Cell Biology & Bacterial Taxonomy

---

1. Taxonomical Hierarchies and Nomenclature

Taxonomy is the science of classifying organisms. In microbiology, we classify bacteria into a strict hierarchy to understand their relationships and pathogenic behaviors.

The Hierarchy of Bacteria

• Kingdom: Prokaryotes (organisms lacking a true nucleus).
• Order: The name always ends with the suffix '-ales'. Example: Enterobacteriales.
• Family: Many families exist in one order. The name always ends with the suffix '-eae'. Examples: Enterobacteriaceae, Pseudomonodaceae.
• Genus: Each family is divided into genera. Example: Within Enterobacteriaceae, you have Escherichia, Klebsiella, Enterobacter.
• Species: The most specific group. Example: Escherichia coli.
• Sub-species: Further divisions based on tiny genetic/antigenic differences (e.g., Salmonella enterica subsp. enterica).
  Clinical Extension: This level is vital for tracking outbreaks! For example, tracking the deadly food-poisoning strain E. coli O157:H7 distinguishes it from the harmless E. coli living normally in your gut.

---

2. Classification of Medically Important Bacteria (Based on Cell Wall)

This is the most critical flowchart in clinical microbiology. Bacteria are primarily classified by their cell wall structure, which dictates what antibiotics will work against them.

A. Lacking Cell Wall

• Genera: Mycoplasma & Ureaplasma

B. Rigid Cell Wall (The vast majority of bacteria)

Divided based on how they live and their shape/staining:

1. Obligate Intracellular Bacteria:
   • Must live inside a host cell to survive (they cannot make their own ATP—they are "energy parasites").
   • Genera: Chlamydia, Rickettsia, Coxiella, Ehrlichia.
2. Filamentous Bacteria (Branching, fungus-like):
   • Genera: Actinomyces, Nocardia, Mycobacteria.
   • Clinical Pearl: Mycobacteria (which causes Tuberculosis) has a rigid wall heavily loaded with mycolic acid (waxy lipids), making it "Acid-Fast" instead of truly Gram-positive or negative. Regular Gram stain bounces right off this wax. We must use the Ziehl-Neelsen (Acid-Fast) stain, where TB appears as bright red snappers.
3. Free-Living, Simple Unicellular (Gram Positive vs Gram Negative):

C. Flexible Cell Wall (Spirochetes)

• Corkscrew-shaped bacteria that move using axial filaments (internal flagella that twist the entire cell like a drill).
• Genera: Treponema (Causes Syphilis), Borrelia (Causes Lyme Disease), Leptospira.
• Diagnostic Note: They are so incredibly thin that they cannot be seen with a normal light microscope. Doctors must use a special "Darkfield Microscope" to see Treponema pallidum swimming in fluid from a syphilis sore.

---

3. Morphology: Size and Shape of Bacteria

Size and Its Importance

Bacterial cell size ranges from 0.1 to 5µm in diameter.

• Cyanobacteria: 5 × 40µm (Huge for bacteria)
• Escherichia coli: 1.3 × 3µm (Average)
• Haemophilus influenzae: 0.25 × 1.2µm (Very small)

Shapes of Bacteria

The shape of the cell directly affects its ecology (how it survives in its environment).

• Coccus (Spheres): Can exist as single cocci, diplococci (pairs, e.g., Neisseria gonorrhoeae looks exactly like two kidney beans facing each other), streptococci (chains), staphylococci (grape-like clusters), tetrads (groups of 4), or sarcina (groups of 8).
• Bacillus (Rods): Coccobacillus (very short, plump rods that look almost like cocci), standard rods (e.g., Bacillus anthracis looks like long boxcars).
• Vibrio: Comma-shaped, curved rods (e.g., Vibrio cholerae, shaped like a comma to rapidly dart through thick intestinal mucus).
• Spirillum / Spirochete: Helical, corkscrew-shaped.
• Filamentous: Long, branching threads (mimicking fungal hyphae to spread through tissue).

---

4. Bacterial Cell Structures (Anatomy of a Prokaryote)

A. Cytoplasm Structures (Inside the cell)

• Nuclear Region (Nucleoid): Prokaryotes do NOT have a true nucleus or a nuclear membrane. Their DNA is a single, long, circular strand that is heavily supercoiled (twisted up tight like a rubber band) to fit inside the tiny cell.
• Ribosomes: Small particles made of protein and rRNA, essential for translation (protein synthesis). Bacterial ribosomes are 70S in size (composed of 50S and 30S subunits).
• Granules (Inclusion bodies): Used to store energy (like Glycogen) or serve as structural building blocks when nutrients are plentiful.
• Plasmids: Extrachromosomal circular DNA. They are entirely separate from the main chromosome and replicate independently. Clinical Importance: Plasmids are the main vehicles for sharing antibiotic resistance genes. A harmless bacterium can pass a "superbug" plasmid to a dangerous bacterium during conjugation!

Note: Prokaryotes LACK all membrane-bound organelles (No mitochondria, no Golgi apparatus, no Endoplasmic Reticulum).

B. Cell Envelope Structures (The border walls)

1. Cytoplasmic Membrane (Inner Membrane)

Because bacteria lack organelles, the cell membrane takes over many critical functions:

• Selective permeability barrier: Controls what enters and leaves.
• Electron transport and oxidative phosphorylation: Since bacteria have NO mitochondria, the enzymes for the respiratory chain (Cytochromes, dehydrogenases) are embedded right here in the cell membrane to make ATP.
• Excretion: Pumps out hydrolytic enzymes and pathogenic toxins into the host's body.
• Biosynthetic function: Contains the enzymes that build the cell wall above it.
• Chemotactic systems: Contains receptors that bind attractants (food) and repellants (toxins). Example: E. coli has 20 different chemoreceptors to navigate the gut.

*Antibiotics targeting the cell membrane: Ionophores and Polymyxins (e.g., Colistin, which acts like a biological detergent to rip open and burst the bacterial membrane. Used only as a last resort due to toxicity!).

2. The Cell Wall (Peptidoglycan)

Lies immediately outside the cytoplasmic membrane. It is made of a complex polymer called Peptidoglycan (Murein). (Note: Archaebacteria and Eukaryotes completely lack peptidoglycan; plants have cellulose, fungi have chitin).

Structure of Peptidoglycan: It consists of 3 parts:

1. A backbone made of alternating sugar derivatives: N-acetylmuramic acid (NAM) and N-acetylglucosamine (NAG).
2. A set of identical tetrapeptide side chains attached to NAM.
3. A set of identical peptide cross-bridges that link the chains together, creating a tough, chain-link fence structure. (Penicillin works by permanently disabling the enzyme—transpeptidase—that builds these cross-bridges, causing the wall to fall apart!)

Functions of the Cell Wall: Maintains shape/cellular integrity (prevents the cell from bursting due to high internal osmotic pressure), essential for cell division, serves as a primer for its own synthesis, and acts as a major antigen determinant.

---

5. Gram Positive vs. Gram Negative Cell Envelopes

This structural difference is the basis of the Gram Stain, invented by Hans Christian Gram.

---

6. Surface Structures and Appendages

A. Capsules and Slime Layers (The Glycocalyx)

A slimy, gummy extracellular polymer secreted on the surface of the bacteria. It is almost always made of polysaccharides.

Exam Exception: The capsule of Bacillus licheniformis (and Bacillus anthracis, the anthrax bacterium) is uniquely made of proteins (poly-D-glutamic acid).

Functions:

• Anti-phagocytic: Makes the bacterium slippery, preventing immune system macrophages from eating it (major virulence factor). Clinical Scenario: We use the thick sugar capsule of Streptococcus pneumoniae to create the Prevnar vaccine, teaching the body to recognize and grab the slippery capsule!
• Attachment: Allows pathogens to stick to hosts. Clinical Scenario: Streptococcus mutans uses its heavy slime layer to stick tightly to tooth enamel, trapping sugar and acid to cause severe dental caries (cavities).
• Antigenic structure: Used by doctors for typing and creating vaccines (e.g., Pneumococcal capsule vaccine).

B. Pili & Fimbriae

Rigid, hair-like surface structures found mostly on Gram-negative bacteria. Shorter and finer than flagella, made of a protein called pilin.

• Fimbriae: Used strictly for adherence (enabling bacteria to stick to human tissues, inert surfaces, or form scums/pellicles on liquids). Example: Uropathogenic E. coli uses fimbriae to hold onto the bladder wall so it doesn't get washed away by urine!
• Pili: Usually longer, and only 1 or a few are present.
• Sex pili (F-pili): Used to attach a donor bacterium to a recipient during Bacterial Conjugation (creating a bridge for sharing DNA/plasmids).
• Antigenic Variation: Pathogens like Neisseria gonorrhoeae constantly change the molecular structure of their pili. Just as the immune system makes antibodies to fight the gonorrhea, the bacterium changes its pili to a new shape, completely evading the immune system!

C. Flagella

Long, thread-like appendages composed entirely of a protein called flagellin arranged in a helical structure (12-13nm in diameter).

• Function: The primary organ of locomotion/motility (swimming). They rotate like boat propellers to push the bacteria toward food or away from poison (chemotaxis). Note: some bacteria move differently, via gliding or gas vesicles.
• Antigenic: The flagellar protein is highly antigenic and is known clinically as the H-antigen.
• Clinical Scenario: Proteus mirabilis is highly flagellated and exhibits "swarming motility". It swims aggressively up the urinary tract, causing severe kidney infections and massive kidney stones.

---

7. Bacterial Endospores (The Ultimate Survival Mode)

Endospores are dormant, incredibly tough, non-reproductive structures formed by a few specific Gram-positive rods (primarily Bacillus and Clostridium species).

• When are they formed? During adverse, harsh environmental conditions (e.g., severe nutritional depletion, extreme heat, drying). The bacterium packs its DNA into a bunker (sporulation).
• How are they released? The vegetative (active) cell undergoes autolysis (bursts open) to release the spore. This process takes time and energy. When conditions become safe again, the spore "germinates" back into an active, dividing bacterium.
• Properties: Extremely resistant to heat, drying, radiation, acids, and chemical disinfectants. (Standard boiling does NOT kill spores; they require autoclaving at 121°C under high pressure for at least 15 minutes).
• Structure: Core, Cortex, Spore Coat, and Exosporium.
• Classification: The location of the spore inside the cell helps identify the bacteria (Central, Subterminal, or Terminal). e.g., Clostridium tetani has a terminal spore, making the cell look exactly like a tennis racket.

---

8. Classification Based on Growth Requirements

A. Based on Oxygen Requirements

B. Based on Temperature Requirements

• Psychrophiles (Cold loving): Optimum temp 10-20°C (can grow <20°C). Example: Pseudomonas fluorescens.
• Mesophiles (Moderate temp): Optimum temp 25-40°C. (All medically important human pathogens are here, as normal human body temp is 37°C!) Examples: E. coli, Salmonella, Staphylococcus.
  Host Defense Note: This is exactly why your body creates a Fever! By raising your body temperature to 39°C or 40°C, your immune system intentionally pushes the Mesophilic bacteria out of their comfortable optimum growth zone, slowing down their replication!
• Thermophiles (Heat loving): Optimum temp 50-80°C. Example: Geobacillus stearothermophilus (used in autoclave testing as discussed).
• Hyperthermophiles (Extreme heat): Optimum temp 80°C or more.

---

Assignment 1: Prokaryotic vs Eukaryotic Cell

A classic exam comparison covering the fundamental differences between bacterial cells and human cells.