Why Petri Dishes are placing in Inverted position

This article contains the description about Petri dishes/Plates position while keeping in incubators for incubations:

A) Why are Petri Dishes/plates placing in Inverted position in Incubators?

When incubating bacterial plates, they are inverted lid facing down, agar facing up for the following reasons:

1.Preventing Condensation on the Agar Surface:

• During incubation, the temperature inside the incubator increases, causing moisture in the agar to evaporate.
• This moisture condenses on the inner surface of the lid due to temperature differences.
• If the petri dish is not inverted, water droplets may fall onto the agar surface, leading to:
  Colony spreading, which results in non-uniform growth. Distorted colony morphology, making it difficult to analyze results.
• Contamination, as the droplets may carry airborne microbes from the lid to the medium.

2.Reducing the Risk of Contamination:

• Inverting the Petri Dish ensures that the agar is protected from airborne contaminants when opening the incubator or handling the petri dishes. This is especially crucial for nutrient-rich media that promote microbial growth.

3.Minimizing Moisture Loss from the Agar Medium:

• Nutrient agar and other media contain a specific amount of water necessary for bacterial growth. Inverting the plate helps retain moisture, preventing the agar from drying out, which could stop bacterial growth or lead to inaccurate results.

4.Preserving Colony Morphology :

Some bacterial studies require well-separated colonies for proper identification. Condensation on the agar can cause colonies to merge or spread, making it difficult to analyze individual colony characteristics.

B) Why Are Surface-Inoculated petri dishes/Plates being Placed Upright?

In some cases, particularly in surface inoculation techniques Spread Plate Method, plates are incubated in their upright position (lid on top, agar on bottom) for the following reasons:

• Ensuring Even Distribution of the Sample, In the Spread Plate Method, the bacterial suspension is spread across the agar surface using a spreader.
• If the Petri dish/plate is inverted too soon, gravity may cause the liquid to shift, leading to uneven bacterial distribution or clumping in certain areas.

1) Allowing the Inoculum to Dry Properly:

After applying the bacterial suspension, the surface needs time to absorb the liquid. If the Petri dish/plate is inverted before drying, the bacteria may spread uncontrollably, leading to irregular colony formation.

2) Enhancing Oxygen Availability for Aerobic Bacteria:

Some bacteria require sufficient oxygen for proper growth. Keeping petri dish/plates in an upright position improves air exchange, supporting better bacterial development.

3) Preventing Colonies from Sticking to the Lid:

If plates are inverted immediately after surface inoculation, some bacterial cells may move with moisture and attach to the lid, leading to loss of colonies or inaccurate results.

General Rule for Plate Incubation and in short summary as below:

For Pour Plate Method (Bacteria Mixed in Agar): Invert the plates to prevent condensation and contamination.

For Spread Plate Method (Surface-Inoculated Bacteria): Keep plates upright to allow proper drying and uniform bacterial growth.