Bacteria and Archaea Versus Eukaryotes

Cellular Organization

Bacteria and archaea are prokaryotic organisms, meaning they lack a nucleus and complex, membrane-bound organelles, unlike eukaryotes. Their DNA is not enclosed within a membrane, and their internal structures are simpler.

• Prokaryotes: No nucleus, no complex organelles

• Eukaryotes: Nucleus present, complex organelles

Bacterial Cell Structure

Overview of Bacterial Anatomy

Bacterial cells possess a variety of structural components that contribute to their survival, pathogenicity, and adaptability. These include the cell envelope, cytoplasm, appendages, and specialized structures.

• Cell envelope: Includes cell wall, cytoplasmic membrane, and sometimes an outer membrane

• Cytoplasm: Contains DNA, ribosomes, and storage granules

• Appendages: Flagella, pili, fimbriae, nanowires

Bacterial Size, Shape, and Arrangement

Size Variation

Bacteria exhibit considerable diversity in size, ranging from ultrasmall bacteria (200–500 nm) to large species such as Thiomargarita namibiensis (100–750 μm).

• Average size: 1 μm

• Ultrasmall bacteria: 200–500 nm

• Largest known bacterium: Thiomargarita namibiensis

Shapes and Arrangements

Bacteria are classified by their shapes and arrangements, which are important for identification and understanding their function.

• Coccus: Spherical

• Bacillus: Rod-shaped

• Curved: Spiral, vibrio, or spirochete

Arrangements include chains, clusters, pairs, and other groupings.

Mycoplasmas

Mycoplasmas are bacteria that lack a cell wall, resulting in extreme variation in shape. This property makes them resistant to antibiotics targeting cell wall synthesis.

Biofilms

Formation and Structure

Biofilms are complex microbial communities that form on surfaces, providing access to nutrients and protection from environmental stress. They are composed of bacteria, archaea, fungi, and algae.

• Formation: Begins with attachment to a moist surface with organic material

• Extracellular matrix: Secreted by colonizing organisms, attracts other microbes

• Stratification: Biofilms vary in thickness and are often layered

Appendages

Types and Functions

Bacterial appendages are specialized structures for motility and attachment.

• Flagella and axial filaments: Provide motility

• Fimbriae, pili, nanowires: Provide attachment or channel formation

Flagella

Flagella are whip-like structures used for movement. Their arrangement varies among species:

• Monotrichous: Single flagellum

• Lophotrichous: Tufts of flagella

• Amphitrichous: Flagella at both poles

• Peritrichous: Flagella dispersed over the cell surface

Chemotaxis and Phototaxis

Bacteria move in response to environmental stimuli. Chemotaxis is movement toward or away from chemicals, while phototaxis is movement toward light.

• Positive chemotaxis: Toward favorable chemicals

• Negative chemotaxis: Away from harmful substances

• Phototaxis: Toward light (in photosynthetic bacteria)

Run Versus Tumble

Bacterial movement alternates between runs (smooth, linear movement) and tumbles (random changes in direction). Attractants increase runs, repellents increase tumbles.

Attachment Appendages

Fimbriae and pili are used for adhesion, not locomotion. Pili are found only in gram-negative bacteria and are involved in conjugation (DNA transfer).

Surface Coatings: S Layer and Glycocalyx

S Layer

The S layer is a proteinaceous surface coating produced in hostile environments, providing protection.

Glycocalyx

The glycocalyx is a polysaccharide layer that can be a slime layer (loose) or capsule (dense). It protects against desiccation and phagocytosis.

• Slime layer: Loosely organized, protects from water/nutrient loss

• Capsule: Tightly bound, denser, protects from phagocytosis

The Cell Envelope

Structure and Function

The cell envelope consists of the cell wall, cytoplasmic membrane, and, in some bacteria, an outer membrane. It acts as a protective unit.

Gram Stain and Cell Wall Types

The Gram stain differentiates bacteria based on cell envelope structure:

• Gram-positive: Thick peptidoglycan wall, inner membrane

• Gram-negative: Thin peptidoglycan wall, inner and outer membranes

Peptidoglycan Structure

Peptidoglycan is a unique macromolecule forming the bacterial cell wall, composed of glycan chains cross-linked by peptides.

• Function: Provides structural support and shape

Gram-Positive Cell Wall

Gram-positive bacteria have a thick, homogenous peptidoglycan layer with teichoic and lipoteichoic acids for cell wall maintenance and division.

Gram-Negative Cell Wall

Gram-negative bacteria have a thin peptidoglycan layer and an outer membrane, which provides flexibility and sensitivity to lysis.

Gram-Negative Outer Membrane

The outer membrane contains lipopolysaccharides (LPS), which act as cell markers and endotoxins, and porin proteins that regulate molecule entry.

• LPS: Stimulates immune responses

• Porins: Defense against antibiotics

Bacterial Internal Structure

Cytoplasm

The cytoplasm is a gelatinous solution containing water, nutrients, DNA, ribosomes, and storage granules. It is the site of biochemical activities.

Bacterial Chromosome

Bacteria possess a single, circular DNA strand located in the nucleoid region. DNA is tightly coiled for efficient packaging.

Ribosomes

Bacterial ribosomes are 70S, composed of 50S and 30S subunits, and are responsible for protein synthesis. Eukaryotic ribosomes are 80S.

Inclusion Bodies

Inclusion bodies are storage sites for nutrients, varying in size and content depending on environmental conditions.

Cytoskeleton

Bacteria have a cytoskeleton composed of actin-like proteins arranged in helical ribbons, contributing to cell shape and potentially serving as antibiotic targets.

Bacterial Endospores

Structure and Function

Endospores are highly resistant, dormant structures formed by some bacteria to survive hostile conditions. They have a two-phase life cycle: vegetative cell and endospore.

• Sporulation: Induced by nutrient depletion

• Resistance: Heat, drying, freezing, radiation, chemicals

Germination

Germination occurs when favorable conditions return, leading to the breakdown of the spore cortex and resumption of growth.

Medical Significance

Endospores are important in medicine and industry due to their resistance to sterilization and their role in diseases such as anthrax, tetanus, gas gangrene, and botulism.

The Archaea

Unique Features

Archaea are prokaryotes more closely related to eukaryotes than bacteria. They possess unique genetic sequences, membrane lipids, and cell wall compositions.

• Cell wall: Lacks peptidoglycan

• Habitats: Extreme environments (heat, salt, acid, pressure)

Types of Archaea

• Methanogens: Produce methane from CO2 and H2

• Extreme halophiles: Require high salt concentrations

• Psychrophiles: Grow at low temperatures

• Hyperthermophiles: Grow at high temperatures

Diagnostic Scheme and Medical Significance

Classification of Bacteria

Bacteria are classified by cell wall structure, shape, arrangement, and oxygen usage. Medically important genera are grouped as gram-positive, gram-negative, or lacking cell walls.

Species and Subspecies

Bacterial species are collections of cells with similar traits. Subspecies, strains, or types differ in characteristics, and serotypes stimulate distinct antibody responses due to surface molecules.

Additional info: These notes cover the essential structural, functional, and classification aspects of bacteria and archaea, as outlined in a typical college microbiology course.