Microbiology: Prokaryotic Cell Introduction

Characteristics of Prokaryotic Cells

• Domains: Bacteria and Archaea.

  • Unlike eukaryotes, prokaryotes are not categorized into kingdoms.

• Absence of Organelles: Lack a true nucleus and other membrane-bound organelles.

• Size: Generally smaller than eukaryotic cells.

Structures of Prokaryotic Cells (General Overview)

External structures: Pilus, Capsule, Cell wall, Flagellum.
Internal structures: Ribosomes, Cytoplasmic membrane, Cytoplasm, Chromosome (DNA), Nucleoid.

External Structures

• Appendages: Two major groups:

  • Motility: Flagella and Axial filaments (periplasmic flagella).

  • Attachment or Channels: Fimbriae and Pili.

• Glycocalyx: A surface coating composed primarily of polysaccharides.

• S-layers and Sheaths: Specialized structures present in some organisms.

Flagella

• Function: Primarily involved in cell motility, spinning like propellers to move the cell.

• Disease Relevance: Some flagella are important in the pathogenicity of diseases.

• Characterization: Their number and arrangement are useful for bacterial characterization.

• Bacterial Flagella Structure: Composed of three main parts:

  • Filament: A long, thin, rigid helical structure made of the protein Flagellin.

  • Hook: A flexible structure that connects the filament to the basal body.

  • Basal Body: A stack of rings firmly anchored in the cell wall, enabling the flagellum to rotate in a 360^ ext{o} motion.

• Arrangement Variations: The number and arrangement of flagella vary by species:

  • Atrichous: No flagella.

  • Monotrichous: A single flagellum at one end.

  • Lophotrichous: A tuft of flagella at one end.

  • Amphitrichous: Flagella at both ends.

  • Peritrichous: Flagella distributed uniformly over the entire cell surface.

• Primary Function: Motility of the cell, allowing it to "swim" through its environment.

• Chemotaxis: Bacteria sense chemicals and move accordingly.

  • Nutrients typically attract the bacteria, while toxins repel them.

  • Movement consists of a series of "runs" (straight movement) and "tumbles" (reorientation).

  • Other taxis responses include:

    • Phototaxis: Movement in response to light.

    • Magnetotaxis: Movement in response to magnetic fields.

Periplasmic Flagella (Axial Filaments)

• Location: These are internal flagella, uniquely found in spirochetes.

• Enclosure: They are enclosed in the space between the outer sheath and the cell wall peptidoglycan.

• Motility Mechanism: They produce cellular motility by contracting and imparting a twisting or flexing motion, particularly effective on surfaces.

Fimbriae and Pili

• Size: Both are shorter than flagella.

• Fimbriae (Somatic Pili):

  • Structure: Fine, proteinaceous, hairlike bristles emerging from the cell surface.

  • Primary Function: Adherence to other cells and surfaces.

  • Motility: Specialized fimbriae (Type IV) are involved in twitching motility and gliding motility, which are forms of surface-only movement.

• Pili (Sex Pili):

  • Structure: Rigid tubular structures composed of pilin protein.

  • Visibility: These structures are primarily visible only in Gram-negative cells.

  • Function: To join bacterial cells for the transfer of DNA in a process called conjugation.

Glycocalyx: Capsules and Slime Layers

• General Description: A gel-like layer located outside the cell wall that offers protection or facilitates attachment to surfaces.

• Capsule: A distinct, gelatinous, and highly organized layer.

• Slime Layer: A diffuse, irregular, and loosely attached layer.

• Composition: Most are composed of polysaccharides, though some can be proteinaceous.

• Functions:

  • Protection: Protect cells from dehydration and loss of nutrients.

  • Adhesion: Often promote adhesion to surfaces.

    • Once attached, cells can grow as biofilms, which are polysaccharide-encased communities (e.g., dental plaque).

  • Immune Evasion: Some capsules enable bacteria to evade the host immune system by inhibiting phagocytosis.

The Cell Envelope

• Definition: The external covering located outside the cytoplasm.

• Composition: Composed of two basic layers:

  • The Cell wall (present in most bacteria).

  • The Cell membrane.

• Role: Maintains cell integrity and acts as a dynamic barrier between the cell's interior and exterior environments.

Cell Wall

• Structure and Function: A strong, rigid structure crucial for preventing cell lysis due to osmotic shock.

• Architectural Significance: The cell wall's architecture distinguishes two main types of bacteria: Gram-positive and Gram-negative.

• Main Component: Peptidoglycan, a unique polymer found exclusively in bacteria, is the primary structural component.

• Structure of Cell Walls Detailed:

  • Determines cell shape and prevents bursting (lysis).

  • Peptidoglycan is the primary component and consists of:

    • Glycan Chains: Formed by an alternating series of two subunits: N-acetylmuramic acid (NAM) and N-acetylglucosamine (NAG).

    • Tetrapeptide Chain: A string of four amino acids that links adjacent glycan chains, providing rigidity.

• Gram-Positive Cell Wall:

  • Peptidoglycan Layer: A thick, homogeneous sheath of peptidoglycan, typically 20 to 80 nm thick.

  • Additional Components: Includes teichoic acid and lipoteichoic acid.

    • Functions: These acids are involved in cell wall maintenance, enlargement during cell division, movement of cations across the cell envelope, and can stimulate specific immune responses.

• Gram-Negative Cell Wall:

  • Peptidoglycan Layer: A thin peptidoglycan layer.

  • Outer Membrane: Possesses a unique outer membrane layer external to the peptidoglycan.

    • The outer membrane contains specific components such as O antigen, lipopolysaccharide (LPS), lipid A, porins, and lipoprotein.

  • Periplasmic Space: Has a true periplasmic space located between the cytoplasmic membrane and the outer membrane, which contains the thin peptidoglycan layer.

Antibacterial Substances That Target Peptidoglycan

• Target Vulnerability: Peptidoglycan is an excellent target for antimicrobial drugs because it is unique to bacteria.

• Mechanism of Action: Weakening the peptidoglycan layer can lead to cell lysis.

• eta-Lactam Antibiotics (e.g., Penicillin):

  • Action: Interferes with peptidoglycan synthesis by preventing the cross-linking of adjacent glycan chains.

  • Efficacy: Usually more effective against Gram-positive bacteria than Gram-negative bacteria due to differences in cell wall structure and outer membrane presence.

• Lysozyme:

  • Action: An enzyme that breaks the bonds linking glycan chains in peptidoglycan.

  • Occurrence: Found naturally in tears, saliva, and other bodily fluids, serving as a first-line defense against bacterial infections.

The Gram Stain

• Type: A differential stain that distinguishes between cells with a Gram-positive cell wall and those with a Gram-negative cell wall.

• Gram-Positive Result: Cells retain the crystal violet stain and appear purple.

• Gram-Negative Result: Cells lose the crystal violet and subsequently stain pink from the safranin counterstain.

• Significance:

  • Classification and Identification: An important basis for bacterial classification and identification.

  • Clinical Aid: A practical tool in diagnosing infections and guiding appropriate drug treatment, largely due to differences in cell wall permeability.

Atypical Cell Walls

• Some bacterial groups lack typical cell wall structures.

• Mycobacterium:

  • Possess a Gram-positive cell wall structure but contain a significant amount of lipid mycolic acid (also known as cord factor).

  • This lipid content contributes to their pathogenicity and a high degree of resistance to certain chemicals and dyes.

  • The presence of mycolic acid is the basis for the acid-fast stain, used for diagnosis of infections such as tuberculosis and leprosy.

• Mycoplasma:

  • These bacteria have no cell wall at all.

  • Their cell membrane is stabilized and strengthened by sterols.

  • Due to the lack of a rigid cell wall, they are pleomorphic, meaning they have no fixed shape and can vary greatly in form.

The Cytoplasmic Membrane

• Boundary Definition: Defines the boundary of the cell, separating the cytoplasm from the external environment.

• Structure: Consists of a phospholipid bilayer embedded with various proteins.

• Function: Serves as a semipermeable, electrochemical barrier, regulating the passage of substances into and out of the cell.

• Proteins: The embedded proteins perform numerous crucial functions, including transport, enzymatic activity, and signal transduction.

• Fluid Mosaic Model: Describes the membrane as a fluid structure where proteins are able to drift about within the lipid bilayer.

Internal Components

• Chromosome(s):

  • Forms a gel-like region within the cytoplasm called the nucleoid.

  • Typically a single, circular, double-stranded DNA molecule.

  • Is tightly packed through the action of binding proteins and supercoiling.

• Plasmids:

  • Small, circular, supercoiled, double-stranded DNA molecules.

  • Usually much smaller than the main chromosome.

  • Contain genes that are not essential for basic cell function, but may confer beneficial traits like antibiotic resistance.

• Ribosomes:

  • Function: Essential for protein synthesis, facilitating the joining of amino acids to form polypeptides.

  • Size (Svedberg): Relative size is expressed in Svedberg (S) units, which reflect density and how fast they settle when centrifuged.

  • Prokaryotic Ribosomes: Are 70S, composed of a 30S subunit and a 50S subunit.

  • Eukaryotic Ribosomes: Are 80S (60S + 40S).

  • Medical Importance: Antibiotics that target the 70S prokaryotic ribosomes generally do not affect the 80S eukaryotic ribosomes, providing a basis for selective toxicity.

• Cytoskeleton:

  • Structure: An internal protein framework, once thought to be absent in bacteria.

  • Proteins: Bacterial proteins are similar to eukaryotic microfilament protein (actin).

  • Role: Likely involved in cell division and controlling cell shape.

• Storage Granules (Inclusions):

  • Composition: Accumulations of various polymers.

  • Formation: Synthesized from nutrients available in excess, serving as reserves.

• Gas Vesicles:

  • Function: Controlled structures that provide buoyancy, primarily in aquatic bacteria, allowing them to adjust their position in the water column.

• Endospores:

  • Nature: A unique type of dormant cell.

  • Producers: Primarily produced by members of the genera Bacillus and Clostridium.

  • Dormancy: Can remain dormant for extended periods of time.

  • Resistance: Extremely resistant to harsh environmental conditions, including heat, desiccation, chemicals, ultraviolet light, and even boiling water.

  • Germination: Endospores that survive adverse conditions can later germinate to become metabolically active vegetative cells.

Bacterial Shapes, Arrangements, and Sizes

• Bacteria vary significantly in shape (morphology), size, and arrangement, but are typically described by a few basic shapes:

  • Coccus: Spherical or roughly spherical shape.

  • Bacillus: Rod-shaped.

    • Coccobacillus: A very short and plump rod, appearing as an intermediate between a coccus and a bacillus.

    • Vibrio: Gently curved rods, often comma-shaped.

  • Spirillum: Spiral or helical shape, often somewhat rigid and S-shaped.

  • Spirochete: Flexible, coiled spirals, known for their corkscrew-like motility.

  • Pleomorphic: Lacking a fixed shape, capable of varying in form.

• Arrangement of Cells: Dependent on the pattern of cell division and how cells remain attached after division:

  • Bacilli Arrangements:

    • Random (Singles): Cells are separate.

    • Diplobacilli: Rods remain in pairs after division.

    • Streptobacilli: Rods form chains after division.

  • Cocci Arrangements:

    • Singles: Individual cells.

    • Pairs (Diplo-): Cells remain in pairs (e.g., diplococci).

    • Chains (Strepto-): Cells form chains (e.g., streptococci).

    • Tetrads: Groups of four cells, arranged in a square.

    • Irregular Clusters (Staphylo-): Grape-like clusters (e.g., staphylococci).

    • Cubical Packets (Sarcina): Cubical packets of eight or more cells.

Using Phenotypic Characteristics to Identify Prokaryotes

• Culture Characteristics: Observable traits when grown in culture.

• Microscopic Morphology: Visual characteristics under a microscope (shape, arrangement, presence of structures).

• Metabolic Capabilities: Biochemical tests to determine metabolic pathways and enzyme activity.

• Serology: Using antibodies to detect specific antigens on bacterial surfaces.

• Fatty Acid Analysis: Examining the composition of fatty acids in the cell membrane/wall.

• Genetic Analysis/PCR: Molecular methods to analyze DNA sequences or amplify specific genes.

Bacterial Taxonomy

• Bergey’s Manual of Determinative Bacteriology:

  • A comprehensive five-volume resource that covers all known prokaryotes.

  • Classification is based on genetic information, used to determine phylogenetic relationships.

  • Recognizes two main domains: Archaea and Bacteria.

  • Further divides into five major subgroups with 25 different phyla.

Species and Subspecies

• Species: Defined as a collection of bacterial cells that share an overall similar pattern of traits, distinguishing them from other bacteria with different patterns.

• Strain: A subspecies culture derived from a single parent that exhibits differences in structure or metabolism from other cultures of the same species.

  • Examples include biovars (biochemical differences) and morphovars (morphological differences).

• Type: A subspecies that shows specific differences in:

  • Antigenic Makeup: Referred to as a serotype or serovar.

  • Susceptibility to Bacterial Viruses: Known as a phage type.

  • Pathogenicity: Designated as a pathotype.

• Medical Relevance: Medical microbiology often requires a more precise identification of bacteria below the level of species for diagnostic and treatment purposes.

Characterizing Strain Differences

• Characterizing the differences between bacterial strains is particularly important in several fields:

  • Foodborne Illnesses: To trace the source of outbreaks and understand transmission.

  • Diagnosing Certain Diseases: To identify specific variants of pathogens that may respond differently to treatment.

  • Forensic Investigations: In cases of bioterrorism or biocrimes, to identify the exact strain used and its origin.