Semester 3 Microbial Physiology Topic 2

Topic 2: Cell Wall and Membrane Structure of Prokaryotes

Notes:
1) There are four structure external to the cell wall:

• Glycocalyx

- substances that surround cell

- it is made inside the cell and excreted to the cell surface

- capsule is called if the substance is organised and is firmly attached to the cell wall

- slime layer is called if the substance is unorganized 

- Functions: protection from phygocytosis, attachement to various surfaces, source of nutrients, and protect a cell against dehydration.

• Flagella

- Long filamentous appendages that propel bacteria

- There are four arrangement of flagella: Monotrichious, Amphitrichous, Lophotrichous and Peritrichous. 

- There are three basic parts in a flagellum:

Filament(contain the globular protein flagellin) , Hook, Basal body(gram-negative bacteria contains two pairs of rings and gram-positive bacteria contains only the inner pair)

• Axial Filaments

- bundles of fibrils that arise at the ends of the cell beneath the outer sheath.
- spiral around the cell
- present in the spirochetes group of bacteria
- e.g. Treponema pallidum and Borrelia burgdorferi

• Fimbriae and Pili

- Hairlike appendages that are shorter, straighter and thinner than flagella

-consist of pilin

- used for motility rather than for motility

- There are two types of fimbriae and pili: Fimbriae can occur at the poles, evenly distributed over the entire surface, and can number anywhere from a few to several hunfred per cell.

- Pili are longer than bimbriae and joint bacterial fells in the perparation for the transfer of 

DNA from on cell to another. 

Pili is used for the transfer of extrachromosomal

2) Cell wall component: Gram-negative and gram positive bacteria

Gram-negative and gram positive bacteria are structurally different and this differences cause them to have different characteristics. Gram-negative bacteria are resistant to antibiotics due to the presence of outer membrane.

3) Archaea have cell walls consisting of polysaccharide, glycoprotein but not peptidoglycan.

They contain a substance similar to peptidoglycan called pseudopeptidoglycan (pseudomurein).

Their cell wall type is S layer which protects against osmotic stress, pH and enzymes.

Archaea are naturally resistant to lysozyme and penicillin.

4) The unique feature found in cell wall of archae is the presence of L-amino acid instead of D-amino acid, the presence of N-acetyltalosaminuronic acid but not N-acetylmuramic acid and the presence of beta(1,3) glycosidic bond instead of beta(1,4)glycosidic bond.

My own exlporace：

1) Mycobacterium does not have any glycocalyx because they do not have cell wall or very little cell wall material.

2) Capsules give better protection than slime layer because capsule is organised and is firmly attached to the cell wall.

3) A spheroplast is a cell from which the cell wall has been almost completely removed, as by the action of penicillin. Spheroplasts are osmotically fragile, and will lyse if transferred to a hypotonic solution.

4) Septic shock is what happens as a complication of an infection where toxins can initiate a full-body inflammatory response. It often occurs in people who are elderly or have a weakened immune system. It is thought that the inflammation resulting from sepsis causes tiny blood clots to form, which can block oxygen and nutrients from reaching vital organs. As a result, the organs fail, causing a profound septic shock. 

Reflection:
Although cell wall and membrane structure of prokaryotes have learned in last two semester, there are few terms that I am not familiar with. Septic shock and spheroplast are new for me. I think to differentiate gram negative and gram positive bacteria have to be very specific to avoid any error since this is the important step to identify the bacteria.