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Introduction to Bacteria

• Bacteria are unicellular organisms classified within the domain of life.

• They possess distinct RNA sequences and cell walls containing peptidoglycan.

• Bacteria are ubiquitous and primarily free-living, with sizes ranging from 0.5 to 10 micrometers in diameter.

Structure of Bacterial Cells

• Cell Organization:

  • Bacterial cells lack a nucleus and membrane-bound organelles (e.g., mitochondria, Golgi bodies).

  • DNA is located in the cytoplasm, not packaged in chromatin as in eukaryotes.

• Cell Components:

  • Flagellum: Locomotive structure.

  • Ribosomes: Responsible for protein synthesis (smaller than eukaryotic ribosomes).

  • Cell Membrane: Encloses cell contents; controls substance transport.

  • Cell Wall: Provides rigidity and shape to the cell.

  • Capsule: Some bacteria have an outer coating that can aid in protection and attachment.

  • Nucleoid: Area containing bacterial DNA (double-stranded).

Classification of Bacteria

• Shape and Arrangement:

  • Cocci (singular: coccus): Spherical shape. Variants include:

    • Diplococci: Cocci in pairs (e.g., Neisseria spp).

    • Streptococci: Cocci in chains (e.g., Streptococcus pneumoniae).

    • Staphylococci: Irregular clusters (e.g., Staphylococcus aureus).

    • Tetrads: Groups of four cocci (e.g., Micrococcus spp).

    • Sarcinae: Cocci in cubes or packets.

  • Bacilli (rod-shaped bacteria): Variants include:

    • Diplobacilli: Bacilli in pairs.

    • Streptobacilli: Bacilli in chains.

    • Coccobacilli: Short, rounded bacilli (e.g., Haemophilus influenzae).

  • Spiral Shapes:

    • Vibrios: Comma-shaped bacteria.

    • Spirilla: Rigid, spiral-shaped.

    • Spirochaetes: Flexible, helical bacteria (e.g., Treponema, Borrelia).

  • Other Shapes: Include filaments and pleomorphic bacteria that can change forms.

Morphological Characteristics and Functions

• Cell Wall Composition:

  • Composed of peptidoglycan (murein), a polymer consisting of sugars (N-acetylglucosamine and N-acetylmuramic acid) and amino acids.

  • Cell wall is critical for determining bacterial shape and is a target for certain antibiotics.

• Gram Staining:

  • Gram-positive bacteria: Thick peptidoglycan layer, stains purple.

  • Gram-negative bacteria: Thinner peptidoglycan layer, stains pink/red due to an outer membrane configuration containing lipopolysaccharides (LPS).

• Cytoplasmic Membrane:

  • Located beneath the cell wall, composed of phospholipids and proteins.

  • Functions in transport, metabolism, and as a barrier to protect the cell.

• Cytoplasm:

  • Contains ribosomes, plasmids (small circular DNA), and nutrients for metabolic processes.

Appendages and Structures

• Flagella:

  • Structures used for movement: types include monotrichous, lophotrichous, amphitrichous, and peritrichous.

  • Functionality in motility and attachment, important for virulence in some species.

• Pili (Fimbriae):

  • Thin, hair-like appendages that facilitate attachment to host tissues.

  • Sex Pilus: Specialized for gene transfer during conjugation.

• Capsules:

  • Protective layers that enhance pathogenicity, hinder phagocytosis, and assist in adhesion.

Special Adaptations

• Spores:

  • Some bacteria can form endospores or exospores, which are dormant forms that enhance survival in harsh conditions.

• Cysts:

  • Thick-walled structures created in response to adverse conditions, allowing for survival until favorable conditions resume.

Conclusion

• Bacterial morphology is integral to their survival, pathogenicity, and adaptability.

• Understanding the structural components of bacteria and their functions is crucial for microbiology.