Question

For obtaining pure cultures of bacteria __________ plate method is not used.

• A. Streak
• B. Spread
• C. Pour
• D. Dip

Hint:

The three fundamental techniques for isolating pure cultures on a Petri plate are Streak, Spread, and Pour. Memorize these three as the primary methods. Any other option listed is likely the answer to a question asking which method is "not used."

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
The question asks to identify which of the listed methods is not a standard laboratory technique for isolating bacteria to obtain a pure culture on a plate. A pure culture contains only one species or strain of bacteria.
Step 2: Detailed Explanation:
The primary goal of plating techniques for pure culture isolation is to separate individual bacterial cells on the surface of a solid medium so that each cell can grow into an isolated colony.
Streak Plate Method: This is the most common technique for isolating pure cultures. A sterile loop is used to streak a sample across the agar surface in a specific pattern, progressively diluting the inoculum to obtain isolated colonies.
Spread Plate Method: A small volume of a diluted liquid sample is pipetted onto the surface of an agar plate and then spread evenly with a sterile L-shaped spreader. This technique is used for both isolating and quantifying bacteria.
Pour Plate Method: A known volume of the sample is mixed with molten agar and then poured into a sterile Petri dish. Colonies develop both on the surface and within the agar. This is also used for isolation and quantification.
Dip Method: This typically refers to a "dip slide," which is a plastic slide coated with agar. It is dipped into a liquid sample (e.g., urine) to estimate the microbial load (bacterial count). While it involves a culture, its primary purpose is quantification and screening, not the meticulous isolation of pure cultures in the same way as the other three methods. It is not considered a standard "plate method" for obtaining pure cultures.
Step 3: Final Answer:
The streak, spread, and pour plate methods are all standard techniques for obtaining pure cultures. The dip method is not used for this purpose.