Oh, no! Contaminated Petri plates!

Just because we are farmers, growing our beloved Dictyostelium discoideum on lawns of bacteria, doesn’t mean we like all microbes. In fact, we fear and detest random microbes, for they contaminate our plates,making how they got there a challenge to figure out. Some places are just not good for growing Dicty. We very much hope St. Louis is not one of them, so we were not happy to see this little spikes turn up in our plates. We had seen them before in Houston, but never this numerous. They are actually in the agar, not on the surface, and we don’t know what they are, except we think they are alive. I’d guess a fungus.

We have to figure out where they came from, so we can figure out how to avoid them. There are a few steps to doing this. The first is to blame whoever is not in the room when you first start talking about the problem. But that approach has a number of problems, and doesn’t usually lead to the true answer. Tracking it down is almost like keying out a plant, with dichotomous branches good. But I won’t write an actual key with branches and steps, but just point out some things to discuss.

First, when was the problem first noticed? In our case, I learned about it a couple days ago.

Second, how is the problem distributed across plates? Is it all or nothing, or do plates vary in how contaminated they are? In our case, it seemed to be all or nothing.

Third, is there any variable that the contaminated plates share, and the uncontaminated plates do not? Date, autoclave run, type of experiment, person using them, are all candidates. In our case, we ruled out person (showed up on some of everyone’s plates), type of experiment (we aren’t doing that much yet), and even autoclave run. In questions of sterility, I’m quick to blame the autoclave, and in this case, we think it was the culprit. We autoclaved our media, SM, in two different volumes, a liter and 1.5 liters. The number of affected plates are consistent with the hypothesis that the larger volume needs to be autoclaved longer.

This doesn’t mean this is the answer, but it does mean we can change this one variable, and hope the problem goes away. Onward!


About Joan E. Strassmann

Evolutionary biologist, studies social behavior in insects & microbes, interested in education, travel, birds, tropics, nature, food; biology professor at Washington University in St. Louis
This entry was posted in Microbes. Bookmark the permalink.

1 Response to Oh, no! Contaminated Petri plates!

  1. David c says:

    I used to get that – maybe check the agar ph

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