Question

1. Why were you instructed to inoculate each plate with three separate streaks, one of each bacterial species, instead of combining all three species and streaking them all onto the plate together?

2. Why did Staphylococcus and Micrococcus fail to grow on the MAC plate? Explain your answer based on cell wall composition and the composition of the media.

3. Which of the bacteria that you tested was a mannitol fermenter? How and why did you conclude that it was a mannitol fermenter?

4. Explain your results from the EMB plate. Which bacteria is a lactose fermenter? How did you come to this conclusion?

5. What is a differential medium? Give an example of how it can be used in diagnosis.

Answer 1

1. It is essential to inoculate each plate with three separate streaks, one of each bacterial species because streaking is a method for separation and isolation of a single strain from a mixture of growing microbes. By streaking, single bacteria is get isolated from a colony and then grow into a separate colony of similar clones. In three streaks, one streak is for a single strain to achieve the objective of separation and isolation of a single bacteria from a colony.

2. The MAC plate or the MacConkey agar is a differential selective media for the Gram negative bacteria and doesn't allow the Staphylococcus and Micrococcus bacteria to grow because they are Gram positive bacteria and they can't grow on a medium containing bile salts as an ingredient. Also Staphylococcus needs blood in the medium to grow as they need special media to grow.

3. The staphylococcus bacteria is a mannitol fermenter as it produces acids as a byproduct when tested for mannitol fomentation. The acid produced is indicated by changing in colour of phenol red, from red to yellow.

4. Staphylococcus is a lactose fermenting bacteria. When we add lactose to a selective media, the staphylococcus bacteria shows the growth which indicates that staphylococcus can utilise lactose as it contains simple sugars as a carbohydrate source. On EMB plate, the Gram positive bacteria get stained as this plate contains eosin and methylene blue in 6 is to 1 ratio which also proves the growth of staphylococcus bacteria which is a Gram positive bacteria.

5. A differential medium is a special type of medium which contains selective nutrients which are used to isolate a single type of bacteria from a mixture of microbes. A differential medium contains some restriction factors which inhibits the growth of unwanted microbes. For example, the penicillin resistant bacteria can you grow in a medium which contains penicillin in it. The penicillin resistant bacteria are are the mutant bacteria which shows no effect of the presence of penicillin in the growth media. Thus, it is a selective media or differential media.