Medical Microbiology: Bacteria Classification

• Introduction to medical microbiology lecture

Case Scenario

• A 3-year-old boy presents with fever, difficulty breathing, and a cough with mucoid sputum.
• Examination reveals diffuse opacities on X-ray film.
• Sputum cultivation shows Gram-negative bacilli with rose-pink mucoid colonies on MacConkey agar.
• The patient was treated with penicillin, but showed no improvement, suggesting penicillin.

Classification of Creatures

• Creatures are classified into 5 kingdoms: Animalia, Plantae, Protista, Fungi, and Monera.
• Animalia, Plantae, Protista, and Fungi are eukaryotic.
• Monera are prokaryotic.
• Examples: Animalia (Helminths), Protista (Protozoa), Monera (Cellular Bacteria, Acellular Viruses).

Prokaryotes vs. Eukaryotes

• Nuclear Membrane: Absent in prokaryotes, present in eukaryotes.
• Chromosomal Number: One copy (haploid) in prokaryotes, two copies (diploid) in eukaryotes.
• Histones: Absent in prokaryotes, present with DNA in eukaryotes.
• Ribosome: 70S in prokaryotes, 80S in eukaryotes.
• Peptidoglycan: Present in prokaryotes, absent in eukaryotes.
• Mitosis: Absent in prokaryotes, present in eukaryotes.
• Membrane-Bound Organelles: Absent in prokaryotes, present in eukaryotes.

General Bacteriology

• Size of Bacterial Cell: Measured by micron (1 micron = 1/1000 mm).
• Smallest Bacterium: Serratia marcescens, 0.4μ.

Shape of Bacterial Cell

• Cocci: Spherical, e.g., Staphylococci.
• Bacilli: Cylindrical, e.g., Diphtheria bacilli.
• Spiral: One curve (Vibrio), more than one (spirochetes).

Bacterial Habitat

• Parasitic: Need a host, includes bacterial flora and pathogenic bacteria.
• Saprophytic: Free-living in soil, air, and water.

Gram Stain

• Gram Positive: Violet.
• Gram Negative: Red.
• Mycobacterium (TB) is not stained by Gram stain but by Ziehl-Neelsen stain.

Gram Stain Experimental Details & Materials

• Crystal violet is the primary stain
• Gram's iodine acts as a mordant.
• Acetone-alcohol functions as a decolorizing agent.
• Diluted carbol fuchsin 0.1% is used as a counterstain.

Gram Stain Procedure

• Crystal violet is applied first
• Then iodine
• Alcohol
• Plus diluted cabrol fuchsin, also called safranin

Bacterial Cell Structure - Surface Structures (Bacterial Envelope)

• Capsule
• Cell wall
• Cytoplasmic membrane

Bacterial Cell Structure - Internal Structures

• Nuclear body
• Ribosomes
• Inclusion bodies
• Flagella
• Fimbria
• Mesosomes

Cell Wall

• The cell wall is the layer outside the cytoplasmic membrane.

Cell Wall - Chemical Structure

• The bacterial cell wall consists of mucopeptide or peptidoglycan.
• Peptidoglycan consists of N-acetyl-glucosamine alternating with N-acetyl-muramic acid, with side chains of 4 amino acids.
• The peptidoglycan layer is present in both Gram-positive and Gram-negative bacteria.

Gram-Positive vs. Gram-Negative Bacteria

• Gram-positive bacteria have a thick peptidoglycan layer (up to 40 sheets), making up 50% of the cell wall material. Teichoic acid is either ribitol or glycerol.
• Gram-negative bacteria have a thin peptidoglycan layer (1 or 2 sheets), making up 5-10% of the cell wall material.
• Gram-negative bacteria have an outer membrane, lipopolysaccharide (lipid A = endotoxin, polysaccharide = somatic antigen), and a periplasmic space containing hydrolytic enzymes and penicillinase.

Functions of the Cell Wall

• Preservation of cell shape.
• Protection against high internal osmotic pressure.
• Antigenic character: Teichoic acid in Gram-positive bacteria, lipopolysaccharide in Gram-negative bacteria (O Ag).
• Toxicity: Lipopolysaccharide acts as an endotoxin.
• Staining reaction.
• Permeability of the cell (non-selectively permeable). -Target action of antibiotics like penicillin, and plays an essential role in cell division.

Cytoplasmic membrane Characteristics & Functions

• A very thin, elastic layer lies immediately under the cell wall.
• Composed of phospholipid and proteins.
• The membrane is responsible for transport and permeability (active, passive, or group translocation), electron transport and oxidative phosphorylation, excretion of hydrolytic enzymes and toxins, and biosynthetic functions (cell wall and membrane lipids).

Capsule

• A gelatinous material outside the cell wall, composed of polysaccharide or polypeptide (in B. anthracis).
• Glycocalyx: A polysaccharide-containing material lying outside the cell.
• Capsule: A condensed, well-defined layer closely surrounding the cell.
• Slim layer: A loosely surround the cell or glycocalyx.

Capsule Functions

• Protects the cell wall against bacteriophages, complement, and lysozymes.
• Protects bacterial cells against phagocytosis.
• Determines the organism's virulence.
• Antigenic.

Flagella

• Flagella are long, thread-like, helical filaments.
• Structure: Composed of contractile protein called flagellin.

Flagella - Types of Bacterial Flagellation

• Monotrichous
• Amphitrichous
• Lophotrichous
• Peritrichous

Flagella Function

• The organ of motility that helps movement toward optimal nutrients and oxygen, aids in colonization and assists pathogenic bacteria in penetration through viscous mucous secretion.
• They are highly antigenic (H-antigens).

Nuclear Bodies

• Bacteria have no true nuclei but a nucleoid region.
• Prokaryotes lack a nuclear membrane.
• The bodies are made of DNA.
• A single chromosome is visible by electron microscopy as a long, thin thread folded.

Fimbriae ("Pili")

• Short, rigid surface appendages formed by many Gram-negative bacteria.
• Two classes: ordinary and sex pili.
• Visible by electron microscope.

Fimbriae ("Pili") - differs from flagellae

• Occur in motile and non-motile strains
• 100-500 per cell
• Shorter and thinner than Flagallae
• more or less straight, flagellae are spiral.
• Not originated from cytoplasm.

Fimbriae ("Pili") - Function

• Ordinary pili facilitate adhesion to each other and to host cells
• Sex pili are responsible for conjugation.
• Fimbriae are antigenic
• Fimbriae is like a capsule that inhibits phagocytosis
• Ordinary pili contribute to virulence (colonization Ag, surface virulent-factor).