Microbial Cell Structure and Function: Study Notes for Microbiology Students

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Structure and Function of Bacterial Cells

Introduction to the Microbial World

The microbial world encompasses a vast diversity of microscopic organisms, including bacteria, archaea, viruses, and microbial eukaryotes. Understanding their structure and function is fundamental to microbiology, as it reveals the unity and diversity of life and the mechanisms by which microbes interact with their environment.

Microbial Cell Structure and Function

The Cell Envelope

The cell envelope is a series of layered structures surrounding the cytoplasm, governing interactions with the environment and preserving cellular integrity. It consists of the cytoplasmic membrane, cell wall, and, in some cases, an outer membrane or S-layer.

Cytoplasmic Membrane

Definition: The cytoplasmic membrane is a selectively permeable barrier that separates the cytoplasm from the external environment.
Structure: It is an 8–10 nm wide phospholipid bilayer containing embedded proteins. The bilayer has hydrophobic fatty acid tails facing inward and hydrophilic head groups exposed to the environment or cytoplasm.
Function:
- Selective permeability: Nutrients are transported in, and waste products are expelled.
- Protein anchor: Holds proteins involved in transport and energy metabolism.
- Energy conservation: Generates proton motive force for cellular energy.

Cryogenic electron tomography of a bacterial cell envelope Phospholipid bilayer membrane structure Structure of the cytoplasmic membrane with embedded proteins Major lipids of Archaea and membrane architecture Functions of the cytoplasmic membrane

Archaeal Cytoplasmic Membranes

Differences: Archaeal membranes have ether linkages in phospholipids and isoprenoid chains, unlike the ester linkages and fatty acids in Bacteria and Eukarya.
Major lipids: Phosphoglycerol diethers and diphosphoglycerol tetraethers, which can form lipid monolayers.

Transporting Nutrients into the Cell

Microbial cells use active transport mechanisms to accumulate solutes against concentration gradients. Transporters are energy-driven and include three main types:

Simple transport: Uses a transmembrane protein and is driven by the proton motive force. Includes symport (solute and H+ cotransported) and antiport (solute and H+ transported in opposite directions).
Group translocation: The transported substance is chemically modified during transport. The best-studied system is the phosphotransferase system in E. coli, which uses phosphoenolpyruvate as an energy source.
ABC transporter systems: Consist of a substrate-binding protein, transmembrane transporter, and ATP-hydrolyzing protein. These systems are ideal for organisms in nutrient-poor environments due to their high substrate affinity.

Three classes of transport systems Simple transport mechanism Group translocation mechanism Phosphotransferase system in E. coli ABC transporter system

The Cell Wall

The cell wall provides structural strength and protects against osmotic pressure. Most bacteria are classified as gram-positive or gram-negative based on cell wall structure and Gram stain reaction.

Gram-Positive vs. Gram-Negative Cell Envelopes

Gram-positive: Cytoplasmic membrane and thick peptidoglycan cell wall.
Gram-negative: Cytoplasmic membrane, thin peptidoglycan cell wall, outer membrane, and periplasmic space.

Cell envelopes of bacteria: Gram-positive and Gram-negative

Peptidoglycan Structure

Definition: Peptidoglycan is a rigid polysaccharide layer found in all bacteria with a cell wall.
Composition: Alternating N-acetylglucosamine and N-acetylmuramic acid joined by β-1,4 linkages, with a short peptide attached to N-acetylmuramic acid.
Cross-linking: Peptidoglycan strands are cross-linked by covalent peptide bonds, providing strength and rigidity.

Structure of the repeating unit in peptidoglycan Peptidoglycan structure in the cell wall

Gram-Positive Cell Envelope Features

Thick peptidoglycan wall, often stabilized by peptide interbridges.
Teichoic acids embedded in the cell wall, lipoteichoic acids covalently bound to membrane lipids.
Peptidoglycan can be destroyed by lysozyme; penicillin blocks peptide cross-link formation.

Structure of the Gram-positive bacterial cell wall

Archaeal Cell Walls

Lack peptidoglycan and typically lack an outer membrane.
Most have an S-layer (protein shell) instead of a polysaccharide wall.
Methanogens have pseudomurein cell walls, which are similar to peptidoglycan but contain β-1,3 glycosidic bonds and L-stereoisomer amino acids.

LPS: The Outer Membrane

Gram-negative bacteria possess an outer membrane composed of a lipopolysaccharide (LPS) layer, which is a major virulence factor and adds structural strength.

LPS Structure: Contains a core polysaccharide, O-polysaccharide, and lipid A (endotoxin).
Function: Facilitates surface recognition, contains porins for solute transport, and anchors to peptidoglycan via Braun lipoprotein.

Gram-negative bacterial cell envelope Structure of bacterial lipopolysaccharide

Diversity of Cell Envelope Structure

S-Layers: Paracrystalline protein or glycoprotein structures that provide strength, shape, and protection.
Alternative Configurations: Some bacteria and archaea lack cell walls but have tough cytoplasmic membranes (e.g., Mycoplasmas, Thermoplasma).

Cell Surface Structures and Inclusions

Cell Surface Structures

Capsules and Slime Layers: Sticky polysaccharide coats outside the cell envelope. Capsules are tightly attached; slime layers are loosely attached. Functions include attachment, biofilm formation, infectivity, and protection from desiccation.
Fimbriae and Pili: Protein structures that enable attachment to surfaces, biofilm formation, and genetic exchange (conjugation). Type IV pili support twitching motility.
Hami: Unique archaeal structures resembling grappling hooks, aiding in surface attachment and biofilm formation.

Cell Inclusions

Energy and Carbon Storage: Poly-β-hydroxybutyric acid (PHB), poly-β-hydroxyalkanoate (PHA), and glycogen are common storage polymers.
Polyphosphate, Sulfur, and Carbonate Minerals: Polyphosphate granules, elemental sulfur, and biomineralized carbonate minerals serve as reservoirs.
Gas Vesicles: Protein-bound structures that confer buoyancy to aquatic microbes.
Magnetosomes: Biomineralized magnetic iron oxides that allow magnetotaxis (migration along magnetic field lines).

Endospores

Definition: Highly differentiated, dormant cells resistant to heat, radiation, chemicals, and desiccation.
Formation: Sporulation is triggered by nutrient limitation, resulting in a heat-resistant structure.
Structure: Multiple layers (core, inner membrane, cortex, outer membrane, coat, exosporium), contains dipicolinic acid and small acid-soluble spore proteins (SASPs).
Function: Survival and dispersal under unfavorable conditions; present only in some gram-positive bacteria.

Characteristic	Vegetative cell	Endospore
Microscopic appearance	Nonrefractile	Refractile
Calcium content	Low	High
Dipicolinic acid	Absent	Present
Enzymatic activity	High	Low
Respiration rate	High	Low or absent
Macromolecular synthesis	Present	Absent
Heat resistance	Low	High
Radiation resistance	Low	High
Resistance to chemicals	Low	High
Lysozyme	Sensitive	Resistant
Water content	High, 80–90%	Low, 10–25% in core
Small acid-soluble spore proteins	Absent	Present

Cell Locomotion

Flagella, Archaella, and Swimming Motility

Flagella: Long, thin appendages anchored in the cell, functioning as rotating machines for swimming. Arrangements include polar, tufts, lophotrichous, amphitrichous, and peritrichous.
Structure: Composed of a filament (flagellin), hook, and basal body (motor).
Archaella: Smaller, unrelated to flagella, more closely related to type IV pili, and driven by ATP hydrolysis.

Surface Motility

Twitching Motility: Requires type IV pili, which extend, attach, and retract to pull the cell forward.
Gliding Motility: Smooth, continuous motion along the cell axis, involving intracellular protein tracks and adhesion proteins.

Chemotaxis and Other Forms of Taxis

Chemotaxis: Directed movement in response to chemical stimuli, involving "run and tumble" behavior in peritrichously flagellated bacteria.
Other Taxis: Osmotaxis (ionic strength), hydrotaxis (water), aerotaxis (oxygen), phototaxis (light), and magnetotaxis (magnetic fields).

Eukaryotic Microbial Cells

The Nucleus and Cell Division

Nucleus: Double membrane-enclosed organelle containing chromosomes. DNA is wound around histones, forming nucleosomes.
Cell Division: Mitosis produces two diploid daughter cells; meiosis produces four haploid gametes.

Mitochondria and Chloroplasts

Mitochondria: Site of respiration and ATP production in aerobic eukaryotes. Contains cristae (folded internal membranes) and matrix (innermost area).
Chloroplasts: Site of photosynthesis in phototrophic eukaryotes. Contains thylakoids (chlorophyll-containing discs) and stroma (with RuBisCO enzyme).
Endosymbiotic Origin: Mitochondria and chloroplasts descended from bacterial cells via symbiosis.

Other Eukaryotic Cell Structures

Cytoskeleton: Internal structural support composed of microtubules, microfilaments, and intermediate filaments.
Endoplasmic Reticulum and Golgi Complex: Networks for protein and lipid synthesis, modification, and transport.
Lysosomes: Membrane-enclosed compartments containing digestive enzymes.
Flagella and Cilia: Motility organelles structurally distinct from prokaryotic flagella, composed of microtubules and driven by ATP.

Example: The presence of mitochondria and chloroplasts in eukaryotic cells is evidence of endosymbiotic events, as these organelles contain their own DNA and ribosomes similar to bacteria.

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