Overview
This lecture covers the structure, diversity, and classification of bacteria and archaea, focusing on cell components, arrangements, staining techniques, and survival adaptations.
Prokaryotic Cell Structure
- Bacteria and archaea are prokaryotes, lacking a nucleus; their DNA is in a nucleoid.
- Prokaryotes do not have membrane-bound organelles (e.g., mitochondria, Golgi apparatus).
- All cells have ribosomes for protein synthesis and a plasma membrane.
- Most bacteria have a cell wall made of peptidoglycan; this is unique to bacteria.
Bacterial Shapes and Arrangements
- Common shapes: coccus (spherical), bacillus (rod-shaped), spirillum (spiral-shaped).
- Arrangements include pairs (diplo-), chains (strepto-), clusters (staphylo-), tetrads, and sarcina (cubical packets).
- Some bacteria lack a fixed shape (pleomorphic) due to absence of a cell wall.
Biofilms
- Biofilms are communities of various microbes in a polysaccharide/protein matrix.
- Bacteria in biofilms cooperate, aiding each other's survival and metabolic processes.
- Dental plaque is a common example of a biofilm.
Bacterial Appendages
- Motility structures: flagella (rotating whip-like tails) and axial filaments (corkscrew motion).
- Attachment structures: fimbriae (bristle-like, for adhesion) and pili (hollow tubes for DNA transfer).
- Pili (specifically sex pilus) enable horizontal gene transfer via conjugation between cells.
Surface Coatings
- S-layers: protein layers providing protection, common in harsh environments.
- Glycocalyx: a polysaccharide coating that can be a loosely bound slime layer or a tightly bound capsule.
- Capsules protect pathogens from host immune responses.
Bacterial Cell Envelope
- Composed of a cell membrane, cell wall (peptidoglycan), and sometimes an outer membrane (in Gram-negative bacteria).
- Gram-positive: thick peptidoglycan layer with teichoic acids; stains purple.
- Gram-negative: thin peptidoglycan, outer membrane with lipopolysaccharides (LPS); stains pink.
- Lipid A in LPS is toxic (endotoxin) and can cause serious health effects.
Gram Stain Procedure
- Steps: crystal violet (primary stain), iodine (mordant), alcohol (decolorizer), safranin (counterstain).
- Gram-positive bacteria retain purple; Gram-negative bacteria appear pink.
Internal Structures
- Cytoplasm is a jelly-like solution where cellular activities occur.
- Bacteria have a single circular chromosome in the nucleoid.
- Plasmids are small, extra-chromosomal DNA with helpful genes (e.g., antibiotic resistance).
- Bacterial ribosomes are 70S (smaller than eukaryotic 80S).
- Inclusion bodies store nutrients.
- Only Bacillus and Clostridium genera produce endospores, which are highly resistant dormant forms.
Endospore Formation and Significance
- Endospores form under nutrient depletion via sporulation.
- Highly resistant to heat, drying, radiation, and chemicals.
- Important in medicine and food safety due to resilience and potential for infection.
Archaea
- Also prokaryotic, but more closely related to eukaryotes than bacteria.
- Lack peptidoglycan cell walls; often found in extreme environments.
Bacterial Classification
- Species: group of cells with a shared pattern of traits.
- Strains or subspecies: members of a species with different characteristics (e.g., toxin production).
Key Terms & Definitions
- Nucleoid — Region in prokaryotes where DNA is located.
- Peptidoglycan — Unique bacterial cell wall polymer of sugars and peptides.
- Biofilm — Cooperative community of microorganisms in a self-produced matrix.
- Flagellum — Whip-like appendage for motility.
- Fimbria — Bristle-like projection aiding in surface adhesion.
- Pilus — Tubular structure for DNA transfer between cells.
- Glycocalyx — Sugar-based outer coating (slime layer or capsule) on some bacteria.
- Endospore — Dormant, highly resistant cell type formed by Bacillus and Clostridium.
- Gram-positive — Bacteria with thick peptidoglycan walls, stain purple.
- Gram-negative — Bacteria with thin peptidoglycan and outer membrane, stain pink.
- Lipopolysaccharide (LPS) — Molecule in Gram-negative outer membrane; lipid A is toxic.
Action Items / Next Steps
- Review the Gram stain procedure and practice identifying cell wall types.
- Memorize key bacterial structures and their functions.
- Study the differences between Gram-positive and Gram-negative bacteria.
- Prepare for quiz questions on endospore formers and bacterial appendages.