I’d thought I’d take a minute to talk about one of the first microbes we isolated that has proven to be quite interesting. The microbacterium TTU3 which was isolated from a toilet biofilm first caught our attention as a brilliantly red colony in the middle of an otherwise rather dully colored plate. Although we know color is not necessarily an indication of environmental importance or any other quality other than color the color itself TTU (as it was called then) stuck in our minds and we paid attention as it went through the sequencing process.
While investigating the ideal growth temperature of TTU one of the things we noticed was that its color was temperature dependent. In a couple of side experiments, David Coil grew 5mL liquid cultures at 37ºC, room temperature and 4ºC and noticed that the colder the temperature was the brighter the red color and that at warmer temperatures the bacteria were whitish-yellow.
TTU’s color is temperature dependent but that’s not the whole story. This week, while growing overnight cultures to verify our stock culture, I noticed that after a dilution, the room temperature culture had turned from its usual pink color to the white-yellow characteristic of the 37º cultures but only at the edges of the biofilm that had collected at the bottom had changed color, the most dense collection of cells was still pink.
Upon further investigation I noticed the bright red plate (also stored at 4ºC) that we had made the overnight cultures from also had patches changed color in some places where it used to be red. What was most interesting about the white-yellow patches was where they were. On the streaked plate, the white appeared only at the beginning of the streak not in the single colonies at the end of the streak unless the colony had been picked for an overnight. In picked colonies, the white-yellow appeared only at the edges where the heat-sterilized wand had touched the colony.
These observations lead me to three conclusions: either the stock and plate have both been contaminated by a similar organism, the color change is also affected by density (perhaps quorum sensing in this organism is somehow tied to environmental temperature) or the color change is oxygen dependent (since we always limit the oxygen exposure of our 4ºC cultures to limit their growth while in storage).
I’m in the process of sequencing the 16s PCR product of these cultures so I will know soon whether the color change is due to contamination or not. If it is not contamination, figuring out the mechanism and conditions under which TTU3 will undergo a color change, may be an interesting side project to work on.
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Jonathan Eisen's Lab 
Have you ever seen multiple kinds of colonies coming from a single streak? Or do all the colonies end up looking the same as each other (and just the dense material at the beginning of the streak looking different?)
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I re-read my post above and noticed that I might not have been very clear. I was using white and white-yellow interchangeably. Any non-red patch I saw was white-yellow/white.
The white-yellow/white patches only show up in the dense material in the beginning of the streak and only after weeks of being in the freezer. The exception to this is the red colonies which have been touched with the heat-sterilized dilution wand. These colonies are at the dilute end of the streak and only the edges where the dilution wand has touched them turn white-yellow/white.
In the denser part of the plate the white-yellow/white patches are roughly circular much like colonies. In the dilution wand colonies the white-yellow/white patches simply border the edges of the red smears left after we used the dilution wand. Other than these differences in shape, all the white-yellow/white colonies appear to be the same.
This plate is the only TTU plate on which I’ve seen this happen. But I wasn’t looking for it before so we might have thrown the other plates out before the white-yellow/white patches had a chance to appear. It would be interesting to try to deliberately create the patches.
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