Question

you performa throat culture on blood agar. After incubation, you notice several B-hemolytic colonies. what would be the best way to determine whether the organism was Staphylococcus aureus or Streptococcus pyogenes, both which B-hemolytic?

a)look for motility

b)check for sensitivity to novobiocin

c)streak a MacConkey plate

d)streak a mannitol salts agar plate

Answer 1

The MacConkey agar is used for the differentiation and isolation of gram negative and Pseudomonas species.Streptococcus pyogenes is a gram positive bacteria so cannotbe used for the isolation

The novobiocin is used for differentiating the coagulase negative Staphylococcus saprophyticus which is novobiocin resistant.S saprophticus causes urinary tract infection in females.

The way to determine that the whether the organism is Staphylococcus aureus or Streptococcus pyogenes is by mannitol salt agar plate.It has high concentration of sodium chloride in the mannitol salt agar prevent the growth of the other bacteria other than Staphylococci.

Due to the fermentation of the mannitol sugar and production of acid which is indicated by the phenol red indicator resulting in yellow colour colonies.Streptococci would have no growth on the mannitol agar plate.

So the right option is d)Mannitol salt agar plate.