Bacterial and Archaeal Cell Structure

Prokaryote Controversy

• The term "prokaryote" is controversial because bacteria and archaea are distinct but grouped together.
• Originally defined by the lack of a membrane-bound nucleus, cytoskeleton, and internal membraneous structures.

Bacterial Diversity and Common Features

• Common shapes include rods (bacilli), cocci (spheres), comma-shaped (vibrio), spiral (spirillum & spirochetes), filamentous, and pleomorphic.
• Frequently observed structures: cell wall, nucleoid, plasma membrane, cytoplasm, inclusions, ribosomes, fimbriae, capsule, and flagella.
• Factors determining size & shape: evolution and surface area to volume ratio (S/V ratio).
• S/V ratio affects nutrient uptake and diffusion efficiency, facilitating rapid growth.

Bacterial Plasma Membranes

• Cell envelope: plasma membrane and surrounding layers.
• Plasma membrane: semipermeable barrier; carries out respiration & photosynthesis; detects chemicals.
• Cell wall: additional layer.
• Bacterial membranes are bilayers of phospholipids with fatty acids connected to glycerol by ester linkage.

Nutrient Acquisition

• Microorganisms need nutrients for energy conservation and biosynthesis.
• Macronutrients are needed in large quantities; micronutrients (trace elements) are needed in small amounts.
• Growth factors (amino acids, purines & pyrimidines, vitamins) are obtained from the environment.
• Passive diffusion: movement down the concentration gradient without energy input.
• Facilitated diffusion: uses transport proteins (channels or carriers) without metabolic energy.
• Group translocation: active transport modifying organic molecules during transport.
• Secretion of siderophores binds ferric irons, enabling iron accumulation.

Bacterial Cell Walls

• Most bacteria have a cell wall outside the plasma membrane for shape and osmotic protection.
• Walls contain peptidoglycan.
• Gram-positive: thick peptidoglycan layer and teichoic acids.
• Gram-negative: thin peptidoglycan layer surrounded by a complex outer membrane containing lipopolysaccharides (LPSs).
• Endotoxin: lipid A portion of LPS, which can lead to septic shock.
• Gram stain mechanism depends on peptidoglycan thickness, preventing its loss during the ethanol wash.

Cell Envelope Layers

• Capsules, slime layers, and glycocalyces protect cells and aid in surface attachment.
• S-layers are the outermost layer in some bacteria and are composed of proteins or glycoproteins.

Bacterial Cytoplasm

• A concentrated solution of biochemicals contains proteins similar to eukaryotic cytoskeletal proteins.
• Some bacteria have internal membrane systems for photosynthesis and respiration.
• Inclusions store organic or inorganic substances (e.g., PHB inclusions, polyphosphate granules, magnetosomes, gas vacuoles).
• Microcompartments contain enzymes for reactions like CO2 fixation (carboxysomes).
• Bacterial ribosomes are 70S in size, composed of proteins & rRNA molecules.
• Nucleoid: contains genetic material (usually a double-stranded, covalently closed circular DNA molecule).
• Plasmids: extrachromosomal DNA that can provide selective advantages; some are episomes that integrate into the chromosome.

External Structures

• Fimbriae (short hair) primarily function in attachment.
• Type IV pili are involved in twitching motility.
• Sex pili participate in DNA transfer.
• Flagella (long hair) are threadlike locomotory organelles.

Bacterial Motility

• Types: swimming, swarming, spirochete motility, twitching, and gliding.
• Swimming: flagellar filament rotates like a propeller.
• Swarming: group movement on moist surfaces, mediated by flagella.
• Spirochete motility: flagella wound around the cell within the periplasmic space rotate.
• Twitching: jerky movement via type IV pili.
• Gliding: smooth movement.
• Chemotaxis: response to gradients of attractants and repellents.

Bacterial Endospores

• Endospores are dormant structures resistant to heat and desiccation, formed within mother cells during adverse conditions.
• Germination is the breaking of endospore dormancy in response to signals.

Archaeal Features

• Many archaea have been identified through 16S rRNA or partial genome sequencing.
• Common shapes: rods, cocci, curved rods, branched, pleomorphic, and spirals (no spirochetes or mycelial forms).
• Similar in size to bacteria, but extremely small or large archaea have also been identified.

Archaeal Cell Envelopes

• Consist of a plasma membrane and cell wall (no peptidoglycan), often with an S-layer.
• Membranes are composed of glycerol diether and diglycerol tetraether lipids.
• Diether lipids form bilayers; tetraether lipids form monolayers.

Archaeal Cytoplasm

• Contains cytoskeletal proteins: FtsZ (tubulin homologue), MreB (actin homologue), and Crenactin (unique archaeal actin homologue).
• Inclusions include gas vesicles.
• Ribosomes are 70S in size and are more similar to eukaryotic ribosomes.
• Genetic material is in a nucleoid (not enclosed by a membrane) and consists of a single, double-stranded, covalently closed, circular DNA molecule.
• Some archaea are polyploid.

Archaeal External Structures

• Have pili (similar to bacterial type IV pili).
• Motile by archaella (flagella).
• Engage in relocate-and-seek swimming behaviors, and archaella-related Type IV Pili may aid in positioning cells optimally in a temperature gradient.
• Archaella: rigid helices that rotate; rotation direction determines forward or backward movement.
• Some are phototactic.
• Rotations are powered by ATP hydrolysis; archaeal taxis machinery is similar to bacterial.