Structure: Long, helical protein filaments (flagellin) extending from the cell surface. Anchored by a basal body and connected by a hook.
Function: Motility (propulsion by rotation, like a propeller), enabling chemotaxis (movement in response to chemical stimuli) and phototaxis.
Arrangements:
- Monotrichous: Single flagellum at one pole.
- Amphitrichous: Single flagellum at each pole.
- Lophotrichous: Tuft of multiple flagella at one pole.
- Peritrichous: Flagella distributed over the entire cell surface.

Pili (Fimbriae):

Structure: Shorter, thinner, and more numerous protein appendages than flagella. Made of pilin protein.
Function:
- Fimbriae: Primarily for adhesion to host cells or surfaces (important for colonization and biofilm formation).
- Sex Pili (F pilus): Longer, less numerous. Involved in conjugation for genetic exchange.

Inclusion Bodies (Storage Granules):

Structure: Intracellular aggregates of various substances (e.g., poly-$\beta$-hydroxybutyrate (PHB) for carbon/energy, glycogen, polyphosphate/volutin granules, sulfur granules).
Function: Storage of nutrients, energy reserves, or metabolic byproducts.

Endospores:

Structure: Highly resistant, dormant structures formed intracellularly by certain Gram-positive bacteria (e.g., Bacillus, Clostridium) in response to nutrient depletion or adverse conditions. Contain a core with DNA, ribosomes, dipicolinic acid, and calcium, surrounded by cortex and spore coats.
Function: Survival mechanism, resistant to heat, radiation, chemicals, desiccation. Can germinate back into metabolically active vegetative cells when conditions improve.

VI. Bacterial Reproduction & Genetic Exchange

A. Asexual Reproduction: Binary Fission

This is the primary mode of bacterial proliferation, resulting in two genetically identical daughter cells.
Process:
1. Cell Elongation & DNA Replication: The cell grows in size, and the circular bacterial chromosome replicates, usually starting from a single origin of replication.
2. Chromosome Segregation: The two identical chromosomes move to opposite ends of the elongating cell, often aided by attachment to the cell membrane.
3. Septum Formation: A new cell wall and cell membrane begin to grow inward from the periphery, forming a transverse septum in the middle of the cell.
4. Cell Division: The septum completes its formation, dividing the parent cell into two separate, genetically identical daughter cells.
Growth Rate: Under optimal conditions, bacteria can divide very rapidly (e.g., E. coli every $20$ minutes).

B. Genetic Recombination (Horizontal Gene Transfer - HGT)

While not sexual reproduction in the eukaryotic sense, HGT allows bacteria to acquire new genetic material from other bacteria, leading to genetic diversity and adaptation.

1. Conjugation ("Bacterial Mating"):
- Definition: Direct transfer of genetic material (plasmid or chromosomal DNA) from a donor bacterium to a recipient bacterium through direct cell-to-cell contact.
- Mechanism:
  - Requires a sex