FOOD MICROBIOLOGY
MICROORGANISMS IN FOOD
|
Question
[CLICK ON ANY CHOICE TO KNOW THE RIGHT ANSWER]
|
|
|
white colonies
|
|
|
colonies with black centre
|
|
|
black colonies
|
|
|
green colonies
|
Detailed explanation-1: -The resulting bacterial colonies will appear colorless with black centers. Shigella do not ferment lactose or produce hydrogen sulfide gas, so the resulting colonies will be colorless.
Detailed explanation-2: -Cultural Characteristics of Shigella: They are aerobes and facultative anaerobes, grow best at 37°, pH 7.4 on ordinary media. Colonies are small, about 2 mm in diameter, circular, convex, smooth and transparent; on MacConkey’s agar medium they are pale or colourless except Sh. sonnei which is late lactose fermenter.
Detailed explanation-3: -To optimize detection of Shigella in stool, samples should be plated on MacConkey and either xylose-lysine-deoxycholate, Hektoen enteric, or deoxycholate citrate agar. Colonies are bluish-green on Hektoen agar and do not have the black center seen with Salmonella, as Shigella do not produce H2S.