Question

Microbiology - Gram Positive (ONLY answer)

[Q1] Mannitol salt agar is both selective and differential.

- True

- False

[Q2] Mueller Hinton medium is a selective and differential medium.

- True

- False

[Q3] 6.5% Salt broth is used to distinguish

- Streptococci and Enterococci

- Streptococci and Staphylococci

- E. coli and Salmonella

- Enterobacter and Streptococci

Answer 1

Q1

Answer: The statement mannitol salt agar is both selective and differential is true. The high concentration of salt in the mannitol agar is selective for most gram negative and positive bacteria but it is inhibitory to salt sensitive bacteria. The media differentiates mannitol fermenting bacteria from non-mannitol fermenting bacteria. Mannitol fermenting bacteria ferment mannitol and produces acid that change the phenol red in the media to yellow. Mannitol non-fermenting bacteria produces red or pink colonies.

Q2

Answer: The statement Mueller Hinton agar is both selective and differential is false. Mueller Hinton agar is non-selective and non-differential media because all the bacteria plated on this media will grows. It is used for antibiotic susceptibility testing.

Q3

Answer: 6.5% salt broth is used to distinguish between streptococci and enterococci. High salt concentration in this broth acts as selective and differential media. Further, high salt concentration interfere with membrane permeability and osmotic equilibrium. Bacteria capable of growing in high salt concentrations such as enterococci ferment sugar in the media to produce acid that changes the purple color of bromcresol purple in the broth to yellow. High salt sensitive bacteria will not grow in the broth and hence, the broth remains purple.