Microbial Cell Structure and Function — Bacterial Walls, Flagella, Endospores & More | Chapter 2 of Brock Biology of Microorganisms

Welcome back to Last Minute Lecture! In this post, we dive into Chapter 2 of Brock Biology of Microorganisms, which provides a comprehensive look at microbial cell architecture. From bacterial envelopes to eukaryotic organelles, this chapter outlines how structure underpins function in the microbial world.

🎥 Watch the full chapter summary here:

Whether you're preparing for an exam or exploring microbiology for the first time, this guide offers a clear and complete overview of microbial cells and their adaptations for life in every environment on Earth.

The Microbial Cell Envelope

Microbial cell envelopes are composed of membranes, walls, and sometimes additional protective layers. These structures are essential for maintaining cellular integrity, mediating transport, and enabling energy generation.

Cytoplasmic Membrane

• Comprised of a phospholipid bilayer with embedded proteins
• Functions in selective permeability, protein anchoring, and energy conservation
• Archaeal membranes use ether-linked isoprenoids; bacterial and eukaryotic membranes contain ester-linked fatty acids
• Some Archaea form lipid monolayers with tetraether lipids for added stability

Nutrient Transport Mechanisms

• Simple transport: Driven by proton motive force
• Group translocation: Solutes are chemically modified during transport
• ABC transporters: High-affinity systems powered by ATP

Bacterial Cell Walls

• Constructed from peptidoglycan, a molecule unique to Bacteria
• Gram-positive: Thick peptidoglycan with teichoic acids
• Gram-negative: Thin wall, outer membrane with lipopolysaccharide (LPS)
• Targeted by antibiotics like penicillin and enzymes like lysozyme

LPS and the Outer Membrane

• Lipid A: Endotoxin component of LPS
• O-antigen and core polysaccharide: Structural and antigenic components
• Porins allow small molecules to diffuse
• Braun’s lipoprotein anchors outer membrane to peptidoglycan

Envelope Variations

• Archaea may have S-layers, pseudomurein, or no wall at all
• Wall-less microbes (e.g., Mycoplasma) rely on resilient membranes
• Ignicoccus has a unique outer membrane unlike that in Gram-negative bacteria

Cell Surface Structures and Inclusions

Adhesion and Defense

• Capsules and slime layers: Protect cells and help form biofilms
• Pili and fimbriae: Enable attachment and genetic exchange (conjugation)
• Hami: Archaea-only grappling hook structures for extreme environments

Cell Inclusions

• PHB and glycogen: Energy storage
• Polyphosphates and sulfur granules: Nutrient reserves
• Gas vesicles: Provide buoyancy
• Magnetosomes: Orient cells using Earth’s magnetic field

Endospores: Survival Specialists

Endospores are highly resilient, dormant forms produced by Gram-positive genera like Bacillus and Clostridium. They form in response to stress and allow cells to survive extreme conditions.

• Contain dipicolinic acid and small acid-soluble proteins (SASPs) for DNA protection
• Return to vegetative growth when conditions improve

Motility and Behavior

Flagella and Archaella

• Rotate like propellers using energy from proton motive force
• Arrangements include polar and peritrichous distributions
• Archaella serve similar functions but have different molecular components

Other Motility and Taxis

• Gliding motility: Surface movement without flagella
• Chemotaxis: Directed movement toward or away from chemicals
• Other stimuli: phototaxis (light), aerotaxis (oxygen), osmotaxis (osmolarity), magnetotaxis (magnetic fields)

Eukaryotic Microbial Cells

Eukaryotic microbes include fungi, algae, and protozoa. They are structurally complex and compartmentalized.

Key Organelles

• Nucleus: Double membrane, nucleolus for rRNA synthesis
• Mitochondria: Sites of aerobic respiration
• Chloroplasts: Photosynthesis centers with thylakoids and stroma
• Both organelles have bacterial origins (endosymbiosis)

Other Eukaryotic Structures

• Endoplasmic Reticulum: Protein and lipid synthesis
• Golgi Apparatus: Protein modification and sorting
• Lysosomes: Cellular digestion
• Cytoskeleton: Microtubules, microfilaments, intermediate filaments
• Flagella and cilia: Dynein-powered, 9+2 microtubule arrangement

Glossary Highlights

• Peptidoglycan: Unique bacterial cell wall polymer
• LPS: Lipopolysaccharide outer membrane with endotoxin activity
• ABC transporter: High-efficiency, ATP-powered transport system
• Endospore: Dormant structure for survival in harsh environments
• S-layer: Crystalline protein surface layer in Archaea and some Bacteria
• Magnetosome: Organelle that helps cells navigate using magnetic fields
• Chemotaxis: Movement in response to chemical gradients
• Mitochondrion/Chloroplast: Endosymbiotic energy-generating organelles
• ER/Golgi: Sites of biosynthesis and molecular transport
• Microtubules: Cytoskeletal filaments supporting structure and movement

Conclusion

Chapter 2 of Brock Biology of Microorganisms reveals the architectural brilliance of microbial cells. These structural insights explain how microbes adapt to diverse environments, interact with hosts, and perform complex behaviors like movement, storage, and survival. The study of microbial cell structure lays the foundation for understanding microbial physiology, genetics, and ecology in later chapters.

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