5 Bacterial cell features

Acid-Fast Bacteria

  • Examples: Mycobacterium and Nocardia

  • Cell Envelope Complexity:

    • Mycolic acids (long, branched fatty acids) are attached to polysaccharide arabinogalactan.

    • Arabinogalactan is bound to peptidoglycan, forming a thick, waxy membrane layer that impedes gram staining.

Acid-Fast Staining Process

  • Primary Stain: Carbolfuchsin (red), which contains phenol and alcohol.

  • Decolorization Step: Acid alcohol removes carbolfuchsin not bound to mycolic acid.

  • Counterstain: Methylene blue, which does not penetrate mycolic acid but provides contrast to non-acid-fast cells.

  • Developed by Paul Ehrlich in 1882.

Structure of Acid-Fast Cell Wall

  • Components:

    • Porins

    • Surface proteins

    • Acyl lipids

    • Mycolic acid

    • Arabinogalactan

    • Lipoarabinomannan

    • Peptidoglycan

    • Cytoplasmic membrane

Additional Bacterial Structures

  • Features of Different Bacterial Groups:

    • Various components serve different functions based on genetic capability.

    • Ability to form non-essential components can be gained or lost over time.

    • Example: Streptococcus mutans forms a capsule only in the presence of sugar.

Capsular Polysaccharide

  • Also referred to as glyocalyx, which is a polysaccharide layer.

  • Capsules can significantly increase in size compared to the cellular structure.

  • Capsules inhibit phagocytosis by immune cells, aiding pathogenic bacteria.

  • Capsules may be lost during culture.

  • India Ink Stain:

    • Negative stain for bacterial capsules, resulting in cells appearing purple and capsules clear against a dark background.

Flagella

  • Etymology: From Latin, flagrum = whip.

  • Function: Motility in bacteria.

  • Structure Differences: Bacterial flagella consist of flagellin; they rotate instead of bending, powered by proton motive force. Used for chemotaxis (movement towards chemical stimuli).

Flagellum Types

  • Types of Flagellar Arrangement:

    • a) Peritrichous

    • b) Polar

    • c) Lophotrichous

  • Structural components include:

    • Filament, Hook, Rings (L, P, MS), Rod, and Mot proteins.

Visualization of Flagella

  • Staining Technique: Leifson flagella stain

  • Method: Electron microscopy used for detailed visualization.

Pili (Fimbriae)

  • Functions:

    • Exchange of genetic material (conjugation).

    • Adhesive tips allow binding to specific receptors on host cells, enhancing pathogenicity and biofilm formation.

Plasmids

  • Definition: Double-stranded circular DNA separate from chromosomal DNA.

  • Independence: Can replicate independently and be passed to daughter cells.

  • Functions: Carry non-essential genes that may confer advantages, like antibiotic resistance or nitrogen fixation.

    • Example: Rhizobium sym plasmids contain nif genes for nitrogen fixation.

Endospores

  • Characteristics: Resilient structures containing bacterial DNA, protecting it under adverse conditions.

  • Formation: Typically produced by Clostridium and Bacillus genera; formation takes around 8 hours.

Components of Endospores

  • Core: Contains genome, ribosomes, enzymes, and dipicolinic acid; has a very low water content.

  • Coats: Three layers, including specific proteins and peptidoglycan.

  • Exosporium: Protein cover.

Resilience of Endospores

  • Endospores resist:

    • Chemicals (acids, alkalis, detergents)

    • Physical factors (heat, irradiation, desiccation)

    • They are extremely resistant due to low water content and the presence of dipicolinic acid.

Germination of Endospores

  • Triggering: Senses nutrient presence through receptors.

  • Activation: Cortex lytic enzymes are activated shortly after triggering.

  • Metabolism Begins: Complete metabolic activity is achieved approximately 1 hour after germination starts.

Importance of Endospore Formation

  • Many pathogenic bacteria form endospores, which are difficult to eliminate.

  • Standard boiling is ineffective; autoclaving is necessary (15 minutes at 121°C, 15 psi).

  • Endospore producers may contribute to food spoilage and poisoning, necessitating management in food production.

  • Example: Bacillus thuringiensis produces Bt-toxin during sporulation, which is effective for biological control of pests.

Questions

  1. Name one group of acid-fast bacteria

  2. How does a bacterial flagellum move?

  3. Who is forming endospores?

  4. When are endospores formed?