Microbial Cell Structure and Function: Study Guide

Microbial Cell Structure and Function

Overview of Microbial Cell Structures

Microbial cells possess a variety of structural components that enable them to interact with their environment, maintain integrity, and perform essential functions. The cell envelope is a key feature, providing protection and mediating exchanges with the surroundings.

• Cytoplasmic (Cell) Membrane: Selective permeability, energy metabolism

• Cell Wall: Maintains shape, rigidity, prevents lysis

• Outer Membrane (Gram-negative only): Protection, virulence

• Capsule: Virulence, immune evasion

• Fimbriae: Attachment

• Pili: Conjugation (DNA transfer)

• Flagella: Motility

• Inclusions: Nutrient and energy storage

• Endospores: Survival under harsh conditions

The Cell Envelope

The cell envelope is a layered system surrounding the cytoplasm, crucial for protection and environmental interaction.

• Cytoplasmic membrane: Present in all cells

• Cell wall: Most bacteria

• Outer membrane: Gram-negative only

• S-layer: Some Bacteria & Archaea

Cytoplasmic (Cell) Membrane

The cytoplasmic membrane is a phospholipid bilayer, typically 8–10 nm thick, with embedded proteins. It is essential for selective permeability and energy metabolism.

• Structure: Hydrophobic fatty acid tails inward, hydrophilic phosphate heads outward

• Functions: Selective permeability, nutrient uptake, waste removal, energy metabolism, protein anchoring

• Membrane Proteins: Integral (transmembrane) and peripheral (loosely attached)

Membrane Differences Across Domains

Bacteria and Eukarya have ester-linked fatty acids, while Archaea possess ether-linked isoprenoid chains, which can form lipid monolayers for extreme stability.

• Bacteria & Eukarya: Ester linkages, fatty acid chains

• Archaea: Ether linkages, isoprenoid chains, lipid monolayers

Cell Wall

The cell wall prevents osmotic lysis and maintains cell shape. In bacteria, it is composed of peptidoglycan, a polymer of alternating N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM) linked by β-1,4 glycosidic bonds and cross-linked peptides.

• Peptidoglycan: Unique to Bacteria

• Absent in: Archaea & Eukarya

Gram-Positive vs Gram-Negative Bacteria

Bacteria are classified based on their cell wall structure, which affects staining and susceptibility to antibiotics.

• Gram-Positive: Thick peptidoglycan (20–35 nm), teichoic acids, no outer membrane (e.g., Staphylococcus)

• Gram-Negative: Thin peptidoglycan, outer membrane with LPS, periplasmic space (e.g., E. coli)

Lysozyme & Antibiotics

Lysozyme and antibiotics target bacterial cell walls, leading to cell lysis.

• Lysozyme: Breaks β-1,4 bonds in peptidoglycan

• Penicillin: Blocks peptide cross-linking

Archaeal Cell Walls

Archaea lack peptidoglycan and often have S-layers. Methanogens possess pseudomurein, which is resistant to lysozyme and penicillin due to β-1,3 bonds and all L-amino acids.

• S-layers: Protein shell

• Pseudomurein: Peptidoglycan-like polymer in methanogenic Archaea

Gram-Negative Outer Membrane (LPS)

The outer membrane of Gram-negative bacteria contains lipopolysaccharide (LPS), which is important for virulence and immune response.

• LPS Components: O-polysaccharide (antigenic), core polysaccharide, lipid A (endotoxin)

• Other Features: Porins (transport), Braun lipoprotein (anchors OM), periplasm (enzymes & proteins)

S-Layers

S-layers are protein or glycoprotein lattices found in some Bacteria and Archaea, providing protection, shape, and adhesion.

• Function: Protection, shape, adhesion

Cell Surface Structures

Microbial cells may possess capsules, slime layers, pili, fimbriae, and hami, which aid in attachment, biofilm formation, and immune evasion.

• Capsules & Slime Layers: Polysaccharide coating, prevent phagocytosis, aid in biofilms

• Pili & Fimbriae: Fimbriae (attachment), sex pili (conjugation), type IV pili (twitching motility)

• Hami (Archaea): Grappling-hook structures for surface attachment

Cell Inclusions

Cell inclusions are storage structures for nutrients and energy, as well as specialized functions like buoyancy and magnetotaxis.

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

Endospores

Endospores are highly resistant survival structures produced by certain Gram-positive bacteria, such as Bacillus and Clostridium.

• Resistance: Heat, radiation, chemicals

• Components: Dipicolinic acid + Ca2+, SASPs (protect DNA)

• Germination Stages: Activation, germination, outgrowth

Flagella & Motility

Flagella are motility structures powered by the proton motive force. Their arrangement varies among species.

• Arrangements: Polar, lophotrichous, amphitrichous, peritrichous

• Energy Source: Proton motive force

Surface Motility

Microbes can move across surfaces using twitching (type IV pili + ATP) or gliding (smooth movement without flagella).

• Twitching: Type IV pili + ATP

• Gliding: Smooth movement

Chemotaxis & Taxis

Chemotaxis is movement toward or away from chemicals, while other taxis include responses to light, oxygen, osmotic pressure, water, and magnetic fields.

• Chemotaxis: Run & tumble (e.g., E. coli)

• Other Taxis: Phototaxis, aerotaxis, osmotaxis, hydrotaxis, magnetotaxis

Endosymbiotic Theory

The endosymbiotic theory explains the origin of mitochondria and chloroplasts in eukaryotes, based on evidence such as circular DNA, bacterial-like ribosomes, and double membranes.

• Mitochondria: Derived from bacteria

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• Chloroplasts: Derived from cyanobacteria

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Microbial Morphology and Arrangement

Bacteria exhibit diverse shapes and arrangements, which are important for identification and classification.

• Cocci: Spherical (e.g., Staphylococcus)

• Rods (bacilli): Elongated (e.g., Escherichia coli)

• Spirilla: Spiral-shaped

• Arrangement: Single, chains, clusters, tetrads

Table: Gram Reaction, Shape, and Arrangement of Common Bacteria

Organism	Gram	Shape	Arrangement
E. coli	−	Rod	Single
Bacillus	+	Rod	Chains
Staphylococcus	+	Cocci	Clusters
Micrococcus	+	Cocci	Tetrads
Streptococcus	+	Cocci	Chains
Vibrio	−	Curved rod	Single
Treponema	−	Spirochete	Single

Prokaryotic vs Eukaryotic Cells

Prokaryotes (Bacteria and Archaea) lack a nucleus and organelles, while eukaryotes (animals, plants, fungi) possess these structures.

• Prokaryotes: Bacteria, Archaea

• Eukaryotes: Animals, plants, fungi

Reproduction

Microbial reproduction varies by domain and group.

• Bacteria: Binary fission

• Fungi: Spores/budding

• Plants: Sexual & asexual

• Animals: Sexual

Endospores vs Fungal Spores

Endospores are survival structures, while fungal spores are reproductive.

• Endospores: Survival

• Fungal spores: Reproduction

Gas Vesicles

Gas vesicles control buoyancy, optimizing access to light and nutrients.

• Function: Buoyancy regulation

Flagella

Flagella are composed of flagellin and provide motility.

• Function: Motility

• Composition: Flagellin

Do Bacteria Have Cilia?

Bacteria do not possess cilia; cilia are exclusive to eukaryotic cells.

Phototaxis

Phototaxis is the movement toward or away from light, an important behavior in some microorganisms.

• Positive phototaxis: Toward light

• Negative phototaxis: Away from light

Key Equations

Peptidoglycan structure:

• $\text{NAG} - \beta-1,4 - \text{NAM}$

Binary fission:

• $\text{N}_t = \text{N}_0 \times 2^n$

Proton motive force:

• $\Delta p = \Delta \Psi - \frac{2.3RT}{F} \Delta pH$

Additional info: Academic context was added to clarify cell envelope structure, membrane differences, and endosymbiotic theory.