Question

1. What type of medium is MSA? What ingredients are responsible for this medium? What would be the result of removing NaCl from the MSA?

2. Give 2 species of a gram pos genus that can be separated using MSA medium and explain how to differentiate between the two species.

3. Is MSA complex or defined? Why?

4. What purpose does Tryptic Soy Agar serve? In what way does it increase validity of the test results?

5. What is a coliform? Give 2 examples of species.

6. Is it possible for gram pos organisms to grow on MC agar? Explain.

Answer 1

1. What type of medium is MSA? What ingredients are responsible for this medium? What would be the result of removing NaCl from the MSA?

Answer: MSA( Mannitol Salt Agar) is a differential and selective medium.

Ingredients:

1. D-Mannitol (sugar, 1%, major ingredient)
2. Sodium Chloride (7.5%)
3. Phenol Red (Indicator)
4. Agar(solidifying agent)
5. Casein's and animal tissues's enzymatic digest as a source of nitrogen, vitamin, and carbon.
6. Beef Extract as a source of nitrogen, vitamin, and carbon.

MSA is used for the selective isolation and differentiation of Staphylococcus aureus so removing NaCl will alter medium sensitivity and hence will prevent the partial or complete inhibition of bacterial organisms(gram negative) other than staphylococci.  

2. Give 2 species of a gram pos genus that can be separated using MSA medium and explain how to differentiate between the two species.

Answer: 1)Staphylococcus aureus (Gram positive) - growth indicationg salt loving with yellow halo indicating acid breakdown from mannitol breakdown.

2) Staphylococcus epidermidis - growth indicationg salt loving but no yellow halo indicating no mannitol breakdown hence no acid production.

3. Is MSA complex or defined? Why?

Answer: MSA is a complex media because it contains materials whose composition is unknown and are of biological origin( beef extract) for providing selective and differential growth medium.  

4. What purpose does Tryptic Soy Agar serve? In what way does it increase validity of the test results?

Answer: Trptic Soy Agar(TSA) are non-selective, enriched media providing enough nutrients for the isolation of  fastidious and non-fastidious microorganisms. Used for culture storage, isolating pure culture, for biochemical testin,cell morphology etc. It increases the validity of test results by showing gram positive as viable organisms. Also determines the halotolerance level of microorganisms. For complete identification all biochemical, immunological, molecular, or mass spectrometry testing be performed on colonies from pure culture.

5. What is a coliform? Give 2 examples of species.

Answer:Coliform bacteias are rod-shaped, gram-negative, non-sporing bacterias found in the intestines and faeces of warm blooded animals(human beings, pigs, cows, dogs etc.) They are the most common water contaminants in the water supply. Examples: Escherichia coli , Citrobacter  amalonaticus.  

6. Is it possible for gram pos organisms to grow on MC agar? Explain.

Answer: No, it is not possible for gram positive to grow on MC agarbecause of the presence of bile salts and crystal violet which makes it a selective medium for isolation of gram-negative bacteria. Both compounds enter inside the cells of gram-positive and are antibacterial in nature which hinder there growth.