II. Cell Surface Structures and Inclusions

2.6 Cell Surface Structures

Microbial cells possess specialized surface structures that play critical roles in protection, attachment, and interaction with their environment.

• Capsules and Slime Layers:

  • Sticky polysaccharide coat outside the cell envelope.

  • Capsule: Tightly attached, forms a dense matrix; visible with India ink staining.

  • Slime layer: Loosely attached, easily deformed.

  • Functions:

    • Assist in attachment to surfaces.

    • Play a role in development and maintenance of biofilms.

    • Contribute to infectivity by protecting against host defenses.

    • Prevent dehydration/desiccation.

• Fimbriae and Pili:

  • Pili: Thin filamentous protein structures (~2–10 nm wide) that enable organisms to stick to surfaces, form pellicles, or biofilms.

  • Fimbriae: Short pili mediating attachment; produced by all Gram-negatives and many Gram-positives.

  • Conjugative/sex pili: Facilitate genetic exchange between cells (conjugation).

  • Electrically conductive pili: Conduct electrons.

  • Type IV pili: Adhere to host tissues and support twitching motility.

2.7 Cell Inclusions

Cell inclusions are intracellular structures that serve as storage sites for nutrients and other essential compounds, helping cells adapt to environmental changes.

• General Features:

  • Function as energy reserves, carbon or phosphorus reservoirs, or have special functions.

  • Enclosed by thin protein membrane, reducing osmotic stress.

• Carbon Storage Polymers:

  • Synthesized when carbon is in excess; broken down as carbon or energy sources when needed.

  • Glycogen: Glucose polymer.

  • Elemental sulfur: Accumulates in periplasmic granules, oxidized to sulfate ().

• Gas Vesicles:

  • Confer buoyancy to aquatic microbes.

  • Conical-shaped, gas-filled structures made of two proteins.

• Magnetosomes:

  • Allow bacteria to orient within magnetic fields.

  • Composed of biomineralized magnetic iron oxides.

  • Enable magnetotaxis: migration along magnetic field lines.

2.8 Endospores

Endospores are highly differentiated, dormant cells formed by certain bacteria to survive extreme conditions.

• Characteristics:

  • Resistant to heat, radiation, chemicals, drying, and nutrient deprivation.

  • Serve as survival structures during unfavorable growth conditions.

  • Life cycle: Vegetative cell → endospore → vegetative cell.

  • Ideal for dispersal via wind, water, or animal gut.

  • Present only in some Gram-positive bacteria (e.g., Bacillus and Clostridium).

• Endospore Formation and Germination:

  • Sporulation: Vegetative cell differentiates into a nongrowing, heat-resistant, light-refractive structure, triggered by nutrient limitation.

  • Endospores can remain dormant for years but convert rapidly back to vegetative cells when nutrients become available.

  • Germination involves three steps: activation, germination, and outgrowth.

  • Special stains (e.g., malachite green) are required for visualization.

• Table: Differences Between Endospores and Vegetative Cells

III. Cell Locomotion

2.9 Flagella, Archaella, and Swimming Motility

Microbial motility is primarily achieved through specialized appendages that enable movement in liquid environments.

• Flagella/Archaella:

  • Assist in swimming in Bacteria and Archaea.

  • Tiny rotating machines that push or pull cells through liquid.

  • Flagella: Long, thin appendages (15–20 nm wide) anchored in cell at one end.

  • Arrangements: polar, lophotrichous (tufts), amphitrichous, peritrichous.

  • Rotational speed depends on strength of proton motive force.

2.11 Chemotaxis

Chemotaxis is the directed movement of cells in response to chemical gradients, allowing microbes to locate nutrients or avoid harmful substances.

• Taxis: Directed movement in response to chemical or physical stimuli.

• Chemotaxis: Response to chemicals, monitored by chemoreceptors that sense attractants and repellents.

• Phototaxis: Response to light.

• Directed movement enhances access to resources or allows avoidance of damage/death.

• Occurs in both Bacteria and Archaea.

• Measuring Chemotaxis:

  • Capillary tube assay: A capillary tube containing an attractant or repellent is inserted into a suspension of motile bacteria.

  • Chemical concentration decreases with distance from tip.

  • Chemotactic bacteria swarm toward attractant, increasing cell numbers in the capillary.

2.12 Other Forms of Taxis

Microbes can respond to various environmental cues through specialized forms of taxis.

• Osmotaxis: Response to ionic strength.

• Hydrotaxis: Response to water.

• Aerotaxis: Response to oxygen.

• Phototaxis: Response to light.

IV. Eukaryotic Microbial Cells

2.13 The Nucleus and Cell Division

Eukaryotic cells are defined by their membrane-bound nucleus and complex organelles, which facilitate compartmentalization and specialized functions.

• Nucleus:

  • Contains chromosomes; DNA is wound around histones forming nucleosomes, organized into chromosomes.

  • Archaea also contain histones and nucleosomes, showing evolutionary relation to Eukarya.

  • Enclosed by two membranes: inner interacts with nucleoplasm, outer with cytoplasm.

  • Nucleolus: Site of ribosomal RNA synthesis.

• Cell Division:

  • Mitosis: Results in two diploid daughter cells (two copies of each chromosome).

  • Meiosis: Specialized nuclear division converting diploid into haploid cells, resulting in four haploid gametes.

2.14 Mitochondria and Chloroplasts

Mitochondria and chloroplasts are energy-converting organelles with evolutionary origins in Bacteria.

• Mitochondria:

  • Site of respiration and oxidative phosphorylation in aerobic eukaryotes.

  • Number per cell varies from few to over 1000.

  • Surrounded by two membranes; cristae are folded internal membranes containing enzymes for respiration and ATP production.

  • Matrix: Innermost area containing citric acid cycle enzymes.

• Chloroplasts:

  • Chlorophyll-containing organelle found in phototrophic eukaryotes.

  • Site of photosynthesis; double membrane.

  • Inner membrane surrounds stroma, which contains large amounts of RuBisCO (key enzyme for Calvin cycle: to organics).

  • Thylakoids: Flattened membrane discs containing chlorophyll and ATP synthetic components, forming proton motive force.

• Endosymbiotic Origin:

  • Mitochondria and chloroplasts descended from respiratory and phototrophic bacterial cells, respectively, via endosymbiosis.

  • Evidence: Circular DNA genomes and ribosomes similar to Bacteria.

  • Eukarya originated from symbiotic fusion of archaeal host and mitochondrial endosymbiont; later, chloroplast endosymbiont was acquired.

2.15 Other Eukaryotic Cell Structures

Eukaryotic cells possess a variety of organelles and structural components that support cellular function and motility.

• Cytoskeleton:

  • Microtubules: Hollow tubes (25 nm diameter) of α- and β-tubulin; maintain cell shape, facilitate motility, move chromosomes and organelles.

  • Microfilaments: 7 nm diameter, polymers of actin; maintain/change cell shape, involved in amoeboid motility and cell division.

  • Intermediate filaments: 8–12 nm diameter, fibrous keratin proteins; maintain cell shape and position organelles.

• Endoplasmic Reticulum (ER), Golgi Complex, and Lysosomes:

  • ER: Network of membranes continuous with nuclear membrane; rough ER has attached ribosomes, smooth ER does not.

  • Smooth ER: Synthesis of lipids and carbohydrate metabolism.

  • Rough ER: Produces glycoproteins and new membrane material.

  • Golgi complex: Stacks of membrane-bound sacs modifying ER products.

  • Lysosomes: Membrane-enclosed compartments containing digestive enzymes and recycling cell components.

• Flagella and Cilia:

  • Present on many eukaryotic microbe surfaces; function in motility.

  • Cilia: Short flagella that beat in synchrony.

  • Flagella: Long appendages that propel cells through whiplike motion.

  • Structurally and functionally differ from prokaryotic flagella.

Additional info:

• Figures referenced in the notes (e.g., Figure 2.16, 2.17, etc.) provide visual examples of the described structures and processes.

• Special stains (e.g., malachite green) are used for endospore visualization due to their resistance to conventional staining methods.