session 1 notes: agar plates, bacterial structure and gram stain

Question 1

what is chocolate agar

Answer 1

agar plate with lysed blood

Question 2

describe the structure of bacteria

Answer 2

• cell envelope- all the material external to and enclosing the cytoplasm so the cytoplasmic membrane, cell wall and the capsule if present.
• cell membrane- composed of phospholipid. acts as a permeability barrier
• cell wall- has a peptidoglycan layer which determines the shape of the cell. helps render the bacterial wall resistance to host peptidases. it is the target for several antibiotics.
• capsule-bacteria secrete sticky, viscous material arhat forms an extracellular coating around the cell. allows cells to adhere to surfaces, protect bacteria from antibodies and phagocytosis and act as diffusion barriers against some antibiotics. capsule can also protect bacteria from desiccation (removal of moisture) or drying which facilitates transmission.

Question 3

name some bacteria with a capsule

Answer 3

streptococcus pneumoniae
neisseria meningitidis
group B streptococcus
haemophillus influenzae type b

Question 4

difference between gram positive and gram negative

Answer 4

gram positive have thick multilayered peptidoglycan cell wall but gram negative are composed of two membranes which are separated by a periplasmic membrane. the peptidoglycan wall is thin and more susceptible to damage. outer membrane has lipolysaccharide embedded in it.

Question 5

what is a gram stain

Answer 5

specimen treated with solution of crystal violet and then iodine staining the bacteria purple. then treated with solvent i.e acetone/alcohol. gram +ve will retain stain and be purple.
addition of counterstain safranin stains gram -ve pink