Overview
This lecture covers the differences between bacteria, archaea, and eukaryotes, focusing on cell structure, shapes, arrangements, staining techniques, and bacterial adaptations like biofilms and endospores.
Prokaryotic Cell Structure
- Bacteria and archaea are prokaryotes (no nucleus or internal membrane-bound organelles).
- Prokaryotes are always single-celled but can form groups (colonies, biofilms).
- Each cell is capable of independent metabolism, nutrient processing, and reproduction.
- They possess a cell membrane but lack organelles like mitochondria or chloroplasts.
Bacterial Shapes and Arrangements
- Main shapes: coccus (round), bacillus (rod), curved (vibrio, spirillum, spirochete).
- Coccus: spherical; bacillus: rod-shaped; pleomorphic: variable shapes.
- Arrangements: diplo- (pairs), strepto- (chains), staphylo- (clusters), tetrads (groups of four), sarcina (cubes).
- Bacilli can form diplo, strepto, or palisade (side-by-side) arrangements.
Biofilms and Quorum Sensing
- Biofilms are communities of microorganisms adhering to surfaces (e.g., dental plaque).
- Biofilms include various species and use sticky coatings (glycocalyx) for adhesion.
- Quorum sensing allows bacteria to communicate using chemical signals in biofilms.
Bacterial Appendages
- Flagella provide motility by rotating (not whipping); patterns: monotrichous (single), lophotrichous (tuft), amphitrichous (both ends), peritrichous (all over).
- Chemotaxis: movement toward (positive) or away from (negative) chemicals.
- Fimbriae are short bristles for adhesion; pili (sex pili) allow DNA transfer between cells (conjugation).
Glycocalyx: Capsule and Slime Layer
- Glycocalyx (sugar coating) helps adhesion and immune evasion.
- Capsule: dense, tightly bound; slime layer: loose, protects from water/nutrient loss.
- Capsules hinder phagocytosis by immune cells.
Cell Envelope and Gram Stain
- Cell envelope includes cytoplasmic membrane, cell wall (usually peptidoglycan), and sometimes an outer membrane.
- Gram-positive: thick peptidoglycan wall, stains purple, contains teichoic acids.
- Gram-negative: thin peptidoglycan, outer membrane with lipopolysaccharide (LPS/endotoxin), stains red.
- Gram stain steps: crystal violet → iodine → alcohol (decolorizer) → safranin (counterstain).
Non-typical Cell Walls and Special Structures
- Acid-fast bacteria (e.g., Mycobacterium): thick peptidoglycan with mycolic acid (waxy); use acid-fast stain.
- Mycoplasmas: no cell wall, pleomorphic shapes.
- Plasmids: small, circular DNA with accessory genes (e.g., antibiotic resistance).
- Ribosomes: site of protein synthesis; size 70S in bacteria.
Endospores
- Made by Bacillus and Clostridium genera for survival in harsh conditions.
- Endospores are resistant to heat, drying, chemicals; not a method of reproduction.
- Sporulation: vegetative cell forms an endospore; germination: endospore returns to active growth.
Archaea and Classification
- Archaea: prokaryotes with unique cell walls (no peptidoglycan); adapted to extreme environments (methanogens, halophiles, hyperthermophiles).
- Bacterial classification by envelope: Firmicutes (thick wall, Gram+), Gracilicutes (thin wall, Gram-), Mendosicutes (archaea), Tenericutes (no wall).
Key Terms & Definitions
- Prokaryote — organism lacking a nucleus and membrane-bound organelles.
- Coccus — spherical bacterial cell.
- Bacillus — rod-shaped bacterial cell.
- Pleomorphic — variable-shaped cells within one species.
- Biofilm — microbial community attached to a surface, often encased in glycocalyx.
- Quorum sensing — chemical communication among bacteria in a group.
- Fimbriae — short hairlike structures for adhesion.
- Pilus — structure for DNA transfer during conjugation.
- Glycocalyx — sugar-based outer cell coating (capsule or slime layer).
- Peptidoglycan — polysaccharide-peptide cell wall structure of bacteria.
- LPS (lipopolysaccharide/endotoxin) — outer membrane component unique to Gram-negative bacteria.
- Endospore — dormant, resistant form produced by certain bacteria.
- Acid-fast — bacteria with waxy cell wall (mycolic acid), stained by acid-fast stain.
Action Items / Next Steps
- Review bacterial cell structure diagrams, focusing on discussed features.
- Practice identifying coccus vs. bacillus shapes and arrangements in lab samples.
- Prepare for the Gram stain and acid-fast stain labs.
- Complete assigned critical thinking questions on bacterial classification.