Air bubbles in my media after inoculation?

[Ryan Ho]

Hello there,
Im doing an undergraduate project on ureolytic bacteria in fish. There seems to be a peculiar result. After innoculating plates with certain tissue samples, air bubbles began appearing on the plates. This could be something happening to the agar but I dont think so. I made the media and left the plates in the fridge for almost a week and no air bubbles. Only after innoculating samples taken from the spleen did air bubbles appear. Also for the higher dilutions, fewer air bubbles appear. However I cant really see any actual colonies in the media, just these bubbles in the agar on the surface. Before you say that they are just air bubbles, the solution that we used to plate the samples on contained lots of urea. Ureolysis releases CO2 which could account for the air bubbles and how at higher dilutions there are less air bubbles because there are less bacteria.

The only problem is that if we arent getting colonies or anything visible, how could such few bacteria cause so many air bubbles? And also wouldn't the CO2 just take the path of least resistance and just flow up from the agar, rather than into the agar?

It doesn't just seem like a coincidence because its happening on just plates inoculated with spleen samples, and only after inoculation. Even though the plates were left for a week no air bubbles.

Anyone have any input?