Microbial Cell Structure and Function

Overview of Cell Structure and Function

This section introduces the fundamental aspects of microbial cell structure and function, focusing on the methods used to visualize and analyze cells, and the diversity of cell morphologies and envelopes in Bacteria and Archaea.

Microscopy: Discovering Cell Structure

Light Microscopy

Light microscopy is a primary tool for observing microbial cells. It allows visualization of cell shape, arrangement, and basic internal structures. Improving contrast is essential for distinguishing cellular features.

• Staining: Dyes such as methylene blue, safranin, and crystal violet bind to specific cellular materials, enhancing contrast.

• Differential Stains: The Gram stain differentiates bacteria into Gram-positive (purple) and Gram-negative (red) groups based on cell wall structure.

• Basic Dyes: Positively charged dyes bind to negatively charged molecules like DNA and cell surfaces.

Non-Destructive Contrast Methods

• Phase-Contrast Microscopy: Enhances contrast in unstained cells by amplifying differences in refractive index.

• Dark Field Microscopy: Illuminates specimens from the side, making them appear bright against a dark background.

• Fluorescence Microscopy: Visualizes specimens that emit light of one color when illuminated with another. Used for natural autofluorescence or after staining with fluorescent dyes (e.g., DAPI binds DNA).

Imaging Cells in Three Dimensions

Confocal scanning laser microscopy (CSLM) uses a laser and computer to focus on single layers of a specimen, compiling them into a 3D image. This technique allows high-resolution imaging of live samples.

Electron Microscopy

Electron microscopes use electrons instead of photons, enabling visualization of structures from 10 nm to 100 µm. Two main types are:

• Transmission Electron Microscopy (TEM): Provides 2D views of thin, stained sections of cells, revealing internal structures.

• Scanning Electron Microscopy (SEM): Coats specimens with heavy metals and scans them with an electron beam, producing 3D images of cell surfaces.

Viruses: Structure and Visualization

Viruses and Their Multiplication

Viruses are not considered cells and lack metabolic abilities. They rely entirely on host cell machinery for replication and infect all cell types, including bacteria (bacteriophages). The smallest viruses are about 10 nm in size.

Cells of Bacteria and Archaea

Cell Morphology

Cell morphology refers to the shape of microbial cells. Major morphologies include:

• Coccus: Spherical or ovoid

• Rod: Cylindrical

• Spirillum: Spiral-shaped

• Other shapes: Spirochete, stalk, hypha, filamentous, budding, and appendaged bacteria

Filamentous Bacteria and Wastewater Treatment

Filamentous bacteria can cause problems in wastewater treatment by affecting sedimentation and floc formation.

Cell Size of Prokaryotes

Prokaryotic cell sizes range from 0.2 µm to over 700 µm. Most cultured rod-shaped bacteria are 0.5–4.0 µm wide and less than 15 µm long. Exceptions include very large (e.g., Epulonipiscium fishelsoni) and ultra-small cells.

• Advantages of Small Size: Higher surface area-to-volume ratio supports greater nutrient exchange and faster growth.

• Lower Limit: Cells smaller than 0.15 µm are rare due to constraints on essential cellular functions.

Ultra-Small Cells

Ultra-small cells can pass through filters with 0.2-µm pores and are often found in environmental samples.

The Cell Membrane and Wall

The Cytoplasmic Membrane

The cytoplasmic membrane is a thin, vital barrier (~8 nm thick) that surrounds the cell, separating the cytoplasm from the environment. It is highly selective, controlling the concentration of metabolites and excretion of waste.

• Phospholipid Bilayer: Composed of hydrophobic fatty acids and hydrophilic glycerophosphates.

• Embedded Proteins: Integral membrane proteins are firmly embedded and serve various functions.

Bacterial Cell Walls: Peptidoglycan

Bacterial cell walls are classified based on Gram stain results:

• Gram-Positive: Thick peptidoglycan layer

• Gram-Negative: Thin peptidoglycan layer and an outer membrane containing lipopolysaccharide (LPS)

Peptidoglycan Structure and Function

Peptidoglycan is a rigid polysaccharide layer providing strength to the cell wall. It consists of:

• N-acetylglucosamine (G) and N-acetylmuramic acid (M): Alternating sugar units

• Amino Acids: Form peptide cross-links

• Cross-Linking: Differs between Gram-positive and Gram-negative bacteria

• Degradation: Lysozyme and beta-lactam antibiotics (e.g., penicillins, cephalosporins) degrade peptidoglycan

Functions of the Cytoplasmic Membrane

• Permeability Barrier: Polar and charged molecules require transport proteins to cross the membrane.

• Protein Anchor: Holds transport proteins in place.

• Energy Conservation: Generation of proton motive force for cellular energy.

Archaeal Cell Membranes

Archaeal membranes differ from those of Bacteria and Eukarya:

• Ether Linkages: Archaeal phospholipids have ether bonds, while Bacteria and Eukarya have ester bonds.

• Isoprene Units: Archaeal lipids contain isoprene instead of fatty acids.

• Lipid Monolayers: Some Archaea have monolayer membranes, which are more heat resistant and found in thermophilic species.

Membrane Function: Conclusion

• Permeability Barrier: Essential for selective transport and accumulation of solutes.

• Protein Anchor: Maintains the position of transport proteins.

• Energy Conservation: Facilitates the generation of proton motive force.

Summary Table: Gram-Positive vs. Gram-Negative Cell Walls

Key Equations

Surface Area to Volume Ratio:

For a sphere:

Conclusion

Understanding microbial cell structure and function is fundamental to microbiology. Microscopy techniques, cell morphology, and cell envelope composition are key to identifying, classifying, and studying microbial life.