Question

Lactic acid bacteria are ________.

• A. Gram -ve, catalase +ve
• B. Gram -ve, catalase -ve
• C. Gram +ve, catalase -ve
• D. Gram +ve, catalase +ve

Hint:

A standard rule of thumb for Lactic Acid Bacteria (LAB) in exams:
Always remember the "three negatives" and "one positive":
- Gram-positive ($+$)
- Catalase-negative ($-$)
- Spore-negative ($-$)
- Motility-negative ($-$) (mostly non-motile)

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
Lactic Acid Bacteria (LAB) are a group of functionally related Gram-positive bacteria that ferment carbohydrates to produce lactic acid as their primary metabolic end-product.
They are widely used in the dairy industry as starter cultures for products like cheese, yogurt, and dahi.
Detailed Explanation:
The taxonomic and physiological classification of Lactic Acid Bacteria is defined by several key microbiological characteristics:
1. Gram Reaction: Lactic acid bacteria are strictly Gram-positive.
They possess a thick peptidoglycan cell wall that successfully retains the crystal violet-iodine complex during the decolorization step of the Gram staining procedure, appearing purple under microscopy.
2. Catalase Reaction: Lactic acid bacteria are catalase-negative.
They lack the heme-containing enzyme catalase, which breaks down hydrogen peroxide ($H_2O_2$) into water ($H_2O$) and oxygen ($O_2$).
Consequently, when mixed with hydrogen peroxide, they do not produce gas bubbles.
Although some strains can produce a non-heme "pseudocatalase" under specific nutritional conditions, they are classified by definition as catalase-negative.
In addition to these traits, LAB are non-spore-forming, acid-tolerant, and aerotolerant anaerobes.
Common genera include Lactobacillus, Lactococcus, Streptococcus, Leuconostoc, and Pediococcus.

Step 2: Final Answer:
Lactic acid bacteria are Gram-positive and catalase-negative.