Capsules, Flagella, Spores, Adhesins, Toxins

Capsules

  • Function: Slimy capsule on surface protects the cell wall from external challenges.

  • Composition: Made up of polysaccharides (sugars) or polypeptides (proteins).

  • Importance: Capsules help bacteria stay hidden from host immune responses by forming various structures:

    • Slime: Blobs of capsule material.

    • Glycocalyx: Small sheets of capsule material.

    • Biofilm: Large thick sheets of capsular material which reinforce bacterial adherence through strong attachments by fimbriae.

Biofilm Formation

  • Example: Streptococcus mutans uses sugars in the mouth to create capsules, which protect the bacteria while releasing acids that damage teeth.

  • Clinical Relevance:

    • Urine catheters can develop biofilms that protect bacteria from antibiotics.

    • Biofilms are a common cause of urinary tract infections (UTIs).

Quorum Sensing in Biofilms

  • Communication: Bacteria in biofilms communicate through quorum sensing, allowing them to coordinate gene expression and functions based on population density.

  • Horizontal Gene Transfer (HGT): This increases genetic diversity and adaptability of bacterial populations.

Flagella

  • Movement: Flagella can rotate clockwise or counterclockwise, affecting bacterial direction.

  • Chemotaxis: The movement of bacteria toward or away from chemical stimuli, guided by flagellar motion.

  • Mechanics:

    • Flagella are attached to the cell wall with rotary rings controlled by rotors powered by proton motive force generated by hydrogen ions.

Spores

  • Germination Process:

    1. Cell stops growing.

    2. DNA replicates.

    3. Septum forms.

    4. Forespore separates.

    5. Engulfment of the daughter spore.

    6. Thickening of peptidoglycan – core wall and cortex formation.

    7. Formation of spore coat from mother cell wall.

  • Examples of Spore Formers: Known from previous study sections, vegetative cells form spores under unfavorable conditions. Spores can remain dormant for years without undergoing binary fission.

Fimbriae

  • Structure: Protein structures aiding in bacterial attachment and twitching movements.

  • Function: Prevents bacteria from being washed away and enhances adherence strength in biofilms, making pathogenic bacteria harder to eliminate.

Defense Enzymes

  • Catalase: An enzyme that converts hydrogen peroxide into water and oxygen; it protects bacteria from oxidative damage. Bacterial colonies that are catalase negative are susceptible to hydrogen peroxide.

Pathogenic Enzymes

  • Hyaluronidase: Breaks down intercellular cement (hyaluronic acid), facilitating bacterial invasion.

  • Collagenase: Degrades the basal layer of collagen, allowing bacterial penetration into tissues.

  • Mucinase: Breaks down mucin, enabling pathogens to burrow deeper into tissues.

  • Example Pathogen: Streptococcus pyogenes breaks down red blood cells and mucin, enhancing its invasive ability.

Microbial Toxins

  • Exotoxins: Highly potent proteins that damage cells by binding to specific receptors and disrupting cellular function.

    • Diphtheria Toxin: Cleaved into two fragments, where fragment B binds to the host cell and fragment A inhibits protein synthesis by ADP-ribosylation of elongation factor 2.

  • Endotoxins: Part of the microbial structure, typically released only when the bacterium is damaged. A notable example is lipopolysaccharide from Gram-negative bacteria.

Phenotype and Genotype

  • Genotype: Refers to all characteristics of microorganisms studied at the DNA or RNA level.

  • Phenotype: Includes all observable characteristics such as morphology, enzymes, and toxin production.