Microbiology Overview and Concepts

Study Notes

Microbiology

Study of microscopic organisms
Single-celled organisms or microbes
Microbial cells carry out all functions to maintain themselves and reproduce
Microorganisms are either Eukaryotes (e.g. fungi) or Prokaryotes (e.g. bacteria)

Eukaryotes vs Prokaryotes

Eukaryotes
- Larger in size
- Have a distinct nucleus surrounded by a nuclear membrane
- Have enclosed organelles, like mitochondria
- Examples include fungi
Prokaryotes
- Smaller in size
- No nucleus
- Lack enclosed organelles
- Examples include bacteria

Viruses

Smallest of the infective agents
Lack cell structure
Obligate intracellular parasites, relying on host cells for replication and survival

General Bacteriology

Morphology of Bacterial Cell

Size: Measured in micrometers (µm), range from 0.2-1.2 µm in width and 0.4-14 µm in length
Shape
- Cocci: spherical or round
- Bacilli: rod-shaped
- Spiral: twisted or curved
- Some bacteria are pleomorphic, meaning they have variable shapes
Arrangement: How bacteria group together
- Cocci
  - Pairs (diplococci)
  - Chains (streptococci)
  - Irregular clusters (staphylococci)
- Bacilli
  - Pairs
  - Chains
  - No special arrangement
Staining
- Different bacteria stain differently with dyes, providing information about their cell wall structure
Motility
- Flagella: enable bacteria to move
- Pili (Fimbriae): short hair-like structures assisting in attachment to surfaces
Capsule: Extracellular layer protecting bacteria and aiding in attachment

Bacterial Structures

Bacterial Coats
- Cell Membrane
  - Innermost layer, composed of a phospholipid bilayer
  - Differs from eukaryotic cell membranes by lacking sterols
  - Functions include transport of molecules, excretion of enzymes, respiration, and cell division
- Cell Wall
  - Surrounds the cell membrane, providing structural support and rigidity
  - Composed of peptidoglycan, a unique molecule that bacteria walls are made of
  - Gram-positive and Gram-negative bacteria differ in the composition of their cell walls
  - Functions include maintaining shape, resisting osmotic pressure, playing a role in cell division, and determining staining characteristics
  - Wall-deficient variants: Mycoplasma, which lack a cell wall, are naturally resistant to cell wall inhibitors like penicillins
- Capsule
  - Extracellular layer made of polysaccharides
  - Forms in the host environment (in-vivo)
  - Functions include adherence to host tissues, contributing to virulence, and aiding in identification
Cytoplasmic Structures
- Nucleoid
  - Contains the genetic information of the bacteria cell
  - Single circular molecule of double-stranded DNA
  - Supercoiled to fit inside the cell
- Plasmids
  - Extrachromosomal DNA molecules
  - Can be present in some bacteria
  - Replicate independently of the chromosome
- Ribosomes
  - Sites of protein synthesis
  - Composed of protein and RNA
  - Smaller in prokaryotes than in eukaryotes
  - Antibiotics can target this difference
- Inclusion Granules
  - Store nutrients, such as phosphates, sulfur, and carbohydrates
External Appendages
- Flagella
  - Enable movement
  - Help in chemotaxis: movement towards nutrients or away from harmful substances
  - Location and number vary by bacterial species
- Pili (Fimbriae)
  - Protein tubes extending from the cell
  - Shorter and thinner than flagella
  - Functions:
    - Adherence to surfaces
    - Conjugation: transfer of DNA between bacteria via a specialized pilus (sex pilus)