Microbiology Lecture 2: Bacterial and Archaeal Cell Structures and Outer Layers and Envelopes
Introduction to Bacterial and Archaeal Microbes
Microbiology METX 119 Lecture 2 focuses on the architectural composition and external layers of bacterial and archaeal cells.
The previous lecture established that the ribosomal RNA sequence can define all of life, grouping microorganisms into Bacteria and Archaea.
While both Bacteria and Archaea are microscopic and require high magnification to be visualized, their internal and external structural components differ significantly.
Case Study: Clinical Significance of Cell Architecture
Initial Symptoms: A college student presented with a fever of , a headache, and a stiff neck.
Preliminary Diagnosis: Health center staff suspected bacterial meningitis, a life-threatening disease, and referred the student to the emergency room immediately.
Diagnostic Procedure: Healthcare workers performed a lumbar puncture to collect cerebrospinal fluid (CSF). The sample was analyzed using a Gram stain to differentiate between Gram-positive and Gram-negative bacteria.
Microscopic Observations:
Large red cells with large red nuclei: These represent the patient's own human cells stained pale red.
Dark purple cells: These represent Gram-positive bacteria.
Outcome: The specific staining pattern (purple color) allowed for a rapid diagnosis and targeted antibiotic treatment.
Key Clinical Questions:
Why is the Gram stain alone sufficient for a diagnosis in this context?
How does cell wall composition dictate the choice of an antibiotic cocktail?
Universal Features of the Cytoplasmic Membrane
All living cells, including those of the Last Universal Common Ancestor (LEUCA), possess a lipid bilayer membrane.
The most basic cellular structure consists of the cytoplasm enclosed by a cytoplasmic membrane.
General composition includes phosphoglycerol diethers or phosphoglycerol diesters.
Membrane Structure:
Hydrophilic Head Groups: Oriented toward the exterior and interior aqueous environments.
Hydrophobic Tails: Oriented toward the center of the membrane, creating a non-polar barrier.
Membrane Structural Variations: Bacteria vs. Archaea
Linkage Differences:
In Bacteria and Eukaryotes, the connection between the hydrophilic head group and the hydrophobic tail is an ester linkage.
In Archaea, this connection is an ether linkage.
Lipid Branching: Archaea possess branched lipids in their membranes, whereas bacterial lipids are typically unbranched.
Bilayer vs. Monolayer:
Bacteria and most life forms utilize a lipid bilayer.
Some Archaea utilize a lipid monolayer, where the hydrophobic tails are fused/connected through the middle. This provides extreme stability and fluidity control, particularly in high-temperature environments.
Rigidity and Fluidity (Sterols and Hopanoids):
Prokaryotes (a term for organisms lacking a nucleus, encompassing both Bacteria and Archaea) add molecules to their membranes to adjust rigidity.
While mammals use cholesterol, prokaryotes use similar molecules called hopanoids (detailed in Box 2.1). These molecules function to modify membrane fluidity.
Functions and Physical Properties of the Cytoplasmic Membrane
Diffusion Barrier: The membrane acts as a primary barrier to separate the internal compartment from the external environment.
Energy Generation: Unlike mammalian cells, which use mitochondria, bacteria and archaea utilize their cytoplasmic membrane as the location for the Electron Transport Chain (ETC).
The membrane maintains a transmembrane ion gradient (proton gradient) used for ATP generation.
Protein Concentration: Prokaryotic membranes tend to have a higher density of proteins than mammalian cells. These proteins serve two main purposes:
Environmental Sensing: Receptors that allow the microbe to respond to its surroundings.
Transport: Facilitating the movement of nutrients across the membrane.
Vulnerabilities:
The membrane follows the Fluid Mosaic Model, meaning it is relatively soft and non-rigid to allow protein movement.
It is vulnerable to toxic substances like detergents and bile salts (natural detergents in the human body that disrupt lipids).
It cannot withstand high internal osmotic pressure on its own.
The Peptidoglycan Cell Wall: Structure and Integrity
Functional Analogy: The cell wall is described as a "chain link fence" because it is rigid yet porous (stiff with large holes).
Osmotic Protection: The primary role of the cell wall is to prevent the cell from bursting due to the high internal osmotic pressure exerted by the cytoplasm's contents (proteins and small molecules).
Porosity: The cell wall allows large molecules to diffuse through, with a size cutoff of approximately .
Chemical Composition: The wall is a sugar polymer composed of two alternating sugars:
N-acetylglucosamine (NAG or G)
N-acetylmuramic acid (NAM or M)
Structure: NAG and NAM form long glycan chains that are connected to one another by amino acid crossbridges. This creates an encompassing "cage" that maintains the cell's shape.
Gram Staining and Cell Wall Differentiation
Gram-Positive Bacteria: Characterized by a thick cell wall consisting of many layers of peptidoglycan.
Gram-Negative Bacteria: Characterized by a thin cell wall consisting of only a few layers of peptidoglycan.
The Gram Stain Process:
Crystal Violet: The primary stain that turns all cells purple.
Mordant (Iodine): Functions as a "stain locker" to fix the crystal violet.
Decolorization (Alcohol): The alcohol removes the stain from Gram-negative cells due to their thin cell wall. Gram-positive cells retain the purple stain because the alcohol cannot penetrate the thick layers.
Safranin: A pink counterstain is applied. Gram-negative cells (which were clear after decolorization) turn pink. Gram-positive cells remain purple, as the light pink stain cannot overcome the dark purple color.
Biosynthesis of Peptidoglycan and Antibiotic Interaction
Biosynthetic Steps:
NAG and NAM are synthesized within the cytoplasm.
They are linked into a disaccharide unit.
A peptide (approximately 4 amino acids long) is attached to the NAM subunit.
This precursor is transported across the membrane via a transporter protein.
The precursor is ligated to the existing glycan chain.
Key Enzymes:
Transglycosylase: Catalyzes the sugar-to-sugar bonds between NAG and NAM.
Transpeptidase (also called Penicillin Binding Protein or PBP): Catalyzes the formation of the peptide crossbridge between chains.
Targets of Inhibition:
Lysozyme: An enzyme found in human secretions (tears, saliva) that cleaves the connection between NAG and NAM sugars.
In an isotonic solution, cells treated with lysozyme lose their shape and round up but do not burst.
In a hypotonic solution (e.g., water), the water rushes into the cell, and without the cell wall cage, the cell bursts.
Penicillin: An antibiotic that binds to the active site of the transpeptidase enzyme, blocking the formation of the peptide crossbridge. This weakens the cell wall, causing the bacteria to burst.
Vancomycin: Inhibits the cell wall by binding directly to the ends of the NAM-peptides, sterically preventing the transpeptidase from forming a crossbridge.
Antibiotic Resistance:
Beta-lactamase: An enzyme acquired by certain bacteria that cleaves and inactivates penicillin.
Structural Resistance: Gram-negative bacteria have an outer membrane that can act as a barrier to certain antibiotics.
Waxy Barriers: Mycolic Acids and Acid-Fast Bacteria
Some bacteria add an additional layer of mycolic acids on top of their peptidoglycan cell wall.
Physical Properties: Mycolic acids are a waxy, hydrophobic layer that is virtually impenetrable to both hydrophobic and hydrophilic molecules.
Acid-Fast Stain: Due to this waxy layer, these bacteria retain stains even after treatment with acid, leading to the term "acid-fast bacteria."
Clinical Example: Mycobacterium tuberculosis is characterized by this waxy coat, making it extremely difficult for antibiotics to penetrate and treat.
The Gram-Negative Outer Membrane and Lipopolysaccharide (LPS)
Gram-negative bacteria possess a second lipid bilayer located outside the thin cell wall called the outer membrane.
Outer Membrane Leaflets:
Inner Leaflet: Composed mainly of standard phospholipids.
Outer Leaflet: Composed of Lipopolysaccharide (LPS).
LPS Structure:
Lipid A: The hydrophobic portion embedded in the membrane.
Core Polysaccharide: The central sugar structure.
O-antigen: Repeating subunits of sugars extending outward into the environment.
LPS Function: LPS is highly hydrophilic, which provides a barrier against hydrophobic toxic substances like detergents.
Transport across the Outer Membrane:
Porins: Protein channels that allow the diffusion of small molecules.
Size Cutoff: Porins have a size limit of approximately .
Periplasmic Space: The area between the cytoplasmic membrane and the outer membrane in Gram-negative bacteria. It contains the cell wall and essential proteins like transpeptidases and transglycosylases.
External Protective Layers: Capsules and Slime
Many bacteria (both Gram-positive and Gram-negative) produce an outermost layer of heavy, gooey polysaccharide or amino acids.
Terminology:
Capsule: If the layer is tightly attached to the cell.
Slime: If the layer is loosely attached.
Functions:
Prevents desiccation (drying out) by retaining water.
Protects against the host immune system.
Acts as a barrier against hydrophobic molecules.
Case Study Resolution: Bacterial Meningitis Diagnosis
The Diagnostic Value of Color: CSF is normally sterile; the presencia of any bacteria is a critical finding. Seeing bacteria definitively ruled out viral meningitis.
Pathogen Identification:
Common causes of meningitis include Neisseria meningitidis (Gram-negative), Haemophilus influenzae (Gram-negative), and Streptococcus pneumoniae (Gram-positive).
Because the stain was purple (Gram-positive), the doctors identified the causative agent as likely being Streptococcus pneumoniae.
Clinical Application: Knowing the specific cell wall structure (thick peptidoglycan, no outer membrane) allowed doctors to select an antibiotic cocktail optimized for Gram-positive organisms, improving the patient's prognosis.