Functional Anatomy of Prokaryotic and Eukaryotic Cells: Study Guide

Functional Anatomy of Prokaryotic and Eukaryotic Cells

Distinguishing Bacterial Genus vs. Shape

In microbiology, the term bacillus can refer to both a bacterial genus (Bacillus) and a rod-shaped morphology. To determine which is meant, context is key:

• Genus: Capitalized and italicized (e.g., Bacillus anthracis).

• Shape: Lowercase, not italicized, refers to rod-shaped bacteria.

• Example: "Bacillus causes anthrax" refers to the genus; "bacillus-shaped bacteria" refers to morphology.

Endospore Formation in Bacteria

Endospore formation is a survival mechanism in certain bacteria, allowing them to withstand harsh conditions. The process involves several stages:

• Sporulation: The process of endospore formation.

• Initiation: Triggered by nutrient depletion or environmental stress.

• Germination: Formation of a new cell from an endospore, triggered by favorable conditions.

Example: Bacillus and Clostridium species form endospores to survive extreme environments.

Structure and Function of Bacterial Cell Walls

Bacterial cell walls are essential for maintaining cell shape, protecting against osmotic lysis, and providing structural integrity. There are several types:

• Gram-positive: Thick peptidoglycan layer, teichoic acids.

• Gram-negative: Thin peptidoglycan layer, outer membrane with lipopolysaccharides.

• Acid-fast: Contains mycolic acids, resistant to decolorization.

• Mycoplasmas: Lack cell walls, have sterols in membrane.

Example: Gram staining distinguishes between gram-positive and gram-negative bacteria based on cell wall structure.

Comparison Table: Bacterial Cell Walls

Type	Main Features	Gram Stain	Drug Sensitivity
Gram-positive	Thick peptidoglycan, teichoic acids	Purple	Sensitive to penicillin
Gram-negative	Thin peptidoglycan, outer membrane, LPS	Pink	Resistant to penicillin
Acid-fast	Mycolic acids, waxy layer	Variable	Resistant to many drugs
Mycoplasmas	No cell wall, sterols in membrane	No stain	Resistant to cell wall-targeting drugs

Gram Stain Mechanism and Drug Effects

The Gram stain differentiates bacteria based on cell wall structure:

• Gram-positive: Retain crystal violet due to thick peptidoglycan.

• Gram-negative: Lose crystal violet, counterstained pink.

• Penicillin: Inhibits peptidoglycan synthesis, effective against gram-positive.

• Gram-negative resistance: Outer membrane blocks penicillin entry.

Example: Penicillin is ineffective against most gram-negative bacteria due to their outer membrane.

Peptidoglycan Structure and Synthesis

Peptidoglycan is a polymer of sugars and amino acids forming a mesh-like layer outside the plasma membrane. Its synthesis is a target for antibiotics:

• Structure: Alternating N-acetylglucosamine (GlcNAc) and N-acetylmuramic acid (MurNAc) with peptide crosslinks.

• Synthesis: Enzymes assemble peptidoglycan precursors and transport them across the membrane.

Osmosis and Effects on Bacterial Cells

Osmosis is the movement of water across a semipermeable membrane. Bacterial cells respond differently to various solutions:

• Isotonic: No net water movement; cell remains stable.

• Hypotonic: Water enters cell; risk of lysis if cell wall is damaged.

• Hypertonic: Water leaves cell; plasmolysis occurs.

Protoplasts and spheroplasts (cells lacking cell wall) are highly sensitive to osmotic changes:

• Hypertonic solution: Cell shrinks (plasmolysis).

• Hypotonic solution: Cell swells and may lyse (cytolysis).

Fimbriae and Other Bacterial Structures

Fimbriae are proteinaceous appendages found on many gram-negative bacteria:

• Function: Attachment to surfaces and host cells.

• Composition: Made of pilin protein.

• Motility: Not used for motility (unlike flagella).

Example: Escherichia coli uses fimbriae to adhere to urinary tract epithelium.

Clinical Applications: Anaerobic Bacteria and Environmental Survival

Clostridium botulinum is a strict anaerobe, killed by oxygen. It survives on plants as endospores, which are resistant to environmental stress. Home-canned foods are a common source of botulism because improper canning fails to destroy endospores, allowing them to germinate in anaerobic conditions.

• Endospores: Allow survival in oxygen-rich environments until anaerobic conditions are met.

• Home-canning: Often lacks sufficient heat to destroy endospores.

Environmental Microbiology: Bacterial Contamination in Water

Bacteria such as Acidithiobacillus can enter household water through biofilms or pipe contamination. Structures like fimbriae and pili facilitate attachment to surfaces, enabling colonization of plumbing systems.

• Biofilms: Communities of bacteria adhering to surfaces, protected by extracellular matrix.

• Fimbriae: Aid in attachment and persistence in water systems.

Analysis and Application Questions

• Epulopiscium fishelsoni: Initially classified as protozoan due to large size; reclassified as gram-positive bacterium based on cell wall structure, DNA, and ribosomal RNA analysis.

• Active Transport of K+ in Hypertonic Solutions: Helps maintain osmotic balance and prevents plasmolysis by increasing internal solute concentration, requiring ATP.

Summary Table: Effects of Solutions on Bacterial Cells

Cell Type	Isotonic	Hypotonic	Hypertonic
Intact cell	No change	Water enters, possible lysis	Water leaves, plasmolysis
Protoplast/Spheroplast	No change	Water enters, lysis	Water leaves, shrinkage