Contamination of media or poor aseptic technique.

Dr. Lee: ‘Let’s examine the affected wells more closely for signs of contamination. Take a look under the microscope and watch for bacterial colonies, which might appear as small, moving dots, or fungal contamination, which often forms filamentous structures.’

The students observe the wells one by one under the inverted microscope. You raise your hand.

You: ‘I see tiny moving dots in one of the wells and floating debris in another. Does that mean contamination?’ 

Dr. Lee: ‘That’s a clear sign. Contamination can happen if aseptic techniques aren’t followed properly. For example, maybe a pipette tip touched a non-sterile surface, or the hood wasn’t cleaned thoroughly before starting. Let’s also check the sterility control wells. If media-only wells are cloudy, it confirms contamination during preparation or handling.’

The control media wells are observed, and cloudiness is confirmed.

Dr. Lee: ‘Alright, the contamination likely occurred due to handling. Review what steps were taken while seeding and adding treatments. For the next experiment, make sure to clean the hood properly, use a fresh pipette tip for each well, and limit how long media bottles are open.’

The group nods, taking notes about observations and improvements for future experiments.

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