CH4 Microbiology Flashcards
what are the smallest and largest known bacteria?
smallest = archaea largest = sulphur bacterium
what are the 3 main shapes of bacteria?
- bacillus or rod shaped e.g. Escherichia: bacillus
- coccus or spherical, e.g. staphylococcus; streptococcus
- spiral or corkscrew-shaped, e.g. spirillum
how is further differentiation of bacteria possible?
the way that bacteria tend to group.
- may be single e.g. helicobacter
- in pairs e.g. diplococcus pneumoniae
- in chains e.g. streptococcus
- in clusters e.g. staphylococcus
what are the colours of the different gram stains?
gram positive = violet
gram negative = red
describe the structure of gram negative bacteria?
a membrane with a thin peptidoglycan layer with an extra lipopolysaccharide layer.
describe the structure of a gram positive bacteria?
a membrane with a thick outer polysaccharide layer.
what are the 4 stages of a gram stain?
- add crystal violet
- add lugol’s iodine
- add acetone-alcohol
- add safranin
what is the function and effect of crystal violet?
basic dye; binds to peptidoglycan so all bacteria stain purple.
what is the function and effect of Lugol’s iodine?
mordant; binds the crystal violet to the peptidoglycan more strongly.
what is the function and effect of acetone-alcohol?
decolouriser; removes unbound crystal violet and lipopolysaccharide layer
- gram negative bacteria lose their stain and become colourless
- gram positive bacteria remain purple
what is the function and effect of safranin?
counter stain;
- gram negative bacteria stain red
- gram positive bacteria remain purple.
name some examples of gram positive bacteria.
bacillus, staphylococcus, streptococcus
explain how the absence of a lipopolysaccharide layer makes gram positive bacteria more prone to collapse?
the antibacterial enzyme lysozyme (in human tears and saliva) hydrolyses the bonds holding the peptidoglycan molecules together.
-penicillin prevents the bonds inter-linking peptidoglycan molecules from forming. when bacteria make new cell walls when they divide the cell walls are structurally weak and prone to collapse. so water uptake by osmosis bursts the cell.
name some examples of gram negative bacteria.
salmonella and E.coli
how are gram negative bacteria resistant to penicillin?
the outer lipopolysaccharide layer protects the peptidoglycan layer. to control them requires a different class of antibiotics, that interfere with the cell’s ability to make proteins.
