Unwanted guests

Microbial contaminants (three species) on a plate

Microbiologists use Petri dishes, filled with a variety of agar media, to grow microbes. Even a small lab can produce hundreds of plates every week… And it is quite common to store in the fridge those of these plates that are not directly inoculated for a future use. Thus, when you open a microbiologist's fridge, you may find columns of Petri plates labeled with the medium and antibiotics they contain, waiting to be covered with a suspension of microbes.

Sometimes, when you store them for long periods, you can have the disagreeable surprise to find unwanted guests on your plates, that is, contaminant microbes!



The pictures show a selection of nicely colored contaminants I found on some of my minimal medium plates (so these guys know how to grow with a minimum supply of nutrients!). We do not work under sterile conditions at all times, so it's not surprising to find such opportunists every now and then. But this should obviously not be too frequent!

The microbes show different colors on the plate, but apart from that they form colonies that are pretty much alike. However, they can be totally different organisms, as the pictures below illustrate. (Pictures are taken with a photonic microscope using phase contrast and a 100X objective.)

 

Orange contaminant (bacteria)

Yellow contaminant (bacteria)

Pink contaminant (yeast)

 Here we can see two species of bacteria (forming orange and yellow colonies on plate). They are made of rods of various shapes. The contaminant that forms pink colonies is a yeast (eukaryote), and its structure is quite different from the two bacterial species: bigger size, cellular compartment, etc.

What are these species? I do not know. At least I know that they grow aerobically and that they can make all of their cellular constituents from scratch (glucose, ammonium, phosphate, and some salts). More elaborate tests, such as provided by BIOLOG, could permit us to identify them. Alternatively, sequencing part of the genome (for instance, the 16S ribosomal RNA gene for the bacteria, the 18S ribosomal RNA for eukaryotes) would tell us what we want to know.