Question

1. Human microbiota. What type of bacteria species are likely to be cultured from your throat swabs on Blood agar? Why?

2. What is the purpose of the candle jar for the cultivation of the Blood Agar plate/throat swabs and what type of metabolism do you think you are targeting?

3.With reference to your two nasal swabs, what is the difference between the two culturing strategies: culture this swab on a mannitol-salt-agar plate and inoculate the nutrient broth containing 7.5% (w/v) NaCl ? Predict what you think you will see when you examine the two cultivations next week？

4. Mannitol-salt-agar is an example of a selective medium. What is the agent that selects for staphylococci in this medium?

5.Mannitol-salt-agar is also a differential medium. What does it differentiate between?

Answer 1

Ans 1. Throat swab has Streptococcus pyogenes. S. pyogenes can be cultured on fresh blood agar plates as Streptococcus pyogenes is a species of Gram-positive bacteria harboring the Lancefield group A antigen, and is often called group A Streptococcus (GAS). Group A streptococci when grown on blood agar typically produces small zones of beta-hemolysis, which is a complete destruction of red blood cells.

Ans 2. Candle jar has 5% CO2. Candle jars are used to grow bacteria requiring an increased CO2 concentration (capnophiles). Candle jars increase CO2 concentrations and still leave some O2 for aerobic capnophiles. Catabolic metabolism is targeted.

Ans 3. In Mannitol Salt Agar which is a selective and differential medium most pathogenic staphylococci, such as Staphylococcus aureus, will ferment mannitol and can grow. Most non-pathogenic staphylococci will not ferment mannitol.

Ans 4. Mannitol Salt Agar has high concentration of salt (7.5%) which selects for members of the genus Staphylococcus, since they can tolerate high saline levels. The nutrient broth containing 7.5% (w/v) NaCl will also show growth of Staphylococcus aureus.

Ans 5.  Mannitol Salt Agar shows differentiation of Staphylococcus aureus from clinical samples.Most pathogenic staphylococci, such as Staphylococcus aureus, will ferment mannitol. Most non-pathogenic staphylococci will not ferment mannitol.