Microorganisms: Types and Classification

Overview of Microbial Diversity

Microorganisms, or microbes, are a diverse group of living entities that include bacteria, archaea, fungi, protozoa, microscopic algae, viruses, and helminths. They play essential roles in ecosystems, human health, and biotechnology. - Bacteria: Prokaryotic, unicellular organisms with various shapes (bacillus, coccus, spiral, star, square). - Archaea: Prokaryotic, often found in extreme environments; cell walls lack peptidoglycan. - Fungi: Eukaryotic, unicellular (yeast) or multicellular (molds, mushrooms); cell wall contains chitin. - Protozoa: Unicellular, eukaryotic; motile via pseudopods, flagella, or cilia. - Algae: Photosynthetic eukaryotes; cell wall contains cellulose. - Viruses: Acellular, contain DNA or RNA, replicate only inside host cells. - Helminths: Multicellular animal parasites; some stages are microscopic.

Classification of Organisms (Woese, late 1970s)

The classification system divides life into three domains based on cell wall composition and molecular features:

• Bacteria: Cell walls contain peptidoglycan.

• Archaea: Cell walls lack peptidoglycan.

• Eukarya: Includes protists, fungi, plants, and animals.

Golden Age of Microbiology

Major Discoveries and Contributors

The period from 1857 to 1914 is known as the Golden Age of Microbiology, marked by foundational discoveries in microbial science.

• Louis Pasteur: Disproved spontaneous generation, developed fermentation and pasteurization.

• Joseph Lister: Introduced antiseptic techniques in surgery.

• Robert Koch: Established methods for linking specific microbes to diseases.

Functional Anatomy of Prokaryotic Cells

Bacterial Cell Wall Structure and Function

The bacterial cell wall is a complex, semirigid structure that determines cell shape, protects against environmental stress, and is a target for antibiotics.

• Peptidoglycan: The main component, consisting of repeating units of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) linked by polypeptides.

• Function: Prevents cell rupture, contributes to pathogenicity, and is targeted by antibiotics like penicillin.

Peptidoglycan Composition and Cross-Linking

Peptidoglycan forms a carbohydrate backbone of 20-65 disaccharide rows, linked by tetrapeptide chains (alternating D and L amino acids). Cross-bridges between tetrapeptides provide structural integrity. - Penicillin: Inhibits cross-bridge formation, causing cell lysis.

Gram-Positive Bacterial Cell Wall

Gram-positive bacteria have thick, multilayered peptidoglycan walls with teichoic acids (wall and lipoteichoic acids) that regulate ion transport and provide antigenic specificity.

Gram-Negative Bacterial Cell Wall

Gram-negative bacteria possess a thin peptidoglycan layer located in the periplasmic space between inner and outer membranes. The outer membrane contains lipopolysaccharide (LPS), lipoproteins, and phospholipids.

• LPS Components: Lipid A (endotoxin), core polysaccharide, O polysaccharide (antigenic).

• Function: Provides barrier to harmful substances, aids in evading phagocytosis, and contains porins for transport.

Staining Techniques: Gram Stain

The Gram stain is a differential staining technique used to distinguish between gram-positive and gram-negative bacteria.

• Process: Crystal violet and iodine stain cytoplasm; alcohol wash decolorizes gram-negative cells; safranin counterstains.

• Result: Gram-positive bacteria retain purple color; gram-negative appear pink/red.

Acid-Fast Cell Walls

Acid-fast bacteria (e.g., Mycobacterium and Nocardia) have cell walls rich in mycolic acid, a waxy lipid that resists decolorization.

• Staining: Carbolfuchsin binds strongly; acid-fast bacteria retain red color after acid-alcohol wash.

Atypical Cell Walls

• Mycoplasma: Lack cell walls; plasma membrane contains sterols for stability.

• Archaea: May lack cell walls or have walls with pseudomurein (NAG + N-acetyltalosaminuronic acid).

Cell Wall Damage and Antimicrobial Agents

Chemicals like lysozyme and antibiotics target bacterial cell walls, often without harming animal or plant cells.

• Lysozyme: Hydrolyzes peptidoglycan, especially in gram-positive bacteria.

• Protoplast: Wall-less cell after lysozyme treatment (gram-positive).

• Spheroplast: Gram-negative cell with damaged wall; membranes remain intact.

• L-cells: Bacteria that lose cell walls and become irregularly shaped; can revert to walled state.

Osmosis and Cell Wall Function

The cell wall protects bacteria from osmotic lysis in hypotonic environments.

• Isotonic solution: No net movement of water.

• Hypotonic solution: Water enters cell; cell wall prevents bursting.

• Hypertonic solution: Water leaves cell; cell shrinks (plasmolysis).

Plasma Membrane: Active Transport

Bacteria use ATP to transport substances against concentration gradients.

• Group translocation: Substance is chemically modified during transport (e.g., glucose phosphorylation).

Bacterial Cytoplasm and Internal Structures

• Cytoplasm: 80% water, contains proteins, sugars, ions, and other molecules.

• Cytoskeleton: Includes MreB, ParM (microfilaments), crescentin (intermediate filaments), FtsZ (microtubules).

• Nucleoid: Region containing bacterial chromosome (double-stranded DNA); not membrane-bound.

• Plasmids: Small, circular DNA molecules; carry genes for antibiotic resistance, toxin production, etc.

Bacterial Ribosomes

Ribosomes are the site of protein synthesis.

• Prokaryotic ribosome: 70S, composed of 30S (small) and 50S (large) subunits.

• Antibiotics: Streptomycin and gentamycin inhibit 30S; erythromycin and chloramphenicol inhibit 50S.

Bacterial Endospores

Certain gram-positive bacteria (e.g., Bacillus, Clostridium) form endospores under stress.

• Structure: Highly durable, dehydrated, thick-walled; contains DNA, RNA, ribosomes, enzymes, dipicolinic acid (DPA) with calcium ions.

• Sporulation: Formation process triggered by nutrient scarcity; involves membrane ingrowth, forespore formation, peptidoglycan deposition, and spore coat development.

• Germination: Endospore returns to vegetative state upon exposure to heat or germinants.

• Difference from fungal spores: Endospore formation is not reproduction; one cell forms one endospore.

Summary Table: Types of Microorganisms

Key Equations

Peptidoglycan Structure

Osmosis Principle

Ribosome Subunit Assembly

Conclusion

These notes provide a comprehensive overview of the microbial world and the functional anatomy of prokaryotic cells, focusing on cell wall structure, classification, staining techniques, and internal cellular components. Understanding these concepts is fundamental for further study in microbiology, microbial genetics, and biotechnology.