Chapter 2

Microbial Cell Structure and Function

The Cell Envelope

The cell envelope is a series of layered structures surrounding the cytoplasm of microbial cells, governing their interactions with the environment. It consists of the cytoplasmic membrane, cell wall, outer membrane, and S-layers. These components provide structural integrity, protection, and mediate transport and communication with the environment.

Cytoplasmic Membrane

The cytoplasmic membrane surrounds the cytoplasm, separating it from the external environment. Its main function is selective permeability, allowing nutrients to enter and waste products to exit. Membrane proteins facilitate these reactions and play a role in energy metabolism.

• Structure: Phospholipid bilayer (8–10 nm wide) with embedded proteins. Contains hydrophobic fatty acid tails and hydrophilic head groups (glycerol + phosphate + functional group).

• Function: Acts as a permeability barrier, protein anchor, and site for energy conservation (proton motive force).

• Membrane Proteins: Integral (embedded), transmembrane (span the membrane), and peripheral (loosely attached).

Archaeal Cytoplasmic Membranes

• Unique Features: Ether linkages in phospholipids (vs. ester in Bacteria/Eukarya), isoprenoid chains instead of fatty acids.

• Major Lipids: Phosphoglycerol diethers with phytanyl side chains, diphosphoglycerol tetraethers with biphytanyl side chains (can form monolayers).

• Function: Many different isoprenoid chains, including ring structures (e.g., crenarchaeol).

Transporting Nutrients into the Cell

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

• Simple Transport: Uses transmembrane transport proteins, driven by proton motive force. Can be symport (solute and proton in same direction) or antiport (opposite directions).

• Group Translocation: Substance is chemically modified during transport. Driven by energy-rich organic compounds (e.g., phosphotransferase system in E. coli).

• ABC Transporter Systems: ATP-binding cassette systems with three components (binding protein, transmembrane transporter, ATP-hydrolyzing protein). Over 200 systems for various compounds.

The Cell Wall

The cell wall provides strength, maintains cell shape, and prevents lysis due to osmotic pressure. Most bacteria are classified as gram-positive or gram-negative based on cell wall structure and Gram stain reaction.

• Gram-Positive: Cytoplasmic membrane + thick cell wall (peptidoglycan).

• Gram-Negative: Cytoplasmic membrane, thin cell wall, outer membrane, periplasmic space.

Bacterial Cell Walls

• Peptidoglycan: Rigid polysaccharide layer found in all bacteria with a cell wall. Composed of alternating N-acetylglucosamine and N-acetylmuramic acid, with short peptides attached.

• Cross-Linking: Peptidoglycan strands run parallel and are cross-linked by covalent peptide bonds. Gram-negative bacteria have single-layer crosslinks; gram-positive have multiple layers and peptide interbridges.

• Teichoic Acids: Acidic molecules embedded in gram-positive cell walls, covalently linked to peptidoglycan or membrane lipids (lipoteichoic acids).

• Destruction: Lysozyme cleaves glycosidic bonds; penicillin blocks peptide cross-link formation.

Archaeal Cell Walls

• Differences: Lack peptidoglycan and typically lack outer membrane. Most have S-layer (protein shell) instead of polysaccharide wall.

• Pseudomurein: Found in methanogens; similar to peptidoglycan but contains N-acetyltalosaminuronic acid instead of N-acetylmuramic acid. Resistant to lysozyme and penicillin.

LPS: The Outer Membrane

Gram-negative bacteria have an outer membrane composed of lipopolysaccharide (LPS), which is a second lipid bilayer external to the cell wall. LPS facilitates surface recognition, acts as a virulence factor, and adds strength. It contains porins for solute transport and Braun lipoprotein for anchoring.

• Structure: Core polysaccharide, O-polysaccharide, and lipid A (endotoxin).

• Periplasm: Space between cytoplasmic and outer membranes, housing extracellular proteins.

Diversity of Cell Envelope Structure

• S-Layers: Paracrystalline protein or glycoprotein structures, always the outermost layer if present. Functions include strength, protection, shape, and facilitating cell surface interactions.

• Alternative Configurations: Some bacteria and archaea lack cell walls but have tough cytoplasmic membranes (e.g., Mycoplasmas, Thermoplasma).

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 preventing dehydration.

• Fimbriae, Pili, and Hami: Protein structures for attachment, biofilm formation, and genetic exchange. Hami are unique archaeal structures resembling grappling hooks.

Cell Inclusions

• Function: Energy reserves, carbon or phosphorus reservoirs, special functions. Enclosed by thin protein membrane to reduce osmotic stress.

• Types: Carbon storage polymers (PHB, PHA, glycogen), polyphosphate granules, sulfur granules, carbonate minerals, gas vesicles (buoyancy), magnetosomes (magnetotaxis).

Endospores

• Specialized Spores: Highly differentiated, dormant cells resistant to heat, radiation, chemicals, drying, and nutrient deprivation. Survival structures for dispersal and endurance.

• Formation: Sporulation triggered by nutrient limitation; germination occurs upon nutrient availability (activation, germination, outgrowth).

• Structure: Multiple layers (core, inner membrane, cortex, outer membrane, coat, exosporium), dipicolinic acid, small acid-soluble spore proteins (SASPs).

• Comparison Table:

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 and Archaella: Structures for swimming motility. Flagella are long, thin appendages anchored in the cell; arrangements include polar, tufts, lophotrichous, amphitrichous, peritrichous. Archaella are smaller and driven by ATP hydrolysis.

• Surface Motility: Includes twitching (requires pili) and gliding (smooth motion without external structures).

• Chemotaxis: Directed movement in response to chemical or physical stimuli. "Run and tumble" behavior in peritrichous bacteria; biased random walk in presence of attractant.

• Other Forms of Taxis: Osmotaxis (ionic strength), hydrotaxis (water), aerotaxis (oxygen), phototaxis (light).

Eukaryotic Microbial Cells

• Nucleus: Double membrane-enclosed, contains chromosomes wound around histones. Site of ribosomal RNA synthesis (nucleolus).

• Cell Division: Mitosis (two diploid cells), meiosis (four haploid gametes).

• Mitochondria: Site of respiration and ATP production; evolutionary roots in Bacteria.

• Chloroplasts: Site of photosynthesis in phototrophic eukaryotes; contain RuBisCO and thylakoids.

• Endosymbiotic Origin: Mitochondria and chloroplasts descended from bacterial cells associating with eukaryal hosts.

• Cytoskeleton: Internal structural support (microtubules, microfilaments, intermediate filaments).

• Other Structures: Endoplasmic reticulum, Golgi complex, lysosomes, flagella, and cilia (motility organelles).