Microbiology Lecture 4: Microbial Morphology

Lecture 4: Basic Microbial Morphology

• Lecture is on basic microbial morphology.
• The lecture covers preparing bacterial cells for microscopy.
• Light microscope resolution is discussed.
• Electron microscopy, including scanning and transmission electron microscopy.
• Domains of Life.
• Types of microorganisms explored.
• Eukaryotic cell structure, compared with prokaryotic cells.
• Prokaryotic cell morphology.
• Bacterial cell structure.
• Bacterial shapes (cocci, bacilli, curved or spiral).
• The Gram stain.
• Gram stain mechanism.
• Bacterial cell structure differences (Gram +ve, Gram-ve, Archaea).
• Cell membrane structure, cell wall and outer membrane.
• Cell appendages.
• Lab instructions for the practical and required materials (print out lab manual, bring a notebook).

Learning Outcomes

• Prokaryotic cell morphology.
• Bacterial cell structure.
• The Gram stain, and Gram stain mechanism.
• Different morphology shapes of bacteria.
• Bacterial cell structure. (focus on Gram-positive and Gram-negative; and Archaea.)
• Cell walls (including cell membrane and outer membrane structures)
• Cell appendages.

Bacterial Cell Structure

• A diagram of basic bacterial cell structure. (Includes cytoplasm, nucleoid, ribosomes, plasmids, cell wall and cytoplasmic membrane.)
• Bacterial cell structure is only 0.5 μm in size. (Microbial size varies)

Gram Stain Procedure

• Gram stain is a crucial differential staining method used in microbiology. (Diagram of the procedure is also available)
• Gram staining distinguishes between Gram-positive and Gram-negative bacteria via differential lipid content. (Process steps are also included)

Gram Stain Mechanism

• The Gram-stain mechanism involves different lipid content in cell envelopes of gram-positive and gram-negative bacteria.
• Gram-positive bacteria have less lipid in the cell-envelope, compared to Gram-negative bacteria.
• Crystal violet-iodine complex is created within the cell (colour = blue)
• Alcohol treatment extracts high lipid content for Gram-negative bacteria, causing the crystal violet-iodine complex diffuse out.
• Neutral red is then used to visualize Gram-negative bacteria.

Microscopy

• Oil must be removed from 100x lens using lens tissue. (This is related to the use of a microscope.)

Streaking out a Mixed Culture procedure

• Steps for streaking out a mixed culture to obtain isolated colonies.

Microbes and their Environments

• Microbes inhabit a wide range of locations. (Air, fingertips, hair, soil, water, blades of grass, and plants).
• Incubate the cultures at 27°C (Incubation temperature is also a point of interest throughout the lab work)

Lab Write Up

• Students are required to write up the lab work and submit it on Brightspace, within one week from the date of the lab.
• Instructions for lab write-up are on page 2 of the practical manual.
• Using Sharpie pens for labeling petri dishes.
• All questions need to be asked to demonstrators during the lab; do not leave the lab before all questions are answered.

Bacteria that don't stain with Gram's Method

• Some bacteria, like Mycobacteria, have a high wax content in the cell envelope preventing the Gram stain technique from working.
• Mycobacteria are stained with a different method (Ziehl-Nielson stain.)
• Mycoplasmas (small bacteria) have no cell walls (so not visible by Gram stain) and special staining techniques are required.

Bacterial Shapes

• Bacteria can be spherical (cocci), Rod shaped (bacilli) or curved / spiral shapes. (Different morphological shapes)

Cell Walls of Archaea

• Archaea cell walls lack peptidoglycan.
• Some Archaea contain pseudopeptidoglycan (repeated sugar units linked with αβ1,3)
• S-layer is another component of walls of Archaea; composed of a protein or glycoprotein layer

Gram-Negative Cell Envelope: Outer Membrane

• Phospholipid-Lipopolysaccharide (LPS) forms the bilayer.
• The outer membrane is the second lipid layer beyond the cell wall in Gram-negative bacteria.
• It offers protection, assists cell adhesion, resist phagocytosis , acts as a molecular sieve.

Bacterial Cell Wall Function

• Bacterial cell walls have high concentrations of dissolved solutes (salts, sugars) which result in high pressure from the cytoplasm pressing against the cell envelope.
• The cell wall allows cells to withstand turgor pressure and it gives the cell shape and rigidity.

Peptidoglycan

• The cell wall's primary structural component is peptidoglycan.
• Peptidoglycan's unique polysaccharide structure ensures the shape of bacteria and prevents osmotic lysis.
• Gram-positive bacteria possess numerous peptidoglycan layers.
• Gram-negative bacteria possess few peptidoglycan layers

Peptidoglycan Monomer Structure

• The structure of peptidoglycan monomer: NAM (N-acetylmuramic acid) and NAG (N-acetylglucosamine) are the parts of the peptide chains.
• Pentapeptide chains contain amino acid links (L-alanine, D-glutamine, L-lysine, and D-alanine).

Additional Structures and Descriptions

• Diagram for gram-positive cell envelope.
• Description for Cell Appendages (flagella, pili).
• Differentiate between different types of flagella (monotrichous, lophotrichous, amphitrichous, and peritrichous.
• Description and Diagram for Pili (adherence and conjugation),

Further Reading Suggestions:

• Microbiology textbook: (Specific title: e.g., Microbiology an Introduction, Tortora, Funke, and Case 12th Ed. Chapter 4 "Functional Anatomy of Prokaryotic and Eukaryotic Cells")

This quiz covers Lecture 4 on Basic Microbial Morphology. Explore key concepts such as the structure of prokaryotic and eukaryotic cells, bacterial shapes, and the Gram staining process. Gain a deeper understanding of microscopy techniques and the types of microorganisms.