Applied Microbiology Lecture Review

Components of Bacteria That Aid in Surface Attachment

• Fimbriae (Pili): Thin, hair-like structures that help bacteria adhere to surfaces, other cells, and tissues.

• Capsules: Gel-like layers surrounding some bacteria, providing protection against phagocytosis and aiding in adhesion.

• S-layer: A proteinaceous layer that can serve as a protective coat and facilitate attachment to surfaces.

Gram Stain Mechanism

• The Gram stain works through a multi-step process:

  • Crystal Violet Staining: Bacteria are first stained with crystal violet, which penetrates both Gram-positive and Gram-negative bacteria.

  • Iodine Treatment: Iodine acts as a mordant, forming a complex with crystal violet that gets trapped in the cell wall.

  • Decolorization: A critical step where Gram-negative bacteria lose the crystal violet stain due to their thinner peptidoglycan layer and the presence of an outer membrane, while Gram-positive bacteria retain it.

  • Counterstaining: A secondary stain, usually safranin, is applied, which colors the decolorized Gram-negative bacteria pink while Gram-positive bacteria remain purple.

• This process differentiates bacteria based on cell wall composition and structure.

Complexity of Flagella in Gram-Negative Bacteria

• Gram-negative bacteria possess a more complex flagellar structure due to their two-membrane system (outer and inner membranes) and a periplasmic space.

• Their flagella consist of additional components, including:

  • Basal Body: Anchored in the cell envelope, which is more intricate due to the need to pass through both membranes.

  • Hook and Filament Structure: They adapt to accommodate the multilayered cell wall and allow for more flexible movement.

• In contrast, the simpler Gram-positive bacteria have a single membrane and a thicker peptidoglycan layer that simplifies flagellar assembly.

Bacterial Endospore Formation

• Endospores are dormant structures formed by some bacteria as a survival mechanism in response to environmental stress (e.g., nutrient depletion, extreme temperatures).

• The endospore protects the bacterial genome and vital cellular components, allowing the bacteria to endure conditions that would normally be lethal.

• Once favorable conditions return, the endospore can germinate, giving rise to a viable bacterial cell.

Mechanisms of Bacterial Pathogenicity

• Bacteria can make us sick through various mechanisms:

  • Invasion of Host Tissues: Some bacteria invade host cells, disrupting normal function.

  • Exotoxin Production: Pathogenic bacteria may secrete potent exotoxins that interfere with cellular processes or damage host tissues (e.g., cholera toxin).

  • Biofilm Formation: Bacteria can form biofilms that protect them from the immune system and antibiotics, increasing persistence and resistance.

  • Immune Evasion: Certain bacteria develop strategies to evade or suppress the host immune response, facilitating infection.

Differences Between Bacteria and Archaea

• Cell Wall Composition: Bacteria typically have peptidoglycan in their cell walls, while archaea possess unique polymers like pseudopeptidoglycan.

• Membrane Lipids: Bacterial membranes consist mainly of fatty acids, whereas archaeal membranes contain ether-linked lipids, providing stability in extreme environments.

• Ribosomal Structure: Differences in ribosomal RNA and protein synthesis machinery contribute to their distinct evolutionary pathways, with archaea more closely related to eukaryotes.

Human Bacterial Pathogens and Their Exotoxins

• Staphylococcus aureus: Produces enterotoxins and toxic shock syndrome toxin (TSST) contributing to food poisoning and severe illness.

• Escherichia coli (E. coli): Certain strains produce Shiga toxin, causing severe gastrointestinal disease and potentially leading to kidney failure.

• Clostridium botulinum: Produces botulinum toxin, the most potent toxin known, leading to botulism, which results in paralysis and can be life-threatening.