Question

1.Before a enteropluri test bacteria are streaked on MacConkey Agar plates. Why? (2)

2 .Rogosa agar contains yeast extract. The pH is adjusted to 5.2 and discourages the growth of many microorganisms that grow above or below this pH; however, all the colonies look similar. The medium is ________ medium. Why? (2)

5.You picked a sterile swab and streaked on a plate. You see 2 separate bacteria (that look different morphologically) on the plate. How can you get pure cultures of both?Draw picture to show how you will do it (2).

Answer 1

Answer 1:

Enteropluri test is a systematic testing of 12 biochemical tests designed for Gram Negative bacteria, aiming at identification of Enterobacteriaceae. As this group of bacteria grow well on MacConkey's Agar and also since we need to ensure that we are inoculating the biochemical system with pure growth, before inoculating the enteropluri system we isolate these colonies on MacConkey's Agar. From this medium then we can pick the colony to be identified and inoculate the biochemicals (enteropluri test).

Answer 2:

Rogosa Agar is a selective medium but is not differential.

Rogosa Agar is used for Lactobacilli. By making stringent requirement of pH, many other flora can be inhibited which can not grow at that pH, making it a selective medium for Lactobacilli. But it does not contain any indicator compounds which will give a change in colour with a particular group of organisms. Hence all those bacteria who can tolerate low pH and can grow at such a low pH will grow on Rogosa agar without giving a differentiating characteristic. All colonies of bacteria will appear similar to each other on this Agar.

Answer 5:

When we streak a sterile swab on a sterile media plate, we expect NOTHING to grow. However, if there are two different bacteria (morphologically) growing from this on the media plate, we will try to isolate them on a separate plates, with help of a sterile nichrome wire loop,  preferably using differential media for primary identification of the group of bacteria. Later study their morphological characteristics on a simple Gram Stain slide. We study colony characteristics and then follow up with biochemicals or sugar tests to identify further if needed.