condensation) Reaction: Joins monomers, releases water $\text{Monomer}_1 + \text{Monomer}_2 \rightarrow \text{Polymer} + \text{H}_2\text{O}$

• Hydrolysis Reaction: Breaks polymers, consumes water $\text{Polymer} + \text{H}_2\text{O} \rightarrow \text{Monomer}_1 + \text{Monomer}_2$

Functional Groups

Functional groups are specific groups of atoms within molecules that determine chemical reactivity and properties. Examples include hydroxyl, carboxyl, amino, and phosphate groups.

Cell Theory and LUCA

Cell Theory

• All organisms consist of one or more cells

• Cells are the basic unit of structure and function for all organisms

• All cells arise from pre-existing cells

LUCA (Last Universal Common Ancestor)

• All living organisms descend from a common ancestor

• LUCA is inferred from shared genetic and biochemical features

Taxonomy and Nomenclature

Taxonomic Hierarchy

• Domain > Kingdom > Phylum > Class > Order > Family > Genus > Species

Naming Organisms

• Binomial nomenclature: Genus species (e.g., Escherichia coli)

• Genus is capitalized, species is lowercase, both italicized

Microbial Communities and Biofilms

Biofilms

Biofilms are structured communities of microorganisms attached to surfaces and embedded in a self-produced extracellular matrix.

• Form on natural and artificial surfaces

• Provide protection from environmental stress and antibiotics

• Enable communication and resource sharing among microbes

Biofilm Formation Steps

1. Attachment: Cells adhere to a surface

2. Colonization: Cells divide and produce extracellular matrix

3. Development: Mature biofilm forms, with complex architecture

4. Dispersal: Cells leave to colonize new sites

Quorum Sensing

Quorum sensing is a cell-to-cell communication mechanism that allows bacteria to sense population density and coordinate gene expression.

• Involves production and detection of signaling molecules (autoinducers)

• Regulates biofilm formation, virulence, and other group behaviors

Additional info: Disrupting quorum sensing is a potential strategy to prevent biofilm-related infections.