The Prokaryotes: Structure, Function, and Diversity

Prokaryotes: General Characteristics

Prokaryotes are a diverse group of unicellular organisms that lack a nucleus and membrane-bound organelles. They reproduce asexually and include the domains Bacteria and Archaea. Their genetic diversity and adaptability allow them to inhabit a wide range of environments.

• No nucleus or membrane-bound organelles

• Unicellular organization

• Asexual reproduction (mainly binary fission)

• Genetic diversity due to mutations, horizontal gene transfer, and rapid reproduction

• Members: Bacteria and Archaea

Bacterial Morphology

Coccus (Cocci) Shape

Cocci are spherical bacteria that can divide in one or more planes, resulting in various arrangements.

• Diplococci: Pairs

• Streptococci: Chains

• Tetrads: Groups of four

• Sarcinae: Cubes of eight

• Staphylococci: Irregular clusters

Bacillus (Bacilli) Shape

Bacilli are rod-shaped bacteria that typically divide in one plane, resulting in limited arrangements.

• Single bacillus

• Diplobacilli: Pairs

• Streptobacilli: Chains

• Coccobacillus: Short, oval rods

Other Bacterial Shapes

• Curved rods: Includes Vibrio (comma-shaped), Spirillum (rigid spiral), and Spirochete (flexible spiral)

• Pleomorphic: Bacteria that can change shape or have irregular forms

Bacterial Cell Structure Overview

Bacterial cells share some similarities with eukaryotic cells but have unique features, especially in their external and internal structures.

External Structures

Glycocalyx

The glycocalyx is a polysaccharide-rich layer outside the cell wall. It can be organized as a capsule or a loose slime layer.

• Capsule: Well-organized, firmly attached; protects against phagocytosis

• Slime layer: Loosely attached; aids in adherence to surfaces

Flagella

Flagella are long, whip-like structures made of flagellin that provide motility. Their arrangement can vary between species and is important for identification.

• Monotrichous: Single flagellum

• Lophotrichous: Tuft at one end

• Amphitrichous: One or more at both ends

• Peritrichous: All over the cell surface

Motility and Taxis

Bacteria move using a "run and tumble" mechanism, allowing them to navigate toward or away from stimuli (taxis).

• Chemotaxis: Movement in response to chemicals

• Phototaxis: Movement in response to light

Fimbriae and Pili

• Fimbriae: Short, hair-like structures for attachment to surfaces and other cells

• Pili: Longer, involved in DNA transfer (conjugation)

Cell Wall

The bacterial cell wall provides shape and protection. It is primarily composed of peptidoglycan, which differs in thickness between Gram-positive and Gram-negative bacteria.

• Gram-positive: Thick peptidoglycan layer

• Gram-negative: Thin peptidoglycan layer, outer membrane with lipopolysaccharide (LPS)

Some bacteria (e.g., Mycobacterium) have mycolic acid in their cell walls and require acid-fast staining.

Plasma (Cytoplasmic) Membrane

The plasma membrane is a phospholipid bilayer that controls the movement of substances in and out of the cell. It is semi-permeable and involved in energy generation and transport.

Osmosis

Osmosis is the movement of water across the membrane from high to low water concentration. Bacterial cells respond differently to isotonic, hypertonic, and hypotonic solutions.

Internal Structures

• Nucleoid: Region containing the single, circular, supercoiled chromosome and plasmids

• Cytoplasm: Gel-like substance containing enzymes, nutrients, and cell structures

• Ribosomes: Sites of protein synthesis (70S type, composed of 50S and 30S subunits)

• Inclusions: Storage granules for nutrients, gases, or metals; some bacteria have magnetosomes for orientation

• Endospores: Highly resistant, dormant structures formed by some bacteria for survival in harsh conditions

Bacterial Diversity: Major Groups

Proteobacteria (Gram-negative)

• Alphaproteobacteria: Low nutrient environments, some fix nitrogen; e.g., Rickettsia typhi (typhus)

• Betaproteobacteria: Use anaerobic byproducts; e.g., Bordetella pertussis (whooping cough), Neisseria gonorrhoeae (gonorrhea)

• Gammaproteobacteria: Diverse; e.g., Pseudomonas aeruginosa (nosocomial infections), Legionella pneumophila (Legionnaire's disease), Salmonella enterica (food poisoning)

• Deltaproteobacteria: Sulfur cycle; e.g., Bdellovibrio bacteriovorus (predatory)

• Epsilonproteobacteria: Slender, helical; e.g., Helicobacter pylori (stomach ulcers)

Non-Proteobacteria (Gram-negative)

• Cyanobacteria: Photosynthetic, often in groups surrounded by glycocalyx

• Chlamydiae: No peptidoglycan, obligate intracellular pathogens; e.g., Chlamydia (STD)

• Bacteroidetes: Anaerobic, found in human body and soil

• Fusobacteria: Anaerobic, spindle-shaped, found in mouth and gums

• Planctomycetes: Aquatic, stalk-producing

• Spirochetes: Tightly coiled, axial filaments for movement; e.g., Treponema pallidum (syphilis)

• Deinococcus: Highly resistant to radiation

Firmicutes (Gram-positive, Low G+C)

• Includes endospore-forming pathogens

• Examples: Clostridium difficile (gastroenteritis), Staphylococcus aureus (MRSA/VRSA)

Actinobacteria (Gram-positive, High G+C)

• Pleomorphic, some produce antibiotics

• Example: Streptomyces (antibiotic producer)

Archaea

Archaea are prokaryotes that often live in extreme environments and have unique cell wall and membrane features.

• Extremophiles: Can survive high salinity (halophiles), extreme pH (acidophiles, alkaliphiles), and temperature (thermophiles, psychrophiles)

• No peptidoglycan in cell walls; may contain pseudopeptidoglycan or other polymers

• Some use ammonia for energy or produce methane (methanogens)

• Example: Pyrodictium abyssi (deep ocean vents, 110°C, disk-shaped with tubules)