Bacterial Cell Structure

D Amino Acids

D amino acids are isomers of amino acids that are not used in enzymes and are found in bacterial cell walls. Their presence helps bacteria resist enzymatic degradation and provides structural stability.

• Isomeric forms: D amino acids are not utilized by enzymes, reducing competition for these isomers.

• Role in cell wall: Contribute to the rigidity and resistance of peptidoglycan.

Gram-Positive vs. Gram-Negative Bacteria

Bacteria are classified based on their cell wall structure, which affects their staining properties and susceptibility to antibiotics.

• Gram-Positive:

  • Simple cell wall structure

  • Thick peptidoglycan layer

  • N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) dimer

  • Beta 1,4 linkage

  • Addition of teichoic acid

• Gram-Negative:

  • Outer membrane contains lipopolysaccharide (LPS) and porins

  • Periplasmic space between outer and cytoplasmic membranes

  • Thin peptidoglycan layer

  • More complex, rough structure

Comparison Table:

Cell Wall Components

• Peptidoglycan: Polymer of sugars and amino acids forming a mesh-like layer outside the plasma membrane.

• Teichoic Acid: Found in Gram-positive bacteria; provides rigidity and regulates cation movement.

• Lipoteichoic Acid: Anchored in cytoplasmic membrane, helps keep cell wall attached.

• Lipopolysaccharide (LPS): Major component of Gram-negative outer membrane; consists of O-polysaccharide, core polysaccharide, and Lipid A.

• Porins: Proteins in Gram-negative outer membrane that form channels for solute transport.

LPS Structure and Function

• O-polysaccharide: Variable, antigenic specificity.

• Core polysaccharide: Conserved, structural role.

• Lipid A: Toxic, triggers immune response (endotoxin).

LPS Table:

Gram Staining

Purpose and Steps

Gram staining is a differential stain used to distinguish Gram-positive from Gram-negative bacteria based on cell wall properties.

• Purpose: Determines bacterial classification for diagnosis and treatment.

• Steps:

  1. Apply crystal violet (primary stain) – all cells turn purple.

  2. Add iodine (mordant) – forms stronger bonds.

  3. Decolorize with alcohol – removes stain from Gram-negative cells.

  4. Counterstain with safranin – Gram-negative cells turn pink, Gram-positive remain purple.

• Color Results:

  • Gram-positive: Purple

  • Gram-negative: Pink

Bacterial Pathogenesis

Streptococcal Diseases

Streptococcus pyogenes is a major pathogen causing diseases such as strep throat, impetigo, and erysipelas. Some strains carry lysogenic bacteriophages encoding toxins responsible for toxic shock syndrome and scarlet fever.

• M protein: Highly variable, anti-phagocytic, adhesive, and invasive properties.

• Superantigen: Overstimulates immune system, can lead to severe disease.

• Toxic Shock Syndrome: Caused by toxins, symptoms include fever, rash, and multi-organ failure.

Glycocalyx and Capsule

• Glycocalyx: External to cell wall, gelatinous and viscous, contributes to virulence and biofilm formation.

• Capsule: Prevents phagocytosis, increases bacterial survival.

Griffith, Avery, MacLeod & McCarty Experiment

Demonstrated transformation in bacteria, showing that DNA is the genetic material.

• Rough strain (nonvirulent): Mouse lives

• Smooth strain (virulent): Mouse dies

• Heat-killed smooth strain: Mouse lives

• Rough strain + heat-killed smooth strain: Mouse dies

Endospores

Formation and Structure

Endospores are highly resistant structures formed by Bacillus and Clostridium species for survival under harsh conditions.

• Structure: DNA, cortex, cell wall, spore coat, exosporium

• Induction: Triggered by oxygen exposure, nutrient deprivation, chemicals, UV light

• Germination: Initiated by water, food, and oxygen removal

• Major events: DNA copied, engulfment, late sporulation, maturation, mother cell lysis, germination

Spore Resistance: Due to dipicolinic acid (DPA), Ca2+, and small acid-soluble proteins (SASPs)

Bacterial Motility

Flagella and Movement

Bacteria move using flagella, which are filamentous appendages powered by proton motive force.

• Types of flagella arrangement:

  • Peritrichous: Flagella all over

  • Monotrichous: Single polar flagellum

  • Lophotrichous: Tuft at one pole

• Flagella structure: Hollow core, powered by hydrogen ion gradient

• Movement: Run (smooth forward), tumble (random direction)

• Rotation: Driven by proton motive force ()

Chemotaxis and Phototaxis

• Chemotaxis: Movement in response to chemicals

• Phototaxis: Movement in response to light

• Run-Tumble Model: Bacteria alternate between running and tumbling to navigate environments

Other Motility Types

• Axial Filament: Found in spirochetes, enables corkscrew movement

• Gliding Motility: Movement across solid surfaces, requires slime layer

• Twitching Motility: Uses type IV pili, powered by ATP hydrolysis

• Archeal Flagellum: Smaller, powered by ATP, simpler structure

Malaria

Overview

Malaria is a protist disease caused by Plasmodium species, transmitted by mosquitoes.

• Lifecycle: Involves human and mosquito hosts, with complex stages in blood and liver

• Diagnosis: Blood tests for Plasmodium antigens

• Symptoms: Fever, chills, splenomegaly, anemia

• Prevention and Treatment: Antimalarial drugs, mosquito control

Additional info: Some explanations and tables were expanded for clarity and completeness based on standard microbiology knowledge.