Plating Methods: Pour Plate vs. Spread Plate

Hi, I’m Kathy with Level Up RN! In this video, we’ll be discussing plating methods, specifically the pour plate method and the spread plate method.

At the end of this video, I’ll quiz you on key points to test your understanding—so stay tuned! If you have our Level Up RN Microbiology Flashcards, grab your card on plating methods and follow along. Pay close attention to the bold red text on the back, as those are the concepts most likely to appear on exams.


What Are Plating Methods?

Plating methods are laboratory techniques used to isolate, count, and grow microorganisms. In many cases, the sample needs to be diluted before plating to ensure a countable number of colonies after incubation. (For a detailed explanation of serial dilution, check out our separate video in this playlist!)

Now, let’s break down each method.

1. Pour Plate Method

With the pour plate method, the bacterial sample is mixed with liquid agar and then poured onto a sterile Petri dish. The plate is swirled to mix, allowed to solidify, and then incubated for colony growth.

Key Characteristics of the Pour Plate Method:

✅ Suitable for both aerobic (oxygen-requiring) and anaerobic (oxygen-free) bacteria
✅ Colonies can form on or beneath the agar surface
✅ The location of bacterial growth depends on oxygen availability

Why Does Growth Location Matter?

  • Aerobic bacteria thrive on the agar surface where oxygen is present. If some get trapped beneath the agar, they won’t grow well.
  • Obligate anaerobic bacteria (which cannot tolerate oxygen) need to be beneath the agar. If they end up on the surface, they’ll be exposed to oxygen and won’t survive.
  • Bottom line: The distribution of bacteria in the agar can affect results, making this an important factor to consider when interpreting growth patterns.

2. Spread Plate Method

With the spread plate method, the bacterial sample is pipetted onto solid agar and spread evenly using a sterile spreader. The plate is then incubated for colony growth.

Key Characteristics of the Spread Plate Method:

✅ Suitable for aerobic bacteria and facultative anaerobes
✅ Colonies only grow on the surface of the agar

Why This Method Works for Specific Bacteria:

  • Aerobic bacteria need oxygen and will thrive on the surface.
  • Facultative anaerobes prefer oxygen but can survive without it, so they can still grow on the surface.
  • Obligate anaerobes cannot grow with this method because they need to be beneath the agar to avoid oxygen exposure.

Quick Quiz!

Question 1:

With which plating method can colonies form on and beneath the agar surface?
✅ Answer: The pour plate method

Question 2:

Which plating method(s) are appropriate for growing obligate anaerobic bacteria?
✅ Answer: The pour plate method only

Question 3:

Which plating method(s) are appropriate for growing aerobic bacteria?
✅ Answer: Both the pour plate and spread plate methods

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