This section of the tutorial covers common errors made in the lab that are easily avoided.
Using contaminated media
A common error is rescuing transformations in contaminated media. The plate at right shows two colony morphologies. The minor population are white E. coli colonies, the majority are yellowish. They came from a bottle of 2YT media that had visible cloudy streaks. Before you use growth media for picking colonies or rescuing a transformation, check to make sure the media has no cloudiness or floating debris. You can't rescue contaminated media, so throw it out. Along these lines, it is important when making media that you autoclave it without significant delay. The powder in the media bottles is not sterile, and will readily grow as soon as you add water. This growth will alter the composition of the media. So, it is important that you kill that stuff by autoclaving before it gets a chance to amplify. Waiting 1 or 2 hours to autoclave is ok. Waiting overnight is fatal.