This question is part of the following Ask The Expert session:
If you experience contamination and thoroughly clean your incubators, water baths, hoods and other work surfaces, and also change any media or reagents that could potentially have been contaminated, you will likely have gotten rid of the source. You may never know exactly what the source was, and it might not be necessary to, but for a few weeks after you should keep a close eye on your cultures and make sure all lab personnel know and follow safe handling processes. It is possible someone noticed that a bottle of media fell over in the water bath and “thought it would be fine” or were in a hurry and neglected to put on their gloves or spray down their work surface, or answered their phone while changing media. After a major contamination event that person will suspect they may have been the cause and there will not be a repeat accident. Most researchers will occasionally take shortcuts in the lab, and find out the hard way that some of these can lead to major problems. If you do notice a second round of contamination close to cleaning up after the first you may need to retrain all of your lab personnel, as well as re-check all media, reagents, and frozen cultures to determine with certainty what the cause was. Communication is essential for working with multiple users in shared cell culture spaces.