Classification Of Bacteria Flashcards
What are the physical and genetic differences in bacteria?
Physical - size, cell wall structure and hence staining characteristics, shape
Genetic - different metabolic features, different surface molecules so different antigenic properties
A genus of bacteria have one of 3 main shapes. What are they?
- bacillus or rod shaped
- coccus or spherical
- spiral or corkscrew shaped
How else might bacteria be differentiated?
Group eg single or in pairs
Chains
Clusters
What colours are gram positive and gram negative stains?
POS = Violet NEG = red
What causes the difference in staining?
Different chemical compositions in the cell walls. Gram positive bacteria have the basic wall structure and gram negative bacteria have an additional outer later of lipopolysaccharide.
What is the structure of bacterial cell walls?
The cell wall is made of polysaccharides and polypeptides called peptidoglycan or murein. The wall protects against swelling or bursting or lysis caused by osmotic uptake of water. It is the cross linking of molecules that provides strength.
What are the 4 stages of staining?
- Crystal violet is a basic dye that binds to peptidoglycan so all bacteria stain purple
- Lugols iodine is a mordant that binds the crystal violet the peptidoglycan more strongly
- Acetone- alcohol is a decoloriser that removes unbound crystal violet and lipopolysaccharide so gpos stay purpose and Gneg go colourless
- Safranin is a counter stain which stains gneg red while gpos stay purple.
What are 2 examples of gpos bacteria and why do they stain purple?
Bacillus and staphylococcus
The absence of outer lipopolysaccharide later in cell walls allows them to bind to stain efficiently
How does penicillin and the enzyme lysozyme work on gpos bacteria?
Lysozyme hydrolyses bonds holding peptidoglycan together
The antibiotic penicillin prevents bonds interlinking peptidoglycan molecules forming - especially significant when new cell walls are made when they divide. The walls are structurally weak and prone to collapse and water uptake by osmosis bursts the cell.
What are examples of gneg bacteria
Salmonella and e. Coli
Why does gneg bacteria stain red?
The outer membrane has large molecules of lipopolysaccharides which protects the cell and excludes dyes like crystal violet. The outer layer protects the peptidoglycan below.
How do you control gneg bacteria, as lysozyme and penicillin don’t work?
Different classes of antibiotics are required which interfere with the cells ability to make proteins. Eukaryotic cells also make proteins, but the protein making cellular machinery is different so antibiotics for work on them
