Question

question 13
image of a petri dish labeled s. aureus and s. epidermidis
pick all the correct statements related to this special media which is extensively used to differentiate staph aureus from staph epidermidis.

• ☐ s epidermidis is not able to ferment mannitol
• ☐ staph epidermidis is able to ferment mannitol
• ☐ this plate is selective for gram positives
• ☐ staph aureus is able to ferment mannitol
• ☐ this plate contains a high amount of salt
• ☐ this plate inhibits gram negatives
• ☐ this plate is selective for halophiles
• ☐ this plate contains phenol red as ph indicator
• ☐ this plate contains crystal violet and bile salts

Explanation:

1. S. epidermidis and mannitol fermentation: Staphylococcus epidermidis does not ferment mannitol, while S. aureus does (causing yellowing in mannitol salt agar, as seen with S. aureus here and no change for S. epidermidis). So "S epidermidis is not able to ferment mannitol" is correct. "Staph epidermidis is able to ferment mannitol" is incorrect.
2. Selectivity for gram positives: Mannitol salt agar (MSA) is selective for gram - positive bacteria (like staphylococci) because the high salt concentration inhibits most gram - negative bacteria. So "This plate is selective for gram positives" is correct.
3. S. aureus and mannitol fermentation: S. aureus can ferment mannitol, producing acid that changes phenol red (the pH indicator) to yellow (seen in the S. aureus colony area). So "Staph aureus is able to ferment mannitol" is correct.
4. High salt content: MSA contains a high amount of salt (7.5 - 10% NaCl), which selects for halotolerant/halophilic organisms (like staphylococci). So "This plate contains a high amount of salt" is correct.
5. Inhibition of gram negatives: The high salt and other components of MSA inhibit gram - negative bacteria. So "This plate inhibits gram negatives" is correct.
6. Selective for halophiles: MSA is selective for halophilic/halotolerant organisms (organisms that can tolerate high salt) since most non - halophilic bacteria are inhibited. So "This plate is selective for halophiles" is correct.
7. pH indicator (phenol red): MSA uses phenol red as a pH indicator. When mannitol is fermented, acid is produced, lowering the pH and changing phenol red from red to yellow (seen with S. aureus). So "This plate contains phenol red as pH indicator" is correct.
8. Crystal violet and bile salts: These are components of MacConkey agar (for gram - negative enterics), not MSA. So "This plate contains crystal violet and bile salts" is incorrect.

Answer:

• S epidermidis is not able to ferment mannitol
• This plate is selective for gram positives
• Staph aureus is able to ferment mannitol
• This plate contains a high amount of salt
• This plate inhibits gram negatives
• This plate is selective for halophiles
• This plate contains phenol red as pH indicator