Question

3) You are given 2 tubes of grown bacterial cultures-1 S. aureus and 1 E. coli. You accidentally wipe their names off of the tubes with alcohol and don't know which tube is which anymore. With only 2 MSA and 2 MacConkey plates you must determine which bacterial strain is in each tube. Explain what you would do and how you would detemine each strain's identity- describe the results on the plates including growth, color changes etc. and what the results indicate. (2 pts)

Answer 1

Answer:

In this case, cultures in both tubes will be streaked into MacConkey and MSA (Mannitol Salt Agar).

MSA or Mannitol Salt agar consist of 3 important things.

1) High salt concentration 2). Mannitol sugar and 3). Phenol red indicator.

Phenol red indicator gives the red colour to the medium. Organisms which can ferment mannitol will produce acidic byproduct, which will change the pH of the medium and hence would turn yellow.

S.aureus can grow in this medium as it can ferment mannitol and appear as yellow colony in this medium. E.coli does not grow in this medium.

MacConkey Agar consist of crystal violet and bile salts which allows the selective growth of gram-negative bacteria only while it prevents the growth of gram-positive bacteria.

E.coli is gram negative while S.aureus is gram positive. Hence E.coli will grow in MacConkey agar plate and appear as pink colonies while S.aureus cannot grow in this.