Microbial Cell Structures: Prokaryotic and Eukaryotic Cells, Bacterial Cell Envelopes, and Specialized Structures

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Microbial Cell Structures

Overview of Prokaryotic and Eukaryotic Cell Structures

Microorganisms are classified based on their cellular organization into prokaryotes and eukaryotes. Understanding the differences in their structures is fundamental to microbiology, as these differences influence physiology, genetics, and responses to antimicrobial agents.

Prokaryotes lack a true nucleus and membrane-bound organelles. Their genetic material is typically a single, circular double-stranded DNA molecule located in the nucleoid region.
Eukaryotes possess a nucleus surrounded by a nuclear membrane and various membrane-bound organelles such as mitochondria, endoplasmic reticulum, and Golgi apparatus.
Both cell types have a cell membrane, cytoplasm, and ribosomes, but differ in ribosome size and structure.

Diagram comparing prokaryotic and eukaryotic cell structures

Key Differences Between Prokaryotic and Eukaryotic Cells

Nucleus: Prokaryotes have no nuclear membrane; eukaryotes have a true nucleus.
Chromosomes: Prokaryotes usually have a single, circular chromosome; eukaryotes have multiple, linear chromosomes.
Cell Wall: Most prokaryotes have a cell wall containing peptidoglycan; eukaryotic cell walls (if present) are chemically distinct (e.g., cellulose in plants, chitin in fungi).
Organelles: Eukaryotes have membrane-bound organelles; prokaryotes do not.
Ribosomes: Prokaryotic ribosomes are 70S (50S + 30S subunits); eukaryotic ribosomes are 80S (60S + 40S subunits).

Bacterial Cell Structures

External Structures

Bacteria possess a variety of external structures that contribute to motility, adherence, protection, and genetic exchange.

Flagella: Long, whip-like appendages used for motility. Structure includes the filament (flagellin protein), hook, and basal body (motor apparatus). Arrangements include monotrichous, lophotrichous, amphitrichous, and peritrichous.
Pili (Fimbriae): Thin protein tubes, primarily in Gram-negative bacteria. Attachment pili (fimbriae) facilitate adherence; conjugation pili transfer DNA during conjugation.
Glycocalyx: Outer coating composed of polysaccharides and/or proteins. Exists as a capsule (organized, uniform) or slime layer (loose, variable). Functions include protection, biofilm formation, and evasion of host defenses.

Electron micrograph of Campylobacter jejuni with bipolar flagella Electron micrograph showing periplasmic flagella Electron micrograph of fimbriae on bacterial surface

Bacterial Cell Envelope

The cell envelope provides structural integrity and protection. It consists of the cell wall and cell membrane, with significant differences between Gram-positive and Gram-negative bacteria.

Cell Wall: Maintains shape, prevents osmotic lysis, and is composed of peptidoglycan (unique to bacteria).
Cell Membrane: Phospholipid bilayer with embedded proteins; functions in selective permeability, energy production, and biosynthesis.

Diagram comparing Gram-positive and Gram-negative bacterial cell envelopes

Gram-Positive vs. Gram-Negative Cell Walls

Gram-Positive: Thick peptidoglycan layer, teichoic acids, no outer membrane.
Gram-Negative: Thin peptidoglycan layer, outer membrane containing lipopolysaccharide (LPS), porin proteins, and a periplasmic space.

Diagram of Gram-positive cell wall structure Diagram of Gram-negative cell wall structure

Peptidoglycan Structure and Synthesis

Peptidoglycan is a mesh-like polymer of sugars and amino acids, providing rigidity to the bacterial cell wall. It consists of repeating units of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) cross-linked by short peptides.

Enzymes called penicillin-binding proteins (PBPs) catalyze the cross-linking of peptidoglycan strands.
Lysozyme cleaves the β-1,4 linkage between NAG and NAM, compromising cell wall integrity.

Diagram of peptidoglycan structure Peptidoglycan synthesis pathway

Lipopolysaccharide (LPS) in Gram-Negative Bacteria

LPS is a major component of the outer membrane of Gram-negative bacteria. It consists of lipid A (endotoxin), a core polysaccharide, and an O-antigen. LPS contributes to structural integrity, protection from chemicals, and triggers strong immune responses in hosts.

Lipid A: Responsible for the toxic effects (endotoxin) when released during bacterial lysis.
O-antigen: Variable region used for serotyping.

Diagram of lipopolysaccharide structure

Specialized Cell Envelopes

Acid-Fast Cell Walls: Contain mycolic acids, making them highly resistant to chemicals and dehydration. Basis for the acid-fast stain (e.g., Mycobacterium).
Bacteria Without Cell Walls: Some bacteria (e.g., Mycoplasma) lack cell walls but have membranes stabilized by sterols, resulting in pleomorphism.

Bacterial Internal Structures

Cytoplasm: Gel-like matrix composed of water, nutrients, and cell components.
Cytoskeleton: Protein filaments (actin-like, tubulin-like) involved in cell shape, division, and intracellular transport.
Chromosome: Usually a single, circular, double-stranded DNA molecule located in the nucleoid.
Plasmids: Small, circular DNA molecules that replicate independently and may carry genes for antibiotic resistance or virulence.
Ribosomes: Sites of protein synthesis; prokaryotic ribosomes are 70S (50S + 30S subunits).
Inclusion Bodies: Storage granules for nutrients and other substances.

Fluorescent micrograph of actin-like filaments in bacteria Diagram of prokaryotic ribosome structure

Endospores

Endospores are highly resistant, dormant structures formed by certain Gram-positive bacteria (e.g., Bacillus, Clostridium) in response to adverse conditions. They allow survival through extreme heat, desiccation, chemicals, and radiation.

Life Cycle: Includes a vegetative cell (active, growing) and an endospore (dormant, resistant).
Sporulation: Process of endospore formation (not reproduction).
Germination: Return to vegetative growth when conditions improve.

Diagram of endospore structure and formation

Summary Table: Comparison of Prokaryotic and Eukaryotic Cell Structures

Feature	Prokaryotes	Eukaryotes
Nucleus	Absent	Present
Chromosomes	Single, circular	Multiple, linear
Cell Wall	Peptidoglycan (most)	Cellulose, chitin, or absent
Organelles	Absent	Present
Ribosomes	70S	80S
Flagella	Simple, flagellin-based	Complex, microtubule-based

Additional info: Some bacteria have sterols in their membranes, and many eukaryotes (plants, algae, fungi) have cell walls and may possess plasmids. The presence and arrangement of external structures such as flagella and pili are important for motility, adherence, and genetic exchange. The Gram stain is a critical diagnostic tool for distinguishing bacterial types based on cell wall structure.