Question

1.what kind of medium is Mannitol Salts Agar (MSA)

a) neither selective nor differential

b)differential

c)selective

d)both selective and differential

2.which of the following is the best method to distinguish Bacillus megaterium from Bacillus Subtilis?

a)use mannitol phenol red

b)streak a MacConkey plate

c)perform an endospore stain

d) perform a gram stain

Answer 1

1. Mannitol salt agar(MSA) is a commonly used selective and differential growth medium in microbiology.

The high concentration of salt(7.5%) selects for the members of genus Staphylococcus,since they can tolerate high saline levels.Most of normal skin microbiota are salt tolerated and will grow on an MSA plate.

MSA is a Differential Medium because of the presence of the sugar mannitol and the pH indicator Phenol Red.

2. Answer of this question is option (c) that is perform an endospore stain.

• The endospore stain is a differential stain used to visualize bacterial endospores.Endospores are formed by a few genera of bacteria, such as Bacillus . By forming spores, bacteria can survive in hostile conditions. Spores are resistant to heat, dessication, chemicals, and radiation.

Now let us check other options and why they are not used for distinguish Bacillus megaterium from Bacillus subtilis.

• Bacillus species weakly grows on MSA medium because they can't tolerate high salt concentration.
• MacConkey agar is an indicator, a selective and differential culture medium for bacteria designed to selectively isolate Gram-negative and enteric (normally found in the intestinal tract) bacilli and differentiate them based on lactose fermentation.Here, Bacillus megaterium is not enteric.
• Both Bacillus megaterium and Bacillus subtilis are gram positive.So gram staining can't be used here.