Question

Which of the following ferments lactose quickly?

Select one: a. Klebsiella b. Brucella c. Moraxella d. Helicobacter pylori

Answer 1

a. Klebsiella can ferment lactose quickly.

Explanation :- Klebsiella is gram negative, enteric, lactose fermenting bacteria. Klebsiella can grow on MacConkey agar because MacConkey agar is selective and differential medium which is mostly used to isolate enteric bacteria with their ability to ferment lactose. It can differentiate between lactose fermenter and non- lactose fermenter. It contains bile salt and Crystal violet which inhibit the growth of gram positive organism. And it also contain neutral red dye as a pH indicator. If lactose is fermented , an acidic environment will be created and pH will drop. Below pH 6.8 the dye turns into red and lactose fermenting bacteria appear as a pink colony. In case of Lactose non-fermenting bacteria it becomes colorless and pH is greater than 6.8 and appear as a white colony. Klebsiella species appear as a pink colony and the dye turns into red therefore it indicates that Klebsiella can ferment lactose quickly.

Whereas Brucella, Moraxella and Helicobactor pylori cannot ferment lactose, although they are gram negative bacteria, they cannot grow on MacConkey agar and appear as white colony at more or less 48hours of incubation. It indicates that pH is increased than 6.8 and they fails to ferment lactose.

Even in broth culture using phenol red lactose broth, after incubation of 24hours phenol red turns to yellow(indicates positive result) for Klebsiella whereas phenol red turns into magenta color(indicates negative result) after 48hours for other three bacteria Brucella, Moraxella and Helicobacter pylori.

Lactose fermentation can be determined when lactose is fermented it produce acid, medium of the pH will drop and pH indicator indicates it by changing color.