Bacterial Cell Biology: Structures, Motility, and Endospore Formation

Public Health and Pathogenic Outbreaks

• It is critical to monitor news and public health announcements regarding outbreaks.
• Case Example: Meat Recalls: Recall for frozen meat might occur months after the sale (e.g., sold in December, recall in March). Most people may have already consumed the product; proactive disposal of potentially contaminated items is necessary.
• Pathogen Examples:     - Salmonella: Associated with outbreaks in various sources, including dog food.     - E. coli O157:H7 (o157:h7): A highly harmful pathogen linked to HUS (Hemolytic Uremic Syndrome).

Comparative Microbiology: Mycobacteria vs. Mycoplasma

• Mycoplasma:     - Size: Tiny bacteria.     - Structure: Lacks a cell wall entirely; possesses only a cell membrane.     - Enviornment: Must live inside other cells (obligate intracellular) to survive osmotic pressures.
• Mycobacteria:     - Distinguishing Feature: Possesses mycolic acid, a waxy substance in the cell envelope.     - Staining Requirements: Cannot be stained using the Gram stain because the mycolic acid acts like wax, preventing dye uptake.     - Acid-Fast Staining: This specific stain is required. In the procedure, the cells must be steamed to melt the wax-like mycolic acid so the dye is absorbed.     - Results: On an acid-fast stain, positive bacteria appear pink against a blue background.     - Genetic Classification: Genetically, Mycobacteria are Gram-positive because they lack an outer membrane. However, they remain "Gram-stain non-reactive" due to the cell wall composition.
• Specific Mycobacterial Pathogens:     - Mycobacterium tuberculosis: Causes tuberculosis.     - Mycobacterium leprae: Causes leprosy.     - Mycobacterium smegmatis: A non-pathogenic species often used in laboratory settings.

Atypical Bacteria: Rickettsia and Chlamydia

• General Characteristics: "Atypical" means not normal. These bacteria often lack standard cell wall features or metabolic independence.
• Nomenclature Notes:     - SPP: Denotes multiple species within a genus.     - ST / SP: Denotes a single species.
• Rickettsia:     - Possesses a highly permeable membrane.     - Exhibits pleomorphism (no fixed shape).
• Chlamydia:     - Energy parasites: They lack the ability to produce their own ATP and must absorb it from the host cell.
• Intracellular Lifestyle: Like Mycoplasma, these are often obligate intracellular pathogens. They grow within the host cytoplasm, which is an isotonic environment (no net gain or loss of water).

Effects of Lysozyme and Penicillin on Cell Walls

• Osmotic Pressure: Most bacteria live in hypotonic environments. Without a cell wall (the "cage"), water rushes in and causes the cell to lyse (explode).
• Lysozyme:     - An enzyme found in innate immunity (mucus, secretions).     - Mechanism: Cleaves the $\beta(1 \rightarrow 4)$ glycosidic bonds between NAM and NAG (the sugars holding peptidoglycan together).     - Effect: Directly breaks existing cell walls, leading to cell lysis.
• Penicillin:     - Mechanism: Inhibits the synthesis of the cell wall (specifically the peptide cross-linking).     - Growth Requirement: Only works on actively replicating bacteria that are currently building new cell walls. It does not break apart pre-existing, stable cell walls.
• Resulting Protoplasts and Spheroplasts:     - Protoplast: A Gram-positive cell that has lost its peptidoglycan layer, leaving only the cell membrane and internal contents.     - Spheroplast: A Gram-negative cell that has lost its peptidoglycan but retains the outer membrane (LPS layer) and the cell membrane. Lysozyme cannot destroy the outer membrane/LPS.

L-form Bacteria

• L-form bacteria have the genetic ability to make a cell wall but can choose to "shut down" the process.
• Strategy: In an isotonic environment (where the "cage" is not needed for protection against osmotic pressure), the bacteria stops making the cell wall to save energy, as synthesizing enzymes, sugars, and amino acids is metabolically costly.
• Reversion: If moved to a hypotonic environment, they resume wall synthesis to prevent lysis.
• Examples: Bacillus anthracis, Treponema pallidum, Mycobacterium, Helicobacter pylori, Rickettsia, and Borrelia.

The Glycocalyx: Capsules and Slime Layers

• The glycocalyx is a polysaccharide layer outside the cell wall.
• Capsules:     - Well-organized and smooth.     - Help with attachment to surfaces.     - Protection: Protect against desiccation (drying out) because they hold water. Colonies with capsules look "wet" or glistening on agar plates.     - Immune Evasion: Protect against bacteriophages (viruses), detergents, and phagocytosis (being eaten by immune cells).
• Slime Layers:     - Unorganized and diffuse (resembling "snot").     - Main function: Attachment and gliding motility (exclusive to slime layers).
• S-Layer:     - Found in both Bacteria and Archaea.     - Composed of self-assembling proteins or glycoproteins; they "snap together" like floor tiles without needing enzymes.     - Functions: Protection, attachment, and potentially useful in nanotechnology.

External Structures: Pili, Fimbriae, and Flagella

• Fimbriae and Pili:     - Short, hair-like extensions used primarily for attachment to hard surfaces.     - Sex Pilus: Used for conjugation, a form of horizontal gene transfer where a donor bacteria shares a strand of a plasmid (such as the R-plasmid for antibiotic resistance) with a recipient.     - Type IV Pili: Used for twitching motility (a "jerking" movement created by polymerizing and depolymerizing the pilus).
• Flagella:     - Long extensions used for motility (swimming and swarming).     - Structure: Consists of a filament (propeller), a hook, and a basal body (motor).     - Energy Source: Powered by the Proton Motive Force (PMF), used in active transport.     - Distribution Patterns:         - Monotrichous: One flagellum at one end.         - Lopotrichous: A cluster ("ponytail") of flagella at one or both ends.         - Amphitrichous: One flagellum at each end.         - Peritrichous: Flagella distributed over the entire perimeter of the cell.

Bacterial Motility and Chemotaxis

• Speeds: Bacteria are among the fastest living organisms relative to body length. Flagellar propellers can rotate up to $1,100$ rotations per second.
• Rotational Directions:     - Counterclockwise (CCW): Results in a Forward Run.     - Clockwise (CW): Results in a Tumble, where the flagella spread out and the cell stops to reorient itself.
• Five Types of Motility:     1. Swimming: Flagellar movement in liquid.     2. Swarming: Rapid movement over semi-solid surfaces (Pseudomonas species are notable swarmers).     3. Spirochete Motility: Flagella (axial fibrils) remain in the periplasmic space wrapped around the cell. As they rotate, the entire cell body rotates like a drill (e.g., Treponema pallidum).     4. Twitching: Requires Type IV pili on a hard surface.     5. Gliding: Requires a slime layer on a hard surface; the cell skates smoothly over the surface.
• Taxis (Directional Movement):     - Chemotaxis: Movement toward a chemical attractant (e.g., glucose) or away from a repellent (e.g., penicillin).     - Phototaxis: Movement toward light.     - Magnetotaxis: Orientation using magnetic fields via magnetosomes.

Bacterial Endospores

• General Facts: Formed by certain Gram-positive bacteria (though not all Gram-positives do so). Spores are highly resistant and biologically dormant.
• Genera that form spores:     - Bacillus (rod shape).     - Clostridium (rod shape).     - Sporosarcina (cocci shape).
• Resistance: Nearly indestructible. Resistant to UV radiation, antibiotics, and bleach. Can only be destroyed by an autoclave.
• Structure:     - Core: Contains the circular chromosome ($1$), essential proteins, and is fully dehydrated (no metabolic activity).     - Calcium-dipicolinic acid (Ca-DPA): Unique to spores.     - SASPs (Small Acid-Soluble Proteins): Bind and protect DNA from mutations.     - Layers: Multiple protective layers including the coat and exosporium.
• Lifecycle:     - Sporulation: Triggered by negative environments. Process takes $8$ to $10$ hours. Starts with chromosome replication. It is a unidirectional process (no turning back once sigma factors are destroyed).     - Endospore: The spore while still inside the mother cell.     - Free Spore: The spore after the mother cell has lysed.     - Germination: The process where a spore returns to being a metabolically active vegetative cell because the environment has improved.
• Critical Principle: Sporulation is NOT a method of replication. One cell produces one spore, and one spore germinates into one cell.
• Historic Examples: "King Tut’s Curse" likely involved aerosolized spores in tombs being inhaled and germinating in the lungs of explorers.

Questions & Discussion

• Question: Can Staphylococcus make spores?
• Answer: No. While Staphylococcus is Gram-positive, it is not on the list of spore-formers (Bacillus, Clostridium, Sporosarcina).
• Question: Is sporulation for replication in bacteria?
• Answer: No. This is a common point of confusion. Unlike yeast, which use spores for replication, bacteria use them strictly for survival. One cell = one spore.
• Question: Does the direction of flagellar rotation apply to all types?
• Answer: Yes, counterclockwise for runs and clockwise for tumbles is standard across flagellated bacteria.
• Question: Which movement type does not use flagella?
• Answer: Twitching (uses Type IV pili) and gliding (uses slime layers).