Fundamentals of Microbiology and Bacterial Cell Structure

Pioneers of Microbiology

• Louis Pasteur: Father of microbiology.
• Alexander Fleming: Discovered penicillin.
• Robert Koch: Proposed the germ theory of disease.
• Joseph Lister: Introduced the use of antiseptics in surgery.

Prokaryotic vs. Eukaryotic Cells

• Prokaryotic cell: Small and simple without any membrane-bound organelles; contains peptidoglycan.
• Eukaryotic cell: Larger and more complex; contains mitochondria but no peptidoglycan.

Bacterial Cell Structures

Bacteria: The most abundant organisms on Earth, usually playing a positive role.

External Structures

• Glycocalyx (sugar coat): An adhering layer of polysaccharides and proteins that protects against dehydration and serves as a source of food.
• Biofilms: Microcolonies of bacteria attached to surfaces.
• Flagella: Long, filamentous structures used for movement, consisting of three basic parts:
  1. Filament: The long, outermost region formed of the protein flagellin.
  2. The Hook: A wider section made of a different protein.
  3. The Basal Body: A small central rod inserted into a series of rings.
• Fimbriae and Pili: Short and thin hair-like fibers formed of pilin protein.
  1. Fimbriae: Used for the attachment of bacteria to surfaces.
  2. Pili: Used in the transfer of genetic material between cells.

The Cell Envelope

• Cell wall: A complex, semi-rigid structure made of peptidoglycan that provides shape and protection to the bacteria.
• Gram-positive bacteria: Characterized by a thick peptidoglycan layer (60–90%) containing teichoic acid, which controls the movement of positively charged ions.
• Gram-negative bacteria: Characterized by a thin peptidoglycan layer (10%) and no teichoic acid.
• Periplasmic space: The space between the outer membrane and the plasma membrane.
• Porins: Proteins that form pores in the outer membrane of Gram-negative bacteria to allow the passage of hydrophilic nutrients.
• Cell membrane: Composed of phospholipids and proteins. It acts as a selective barrier for materials entering or exiting the cell. Its functions include:
  • Production of energy by enzymes.
  • Contains enzymes needed for the synthesis of the cell wall.

Internal Structures

• Cytoplasm: Contains proteins, amino acids, sugars, nucleotides, salts, and vitamins.
• Nucleoid: A closed loop of DNA; haploid (1N); undergoes no meiosis, only binary fission.
• Plasmids: Smaller than the bacterial chromosome; extrachromosomal elements.
• Ribosomes: Located in the cytoplasm; synthesize proteins; composed of RNA (note: human ribosomes are 80S).

Phases of Bacterial Growth

• Lag phase: No cell division occurs. This is an adaptation phase where bacteria grow in size, store nutrients, and synthesize enzymes.
• Log phase: Binary fission takes place, and the number of bacteria doubles.
• Stationary phase: Nutrients decrease, wastes increase, and the reproduction rate equals the death rate.
• Decline phase: The death rate is higher than the reproduction rate.
• Generation time: The time interval required for bacteria to divide (differs from one species to another).