Microbiology: MacConkey Agar

The video tutorial explains the use of MacConkey Agar, a selective and differential medium, to identify gram-negative bacteria capable of fermenting lactose into lactic acid. The presence of bile salts and crystal violet inhibits gram-positive bacteria, allowing only gram-negative bacteria to grow. The medium contains a pH indicator that turns hot pink when lactose is fermented into acid end products. The biochemical pathway involves lactose being hydrolyzed into glucose and galactose, which enter glycolysis to form pyruvate, eventually converting to lactic acid. The video also covers interpreting results on MacConkey Agar plates, identifying lactose fermenting and non-fermenting colonies.

What is the primary purpose of MacConkey Agar in bacterial culture?

• To promote the growth of gram-positive bacteria

• To select for gram-negative bacteria and differentiate lactose fermenters

• To differentiate between aerobic and anaerobic bacteria

• To identify bacterial spores

Which component in MacConkey Agar is responsible for indicating lactose fermentation?

• Lactate dehydrogenase

• pH indicator

• Crystal violet

• Bile salts

What are the monosaccharides produced from the hydrolysis of lactose?

• Fructose and sucrose

• Glucose and galactose

• Maltose and ribose

• Lactose and sucrose

Which enzyme converts pyruvate into lactic acid during lactose fermentation?

• Lactate dehydrogenase

• Beta-galactosidase

• Amylase

• Protease

How can you identify non-lactose fermenting colonies on MacConkey Agar?

• They turn the agar black

• They produce a green sheen

• They appear colorless or pale

• They appear hot pink