A. Mannitol salt selects for salt tolerant microbes. Acid from mannitol-fermenting bacter will turn the indicator from red to yellow.
B-D. Nutrient agar contains peptone and beef extract. Many common heterotrophs will grow on this medium.
B. One of the bacteria on this plate is producing a water-soluble pigment.
D. These bacteria are producing a nonwater-soluble pigment.
The answer: Plate D is a pure culture of Serratia marcescens. Look for identical colonies on the plate.
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