Overview
This lecture covers key structures in prokaryotic (bacterial) cells, mainly focusing on the cell wall, external features, and internal components, with emphasis on differences between Gram-positive and Gram-negative bacteria.
Bacterial Cell Wall Structure
- The bacterial cell wall provides support and protection.
- It is mainly composed of peptidoglycan, a unique molecule of repeating sugars (NAG and NAM) and amino acid peptides.
- Gram-positive bacteria have a thick peptidoglycan layer containing teichoic acids that bind positive ions.
- Gram-negative bacteria have a thin peptidoglycan layer and an additional outer membrane called lipopolysaccharide (LPS), which serves as an endotoxin.
Gram Staining and Classification
- Gram-positive bacteria are characterized by a thick peptidoglycan layer.
- Gram-negative bacteria have a thin peptidoglycan layer and an LPS outer membrane.
- The outer membrane contains Lipid A (triggers immune response) and O antigen (varies between strains).
Attacks on Cell Wall
- Peptidoglycan can be broken down by penicillin (antibiotic) and lysozyme (enzyme found in secretions).
- These agents are more effective against Gram-positive bacteria due to their thick peptidoglycan layer.
- Some bacteria like Mycoplasma lack a cell wall and instead contain sterols in their membrane.
Structures Outside the Cell Wall
- The glycocalyx (sugar coat) can form a capsule (well-defined) or slime layer (irregular), aiding in protection, attachment, and water storage.
- Capsules are important in pathogenic bacteria and biofilm formation.
- Flagella are used for movement, composed of flagellin protein, and function by rotation powered by ATP.
- Types of taxis (directed movement) include chemotaxis (chemicals), aerotaxis (oxygen), magnetotaxis (magnetic fields), thermotaxis (temperature), and phototaxis (light).
- Pili are short appendages used for attachment (fimbriae) or gene transfer (sex pilus).
Internal Structures of Prokaryotic Cells
- The nucleoid contains the main circular chromosome and possibly plasmids (extra DNA providing advantages).
- Ribosomes (70S) in the cytoplasm are the sites of protein synthesis and are targets for antibiotics.
- Some have a cytoskeleton for support and cell division.
- Gas vesicles provide buoyancy in aquatic bacteria.
- Endospores (in Bacillus and Clostridium, both Gram-positive rods) are dormant, resistant structures formed during tough conditions; sporulation produces spores, which can later germinate.
Key Terms & Definitions
- Peptidoglycan — a polymer of sugars and amino acids forming the bacterial cell wall.
- Gram-positive bacteria — bacteria with a thick peptidoglycan cell wall.
- Gram-negative bacteria — bacteria with a thin peptidoglycan layer and an LPS outer membrane.
- LPS (lipopolysaccharide) — outer membrane of Gram-negative bacteria, acts as an endotoxin.
- Capsule — well-defined, protective sugar coat outside some bacterial cells.
- Slime layer — irregular, less-defined glycocalyx outside some bacterial cells.
- Flagella — protein structures used for movement.
- Pili (fimbriae) — short structures for attachment or gene transfer.
- Nucleoid — region containing bacterial chromosome.
- Plasmid — small, extra-chromosomal DNA.
- Endospore — dormant, resistant bacterial structure for surviving adverse conditions.
- Teichoic acids — negatively charged molecules in Gram-positive cell walls.
Action Items / Next Steps
- Review differences between Gram-positive and Gram-negative cell wall structures.
- Learn the function and importance of external and internal bacterial structures.
- Study the process of sporulation and factors leading to endospore formation.