Microbial Cell Structure and Function

Overview of Cell Structure and Function

This section explores the diversity of microbial cell structures, focusing on the composition and function of cell walls, surface features, and mechanisms of locomotion. Understanding these features is essential for distinguishing between major groups of bacteria and archaea, as well as for appreciating their ecological and medical significance.

Cell Walls of Bacteria and Archaea

Gram Stain and Cell Wall Types

The Gram stain is a differential staining technique that classifies bacteria into two groups based on cell wall structure: Gram-positive and Gram-negative. This distinction is fundamental in microbiology for diagnosis and taxonomy.

• Gram-positive bacteria: Thick peptidoglycan layer, retain crystal violet dye (appear purple).

• Gram-negative bacteria: Thin peptidoglycan layer, outer membrane present, lose crystal violet and retain safranin (appear pink).

Peptidoglycan Structure

Peptidoglycan is a unique polymer forming the main component of bacterial cell walls. It consists of repeating units of N-acetylglucosamine (NAG) and N-acetylmuramic acid (NAM), cross-linked by short peptides.

• Provides structural strength and shape to the cell.

• Target for antibiotics such as penicillins and lysozyme.

Gram-Positive Cell Walls

Gram-positive bacteria have a thick peptidoglycan layer (up to 90% of the cell wall) and contain teichoic acids, which are important for cell wall maintenance and ion transport.

• Teichoic acids: Polymers of glycerol or ribitol phosphate, covalently linked to peptidoglycan.

• Lipoteichoic acids (LTA): Anchored in membrane lipids, contribute to cell wall structure.

Gram-Negative Cell Walls

Gram-negative bacteria have a more complex cell wall structure, with a thin peptidoglycan layer and an outer membrane containing lipopolysaccharide (LPS).

• Outer membrane: Phospholipid bilayer with embedded proteins and LPS.

• Lipopolysaccharide (LPS): Composed of Lipid A (endotoxin), core polysaccharide, and O-specific polysaccharide.

• Periplasm: Gel-like space between the cytoplasmic and outer membranes, containing various proteins.

• Porins: Channels that allow passage of small molecules.

Clinical Relevance: Escherichia coli O157:H7

Escherichia coli O157:H7 is a pathogenic Gram-negative bacterium that produces Shiga toxin, causing severe foodborne illness. It is identified by its O (somatic) and H (flagellar) antigens.

• Causes watery diarrhea, hemorrhagic colitis, and hemolytic-uremic syndrome (HUS).

• Transmitted through contaminated food, especially undercooked meat and unpasteurized milk.

Mollicutes: Bacteria Without Cell Walls

Mollicutes, such as Mycoplasma species, are prokaryotes that lack cell walls. They are among the smallest free-living organisms and are resistant to antibiotics targeting peptidoglycan synthesis, such as beta-lactams (e.g., amoxicillin).

• Mycoplasma pneumoniae: Causes walking pneumonia.

• Thermoplasma: Archaeal species without peptidoglycan.

Archaeal Cell Walls

Archaea have diverse cell wall structures, typically lacking peptidoglycan and outer membranes. Some possess pseudomurein, a polysaccharide similar to peptidoglycan but resistant to lysozyme and penicillin. The most common cell wall type among Archaea is the S-layer, composed of protein or glycoprotein.

• S-layer: Provides structural support and protection.

• Pseudomurein: Found in some methanogens, not affected by antibiotics targeting peptidoglycan.

Cell Surface Structures and Inclusions

Cell Inclusions

Cell inclusions are storage granules found in many prokaryotes, serving as reserves of carbon, energy, or other essential elements.

• Poly-β-hydroxyalkanoates (PHA): Lipid storage granules, biodegradable plastics.

• Glycogen: Glucose polymer, energy storage.

• Polyphosphates: Inorganic phosphate storage.

• Sulfur globules: Elemental sulfur storage, common in sulfur bacteria.

• Magnetosomes: Magnetic inclusions containing magnetite (Fe3O4), allow orientation in magnetic fields.

• Gas vesicles: Gas-filled structures that confer buoyancy, allowing planktonic cells to float and access light.

Capsules and Slime Layers

Capsules and slime layers are gelatinous polysaccharide layers external to the cell wall. They play important roles in attachment, biofilm formation, protection from desiccation, and evasion of host immune responses.

• Assist in attachment to host cells and surfaces.

• Protect against phagocytosis and desiccation.

• Component of some vaccines (e.g., pneumococcal vaccine).

Fimbriae and Pili

Fimbriae and pili are filamentous protein structures on the cell surface. Fimbriae are shorter and more numerous, aiding in attachment and biofilm formation. Pili are longer, involved in attachment, genetic exchange (conjugation), and certain types of motility (twitching).

• Fimbriae: Enable adherence to surfaces and other cells.

• Pili: Facilitate conjugation (DNA transfer) and twitching motility.

Cell Locomotion

Flagella and Swimming Motility

Flagella are helical, whip-like structures used for locomotion in many bacteria. They are powered by a proton motive force, rotating like a propeller to move the cell. Flagellar arrangements include peritrichous (all over), polar (one or both ends), lophotrichous (tuft at one end), and amphitrichous (both ends).

• Swimming speed can reach up to 60 cell lengths per second.

• Flagellar rotation direction determines movement: counterclockwise (run), clockwise (tumble).

Gliding Motility

Some bacteria move across surfaces without flagella, using gliding motility. This movement is slower and smoother than swimming and requires surface contact. Mechanisms include secretion of polysaccharide slime, movement of surface proteins, or pili-mediated twitching.

• Important for colonization and biofilm formation.

• Observed in genera such as Flavobacterium.

Chemotaxis and Other Taxes

Chemotaxis is the directed movement of bacteria in response to chemical gradients. Other taxes include phototaxis (light), aerotaxis (oxygen), osmotaxis (ionic strength), and hydrotaxis (water). Bacteria sense gradients via chemoreceptors and adjust their movement accordingly, exhibiting 'run and tumble' behavior.

• Attractants increase run duration; repellents increase tumbling.

• Measured using capillary assays with attractants or repellents.

Endospores

Endospore Structure and Function

Endospores are highly differentiated, dormant cells formed by some Gram-positive bacteria. They are resistant to heat, chemicals, and radiation, allowing survival in harsh environments for extended periods. Endospores are important in the transmission of diseases such as anthrax and botulism.

• Formed by genera such as Bacillus and Clostridium.

• Contain protective layers, dipicolinic acid, and small acid-soluble spore proteins (SASPs).

• Germinate into vegetative cells when conditions improve.

Summary Table: Comparison of Cell Wall Types