Question

1. Based on your results and the information in the lab manual, fill in the table below. Predicted Gram Reaction Ability to ferment lactose/sucrose Ability to ferment mannitol Coliform or Non-coliform S. aureus E. coli P. aeruginosa M. luteus 2. What general type of growth medium would you use to: (a) grow one type of bacteria but inhibit the growth of another type? (b) discriminate between different types of bacteria? 3. Why is it necessary to sterilize the loop between streaks when streaking for single colonies?

Answer 1

Q.1 S. aureus

1. Gram reaction- Gram positive

2. Ability to ferment lactose/sucrose- Positive

3. Ability to ferment mannitol- Positive

4. Coliform or non - coliform - Non- coliform

E. coli

1. Gram reaction- Gram negative

2. Ability to ferment lactose/sucrose- Positive

3. Ability to ferment mannitol- Positive

4. Coloform or non- coliform - Coliform

P. aeruginosa

1. Gram reaction- Gram negative

2. Ability to ferment lactose/sucrose- Negative

3. Ability to ferment mannitol- Positive

4. Coliform or non- coliform - Non- coliform

M. luteus

1. Gram reaction- Gram positive

2. Ability to ferment lactose/sucrose- Negative

3. Ability to ferment mannitol- Positive

4. Coliform or non- coliform- Non- coliform

Q.2

a. Selective media- It is composed of specific ingredients which inhibit the growth of certain species while allowing others to grow. For eg. Mac conkey agar is selective for gram negative bacteria. Mannitol salt agar is selective for gram positive bacteria.

b. Differential media- It contains some specific ingredients/chemicals to visually distinguish b/w species which do/don't carry out a specific biochemical process. For eg. Eosin methylene blue and MacConkey agar is differential for lactose fermentation. Mannitol salt agar is differential for mannitol fermentation.

Q.3 It is necessary to do so to reduce even the slightest possibility of contamination. It is also done to reduce the number of bacteria being streaked on the plate. Only those bacteria which are originally placed in the plate can be transferred to next section.