Overview
This lecture covers the structure and function of bacterial cells, highlighting differences from eukaryotic cells and the roles of their key components in health and disease.
Bacterial Cell Structure
- Bacteria are prokaryotic cells lacking a nucleus and membrane-bound organelles.
- Bacteria have a cell envelope made up of several protective layers.
- The cell wall protects bacteria, maintains shape, and is composed of peptidoglycan.
- Gram-positive bacteria have a thick peptidoglycan layer, while gram-negative bacteria have a thin one.
- Bacterial shapes include cocci (round), bacilli (rod-shaped), spirilla (spiral), and pleomorphic (variable shape).
Cell Envelope Components
- Some bacteria have a capsule outside the cell wall that protects against phagocytosis and helps surface adhesion.
- The capsule increases bacterial virulence; strains lacking it are less virulent.
- The plasma membrane is beneath the cell wall and encloses the cytoplasm, acting as a selective barrier.
Cytoplasm and Internal Components
- The cytoplasm is a water-based gel containing enzymes, molecules, and cell structures.
- Ribosomes in the cytoplasm synthesize proteins, either for internal use or for export, depending on their location.
- The nucleoid is an irregular region containing the bacterial chromosome, usually a circular double-stranded DNA molecule.
- Some bacteria have more than one chromosome or a linear chromosome.
Plasmids
- Plasmids are extra-chromosomal, small DNA molecules, circular or linear, providing selective advantages like antibiotic resistance.
- Plasmid genes are not essential for basic bacterial survival.
Surface Structures and Motility
- Pili (hair-like) and fimbriae (shorter) help bacteria attach to surfaces and other cells.
- Flagella are long whip-like structures enabling bacterial motility.
Key Terms & Definitions
- Prokaryotic — cell type lacking a nucleus and membrane-bound organelles.
- Peptidoglycan — a polymer forming the bacterial cell wall.
- Gram-positive — bacteria with a thick peptidoglycan cell wall that retains dye.
- Gram-negative — bacteria with a thin peptidoglycan layer, does not retain dye.
- Capsule — an external polysaccharide layer enhancing protection and virulence.
- Plasma Membrane — selectively permeable barrier enclosing the cytoplasm.
- Ribosome — site of protein synthesis.
- Nucleoid — region containing bacterial chromosome(s).
- Plasmid — small, extra-chromosomal DNA molecule with non-essential genes.
- Pili/Fimbriae — structures aiding in attachment.
- Flagella — structures enabling movement.
Action Items / Next Steps
- Review bacterial cell diagrams and identify major structures.
- Read about Gram staining and its clinical significance.