Question

Scenario 1: A bacterium isolated from a wound sample gave the following results: MacConkey agar = no growth Mannitol salt agar - good growth, color of medium unchanged Based on these results, which of the following statements is/are true? (Choose ALL true statements.) A. The organism is most likely Gram positive. B. The organism is most likely Gram negative. C. The organism is most likely a bacillus. D. The organism is most likely a coccus. Which of the following most likely to be the identity of this organism? A. Staphylococcus aureus B. Staphylococcus epidermidis C. Escherichia coli D. Bacillus cereus E. Salmonella sp. Explain your answers to both questions. Scenario 2: A bacterium isolated from a stool sample gave the following results: MacConkey agar = good growth, pink colonies Mannitol salt agar = no growth Based on these results, which of the following statements is/are true? (Choose ALL true statements.) A. The organism is most likely Gram positive. B. The organism is most likely Gram negative. C. The organism is most likely a bacillus. D. The organism is most likely a coccus. Which of the following most likely to be the identity of this organism? A. Staphylococcus aureus B. Staphylococcus epidermidis C. Escherichia coli D. Bacillus cereus E. Salmonella sp. Explain your answers to both questions.

Answer 1

Scenario 1:

PART 1- ANS - (A) The organism is most likely Gram Positive because the MacConkey Agar media contains crystal violet which is known to inhibit the growth of Gram Positive Bacteria whereas the Mannitol Salt Agar media contains NaCl in high concentration that makes the Mannitol Salt Agar media selective to the Gram Positive Bacteria and the colour of media remains unchanged.

PART 2- ANS - (A) Staphylococcus aureus is the most likely to be that organism because Staphylococcus is a Gram positive cocci which makes it unable to grow on the MacConkey Agar media whereas on the Mannitol Salt Agar media, the bacteria grows selectivity due to the high NaCl concentration.

SCENARIO 2:

PART 1 - ANS - (B) The organism is most likely Gram Negative because the MacConkey Agar media is mainly used for these Gram Negative Cocci and it contains crystal violet which is selective to Gram Negative Cocci whereas on the Mannitol Salt Agar media, the growth of Gram Negative cocci is inhibited due to the high salt concentration.

PART - 2 - ANS - (C) Escherichia coli is the most likely to be that organism due to the presence of crystal violet which is selective to this bacteria whereas this organism growth is inhibited on the Mannitol Salt Agar media due to its high salt concentration. Also the colour of MacConkey Agar media changes to pink due to acid produced from lactose.