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why was the water bath set at 50

Yes, sometimes both of them are combined in a single medium. Ex: If we want to isolate the common bacterium staphylococcus aureus, found in the nasal passages. This organism has a tolerance for high concentrations of sodium chloride; it can also ferment the carbohydrate mannitol to form acid. Mannitol salt agar contains 7. 5% sodium chloride, which will discourage the growth of competing organisms and thus select for (favor the growth of) S. aureus. This salty medium also contains a pH indicator that changes color if the mannitol in the medium is fermented to acid; the mannitol-fermenting colonies of S. Aureus are this differentiated from colonies of bacteria that do not ferment mannitol.

Bacteria that grow at the high salt concentration and ferment mannitol to acid can be readily identified by the color change. These are probably colonies of S. aureus, and their identification can be confirmed by additional tests.
Soft agar is poured over the hard tryptone agar plates with the phage and E. coli B host mix It's important to use hard agar with soft agar overlay because The hard agar underneath the soft agar overlay is where you make a lawn streak of your bacteria.

Since phage can only grow in the presence of bacteria, this is the only way you can visualize plaques. When the phage lyse the bacteria, you'll see a zone of inhibition on the plate where the phage in the soft agar inhibit the bacteria on the hard agar from growing.

The hard agar is necessary to suspend the soft virus-bacteria agar mixture. If we did a soft agar on top of another soft agar layer, the two layers would mix. This would make it hard to count if not impossible. The relative concentrations of the phage and bacteria would no longer be known. Placing a soft agar on top of a hard agar ensures only one layer is growing and one area will need interpretation. + 6 pts

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