Bacterial Structure and Organization

Bacterial Structure & Organization

• The objectives for the lecture include identifying bacterial shapes/arrangements, differentiating bacteria using Gram's stain, describing cell structure, identifying cell component functions, defining cell-wall deficient bacteria, discussing structures external to the cell wall, and comparing capsules with spores.

Bacterial Morphology

• Size: Bacteria are measured in micrometers (1 μm = 1/1000 mm)
• Shape: Bacteria can be spherical (cocci), rod-shaped (bacilli), or helical (spiral). Spiral bacteria can be rigid (spirilla) or flexible (spirochetes).
• Arrangement: Bacterial arrangement is determined by their division planes. Bacteria can exist in pairs (diplo), chains (strepto), or clusters (staphy).
• Staining Characteristics:
  • Simple stain uses one dye to show one morphology. Methylene blue is an example.
  • Differential stain uses two dyes to show two characters for morphology and type
    • Gram stain & Ziehl-Neelsen

Gram's Stain & Ziehl-Neelsen Stain

• Gram-positive bacteria stain purple.
• Gram-negative bacteria stain pink.
• Ziehl-Neelsen Stain is used for bacteria that cannot be stained with Gram's stain due to the high lipid content of its cell wall,
  • Acid-fast bacteria, such as Mycobacterium tuberculosis, appear pink.
  • Non-acid-fast bacteria appear blue.

Bacterial Ultra-structures

• Essential structures: cell wall, cytoplasmic membrane, cytoplasm & cytoplasmic organelles (e.g., ribosomes), and chromosome (nucleoid)
• Additional structures: flagella, pili (fimbria), capsule, plasmid, and spore

Cell Wall

• A rigid structure that surrounds the cytoplasmic membrane.
• Rigidity is caused by peptidoglycan.
• Peptidoglycan is a polymer made of N-acetylglucosamine [NAG], N-acetylmuramic acid [NAM], and tetrapeptide side chains attached to NAM.
• Bacteria classification is Gram-positive or Gram-negative based on cell wall structure
• Gram-positive bacteria have a thick peptidoglycan layer and teichoic acids.
• Gram-negative bacteria have a thin peptidoglycan layer, periplasmic space, and an outer membrane composed of lipopolysaccharide (LPS).
• LPS includes lipid A (endotoxin) & polysaccharide (Somatic O antigen)
• Cell Wall:
  • Maintains the characteristic shape of the bacterium.
  • Supports the cytoplasmic membrane against osmotic pressure.
  • Plays a role in cell division.
  • Determines the staining affinity of the organism.

Cell-Wall Deficient Variants

• Cell-wall deficient variants do not stain by Gram's staining.
• Cell-wall deficient variants are resistant to cell wall inhibitors like Penicillin.
• Mycoplasma:
  • Naturally present
  • Cannot resume a walled form
• L-Forms:
  • Artificially induced by treating the cell wall with antibiotics or lysozyme
  • May or may not resume the walled form

Cell (Cytoplasmic) Membrane

• Composed of a phospholipid-protein bilayer similar to eukaryotic cells, but without sterols
• Functions: selective transport, respiration, cell wall biosynthesis, reproduction, chemotactic system, and excretion of extracellular enzymes

Cytoplasm & Cytoplasmic Contents

• Chromosome (Nucleoid): Single chromosome, circular double-stranded DNA (dsDNA) that carries the essential genetic information of the bacterial cell.
• Plasmids: Extra-chromosomal, small, circular dsDNA that replicates autonomously. They carry non-essential genes and virulence/antibiotic resistance genes.
• Bacterial ribosomes are 70s, while eukaryotic ribosomes are 80s making them a selective target for antibiotics
• Inclusion Granules: Stores nutrients & energy
• Mesosomes: Inward invaginations of the cell membrane into the cytoplasm, with functions in cell division, cell sporulation, and respiration.

External Appendages

• Flagella: Long, thick filaments made of flagellin, embedded in the cell membrane, and extend through the cell wall. Flagella are antigenic (Flagellar or "H" antigen).
• Flagella arrangement: all around a cell, at one pole or both poles.
• Motility function: Positive chemotaxis (towards nutrients) and negative chemotaxis (away from disinfectants)
• Axial filaments (Endoflagella): Found only in spirochetes. Fibers are located within the cell and overlap in the middle. Spirochetes are motile by axial filaments, rotating around the longitudinal axis.
• Pili or fimbriae: Short hair-like projections formed of pillin that is antigenic (fimbrial antigen)
  • Ordinary pili enable the organism to adhere to the host surfaces.
  • Sex pilus involved in the transfer of DNA between bacteria (conjugation).

Bacterial Capsule

• Extra-cellular polymer, outside the cell wall, formed only inside the host.
• Usually polysaccharide except for Bacillus anthracis that's formed of polypeptide.
• Gram staining will not stain the bacterial capsule and appears as unstained halo around the organism.
• Types: Capsule (well-defined, thick) or slime layer (not well defined, thin).
  • Capsule Function: A virulence factor to escape phagocytosis.
  • Slime Layer Function: Adherence and protects the cell from dehydration.
• The bacterial capsule assists protection from the host immune system/antibacterial agents and helps with attachment to host surface to colonize and establish infection.

Clinical Value (use) of Bacterial Capsule

• Vaccines: Bacterial capsules can stimulate the production of anti-capsular antibodies(e.g., Meningococcal vaccine, Pneumococcal vaccine, and Haemophilus influenzae vaccine)

Bacterial Spores (Endospores)

• Highly resistant resting phase of some bacteria, formed in response to unfavorable environmental conditions.
• Spores do not grow or reproduce and can remain dormant for many years.
• Spore-forming bacteria: Bacillus spp and Clostridium spp
• Spore structure: DNA + a small amount of cytoplasm covered by many layers (spore wall, cortex, spore coat).
• Sporulation: A single vegetative bacterium forms a single spore on exposure to unfavorable external environmental conditions. -Germination: Endospores can quickly respond, lose the coats and revert to the vegetative form when conditions improve. -Bacterial sporulation: Not a method of replication
• Spore Structure: The spore morphology can be central, terminal or sub-terminal location.
  • Spores are unstained by Gram's stain, but can be stained using special stains.
  • The spores can be oval or rounded depending on the species/strain of the bacteria.
  • Size is characteristic of the species & help in organism identification/classification
• Spore Viability & Resistance: Is due to to high content of Ca++ & dipicolinic acid, impermeability of their cortex & outer coat, low content of water, and low metabolic & enzymatic activity. Moist heat at 121°C for 20 minutes kills spores, while 60°C kills vegetative bacteria

Comparison of the bacterial capsule with the bacterial spore.

• Capsule forms inside the host, bacterial spore forms outside the host.
• Capsules can replicate with the cell, spores cannot replicate.
• Capsule Functions: Protection from the host's immune system
• Spores Function: Protection from bad external conditions of the environment.