D Amino Acids

Role in Bacterial Cell Walls

D-amino acids are isomers of amino acids not used in enzymes and are incorporated into bacterial cell walls, providing resistance to enzymatic degradation.

• Isomeric forms: D-amino acids differ from L-amino acids, which are commonly found in proteins.

• Function: D-amino acids are not substrates for most enzymes, making the cell wall more resistant to attack.

• Competition: There is no competition for these isomers in cell wall synthesis.

Gram-Positive vs. Gram-Negative Bacteria

Major Structural Differences

Bacterial classification into Gram-positive and Gram-negative is based on differences in cell wall structure, which affects staining, antibiotic susceptibility, and pathogenicity.

• Gram-Positive:

  • Simple cell wall structure

  • Thick peptidoglycan layer

  • NAG-NAM dimer with beta 1,4 linkage

  • Addition of teichoic acid

  • Single cytoplasmic membrane

• Gram-Negative:

  • Complex cell envelope

  • Thin peptidoglycan layer

  • Outer membrane containing lipopolysaccharide (LPS) and porins

  • Periplasmic space between outer and inner membranes

  • Inner cytoplasmic membrane

• Peptidoglycan: Identical chemical structure in both types, but thickness and associated molecules differ.

Gram Staining

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

• Purpose: Determines Gram type for identification and treatment decisions.

• Steps:

  1. Apply primary stain (Crystal Violet): stains all cells purple by binding peptidoglycan.

  2. Add mordant (Iodine): forms stronger bonds.

  3. Decolorize (Alcohol): removes stain from Gram-negative (thin wall), Gram-positive retains stain.

  4. Counterstain (Safranin): stains Gram-negative pink, Gram-positive remains purple.

• Results:

  • Gram-positive: purple

  • Gram-negative: pink

Bacterial Cell Wall Components

Peptidoglycan

Peptidoglycan is a polymer of sugars and amino acids forming a mesh-like layer outside the plasma membrane.

• Structure: N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) linked by beta 1,4 glycosidic bonds.

• Function: Provides rigidity and protection against osmotic pressure.

Teichoic and Lipoteichoic Acids (Gram-Positive)

• Teichoic acid: Anchored to peptidoglycan, provides antigenic specificity and regulates cation movement.

• Lipoteichoic acid: Anchored in cytoplasmic membrane, helps anchor cell wall, not accessible to outside.

Lipopolysaccharide (LPS) (Gram-Negative)

LPS is a major component of the outer membrane of Gram-negative bacteria, contributing to structural integrity and pathogenicity.

• Three portions: O-polysaccharide, Core polysaccharide, Lipid A

• Lipid A: Toxic component, acts as endotoxin, triggers immune response (septic shock)

• O-polysaccharide: Highly antigenic, variable, used for bacterial identification

• Core polysaccharide: Conserved structure

Porins

• Location: Outer membrane of Gram-negative bacteria

• Function: Channels for solutes to enter and exit, contribute to antibiotic resistance

Antibiotics

Types and Mechanisms

• Bacteriostatic: Stops bacterial growth, does not kill

• Bactericidal: Kills bacteria directly

Streptococcal Diseases and Pathogenicity

Streptococcus pyogenes

• Causative agent: Strep throat, skin infections (impetigo, erysipelas), and other diseases

• Virulence factors: M protein (antiphagocytic, highly variable), superantigens

• Complications: Untreated infections can lead to toxic shock syndrome and scarlet fever

Toxic Shock Syndrome

• Cause: Toxins (e.g., Streptococcal Toxic Shock Syndrome Toxin)

• Symptoms: Headache, fever, red eyes, rash, pain in hands/feet, high fatality rate in severe cases

Superantigens

• Definition: Proteins that overstimulate the immune system, leading to harmful inflammation and cell death

• Effect: Can cause immune system to attack host tissues

Glycocalyx, Capsules, and Slime Layers

Structure and Function

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

• Capsule: Well-organized glycocalyx, prevents phagocytosis

• Slime layer: Loosely organized, aids in adherence

Griffith, Avery, MacLeod & McCarty Experiment

• Context: Demonstrated transformation in bacteria using rough (nonvirulent) and smooth (virulent) strains of Streptococcus pneumoniae

• Findings: Heat-killed smooth strain + live rough strain = mouse dies, showing genetic material transfer

Endospores

Formation and Structure

• Produced by: Bacillus and Clostridium species

• Function: Survival under harsh conditions (heat, chemicals, UV, dehydration)

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

• Resistance: Highly resistant due to dipicolinic acid (DPA), Ca2+, and small acid-soluble proteins (SASPs)

Sporulation and Germination

• Induced by: Oxygen exposure, nutrient deprivation, chemicals, UV light

• Germination: Triggered by water, food, and removal of environmental stress

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

Bacterial Motility

Flagella

• Structure: Filamentous appendages made of flagellin, powered by proton motive force

• Types:

  • Peritrichous: flagella all over cell

  • Monotrichous: single polar flagellum

  • Lophotrichous: tuft at one pole

• Function: Propels bacteria, enables chemotaxis and phototaxis

Flagella Biosynthesis and Rotation

• Biosynthesis: Hollow core, flagellin transported through channel

• Rotation: Driven by proton motive force (), ions flow through channel, causing rotation

Movement Patterns

• Run-Tumble-Run: Run: smooth forward motion; Tumble: stops and jiggles, changes direction

• Chemotaxis: Movement in response to chemicals; phototaxis: response to light

Other Motility Types

• Axial filament: Endoflagella in spirochetes, corkscrew motion

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

• Twitching motility: Uses type IV pili, energy from ATP hydrolysis

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

Malaria

Overview

• Cause: Protist disease caused by Plasmodium

• Transmission: Mosquito vector

• Symptoms: Fever, chills, splenomegaly, anemia

• Diagnosis: Blood smear, detection of Plasmodium in red blood cells

• Prevention/Treatment: Antimalarial drugs, mosquito control

Lifecycle

• Stages: Mosquito injects sporozoites, infects liver, multiplies, enters bloodstream, infects red blood cells

• Co-evolution: Human genetic traits (e.g., sickle cell) confer resistance

Additional info: Some explanations and terminology have been expanded for clarity and completeness. Tables from the original notes have been described in text for accessibility.