Lecture 2 - Basic Bacterial Structure + Gram Staining & Morphology Flashcards
In a basic bacterial structure, describe the cell envelope
The cell envelope consists of several chemically and functionally distinct layers like the Cytoplasmic Membrane and Cell Wall.
Describe the cytoplasmic membrane in bacterial cell structure
- Some bacteria cell membranes contain hopanoids (These are sterol based compounds that add to the structural integrity)
- Eukaryotes often have sterols in their cell membrane in the form cholesterol.
Describe the membrane proteins in bacterial cell structure
- Important for cellular processes like Metabolism and Transport of molecules
What are some major functions of the cytoplasmic membrane?
- permeability barrier (prevents leakage and functions as a gateway for transport of nutrients into, and waste out of, the cell)
- protein anchor (site of proteins that participate in transport, bioenergetics, and chemotaxis)
- energy conservation (site of generation and dissipation of the proton motive force)
In bacteria with a cell wall, what is the cell wall composed of?
It is composed of a rigid polysaccharide called peptidoglycan.
Describe the chemical structure of peptidoglycan
- polysaccharide backbone: repeating alternating glucose residues of N-acetylmuramic acid (NAM/M) and N-acetylglucosamine (NAG/G), along with a tetrapeptide
- the basic unit of peptidoglycan is called Glycan Tetrapeptide
What does the tetrapeptide component consists of?
L-alanine, D-alanine, D-glutamic acid, L-lysine or Diaminopimpelic acid (DAP)
What are the use of peptide chains in peptidoglycan?
The peptide chain serves to cross link adjacent polysaccharide chains (links the N-acetylmuramic acid units). This creates a strong rigid structure.
What are interbridges in peptidoglycan?
Some bacteria have “interbridges” that link together polysaccharide layers together. For example: Staphylococcus aureus has a penta-glycine bridge
What is the purpose of using a gram stain?
To differentiate between types of bacteria. Bacteria can be separated into two specific groups: Gram Positive Bacteria (appear purple/violet) & Gram Negative Bacteria (appear pink)
Describe the Gram Staining Procedure
- Flood the heat-fixed smear with crystal violet for 1 min
- Add iodine solution for 1 min
- Decolourize with alcohol briefly - about 20 sec
- Counterstain with safranin for 1-2 min
* G+ cells are purple, G- cells are pink
Why is there a difference in colour when staining bacteria?
Crystal violet detects peptidoglycan. Some bacteria (G+) have very thick peptidoglycan walls.
G- bacteria have very thin peptidoglycan walls. The thin layer gets dissolved with the addition of alcohol and therefore it loses its colour.
What is in the Gram Positive cell wall?
The gram positive cell wall contains teichoic acid. Many teichoic acids are covalently linked to peptidoglycan. Some are covalently linked to cytoplasmic membrane lipids, called Lipoteichoic acids.
Why are teichoic acids important to the G+ cell wall?
- it adds rigidity to the cell wall
- contributes to the negative charge of the cell wall
- attracts cations (Ca2+ and Mg2+) prior to their transport into the cell to be used in metabolic processes
Describe the Gram Negative cell envelope
- Contains a thin layer of peptidoglycan and an outer membrane.
- contains a space between the inner and outer membrane called periplasm.
Describe the outer membrane in a G- cell
- the outer membrane contains phospholipids, proteins, and polysaccharides that are attached to the lipids (lipopolysaccharides (LPS))
- does not confer much structural stability
How are the polysaccharides (that are attached to the lipids) relevant in the outer membrane of G- cells?
- The polysaccharides that are attached to the lipids are used to differentiate between subspecies of G- bacteria.
- the polysaccharides tend to be polar and attract water because they are carbohydrates. this can prevent lipophilic(fat soluble) substances from penetrating
Describe the structure of lipopolysaccharides
- polysaccharide component: O-specific polysaccharide (a repeating long sequence of sugars) and Core polysaccharide (a short sequence of sugars connected directly to the lipid)
- lipid component: Lipid A (embedded in the membrane and is toxic to animals and humans (endotoxin)). If the bacteria is killed lipid A can be released and cause illness!
What is the periplasm and porins in the G- cell wall?
Periplasm is the space between the cytoplasmic membrane and the inner membrane.
Porins allow for permeability to small molecules and solutes of the outer membrane
What is morphology?
Morphology refers to the shape of a cell.
What are the 3 most common bacterial cell morphology?
- Coccus (cocci) –> Round or spherical or ovoid
- Rod (rods) –> rod shaped (bacilli)
- Spirillum (spirilla) –> loose curves/spirals
- Spirochete – > long and coiled, very thin
- Stalk/Hypha –> budding and appendaged
- Filamentous –> long chains, not coiled
What are other important clues to identifying bacteria?
- arrangement of specific bacteria after replication are often characteristic and can give clues to their species
For example:
- clusters/groups (Staphylococcus)
- chains (streptococcus)
- pairs (neisseria)
