Question 1

Why is streak plating used instead of spreading bacteria across multiple plates?

Accepted Answer

Streak plating achieves progressive dilution of a bacterial sample on a single Petri dish, making it far more practical than using multiple plates. A loopful of mixed culture can contain up to one trillion bacteria. Spreading this across enough plates to isolate single colonies would require encircling Liberty Island. Streak plating solves this by systematically diluting bacteria across five sections using a sterile loop between each streak.

Question 2

What happens when you flame sterilize an inoculation loop between streaks?

Accepted Answer

Flaming the wire loop until it glows red hot removes any remaining bacterial cells from the previous streak, assisting in dilution. The hot loop is then cooled in air for a few seconds before being dragged through the next section. This cycle of streaking and sterilizing ensures that each subsequent section contains progressively fewer bacteria, eventually yielding isolated colonies.

Question 3

How does streak plating create isolated bacterial colonies from a mixed sample?

Accepted Answer

Streak plating divides a Petri dish into five sections and progressively dilutes bacteria using a sterile loop. Each section is streaked with fewer bacteria than the previous one. By the final section, only a few discretely located bacteria remain. Upon incubation, these individual bacteria multiply through binary fission to produce isolated clonal colonies, referred to as Colony Forming Units or CFUs.

Question 4

What is a pure culture and why is it important in microbiology?

Accepted Answer

A pure culture consists of only one bacterial type derived from a single mother cell, making all cells genetically similar. Pure cultures are essential for genetic and proteomic examinations, analyzing sample purity, and identifying infectious agents from clinical samples. Re-streaking isolated colonies ensures subsequent work involves only a single bacterial type suitable for further characterization.

Question 5

How should agar plates be prepared and stored before streak plating?

Accepted Answer

Prepare media by autoclaving at 121 degrees Celsius for twenty minutes. Pour 20-25 milliliters of approximately 45 degree Celsius media into sterile Petri dishes, avoiding bubble formation. Allow agar to solidify at room temperature for at least two hours or overnight. Store solidified plates upside down at four degrees Celsius to minimize condensation on the medium's surface.

Question 6

What aseptic techniques must be followed when performing streak plating?

Accepted Answer

Sterilize the workspace using 70% ethanol before beginning. Use sterile, disposable loops or flame-sterilize wire loops between each streak. Work quickly when removing plate lids and inoculating. Replace lids immediately after streaking to prevent contamination. Seal plates with parafilm if appropriate, and incubate upside down. All media, glassware, and plasticware should be autoclaved before use.

Question 7

Can streak plating be used for purposes other than isolating single colonies?

Accepted Answer

Yes, streak plating is also used to select media-specific strains, determine bacterial colony morphology, and identify different bacterial species. The technique can be combined with selective and differential media to inhibit certain organisms or distinguish between them. This versatility makes streak plating a fundamental tool in microbiology for various diagnostic and research applications.

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