Eggs. Fridge or not fridge??

Certainly if an egg floats it's a sign of a potential heavy bacterial infestation and you certainly should discard it however the objective of any storage system is to have the very minimum of bacteria growth in the egg.  It's not really possible to gauge a safe level of bacteria because people's susceptibility varies as previously mentioned.

The floating egg test is just a very rough rule of thumb and won't provide a precise indication of an eggs freshness.   Basically as an egg gets older it loses moisture and the air pocket gets bigger so making it more buoyant.   It's not primarily to do with bacteria creating carbon dioxide making the egg more buoyant.  In fact as an eggshell isn't flexible creating pressure within the egg won't make the egg expand which it would need to do to make it more buoyant.  

Eggs can however explode due to the rapid growth of bacteria producing CO2 and this can happen in an incubator if infertile eggs aren't removed.  I'm sure you will have noticed that boiled eggs sometimes crack and expel the interior due to the air pocket expanding (pricking the blunt end prevents this).

A slow growth of bacteria in an egg in storage however will produce CO2 which will pass through an eggs porous shell without building up a substantial internal pressure.

Even in fresh eggs the air pocket can vary a great deal depending on the age of the hen.  Older hens produce larger air pockets.  Commercial egg producers check the size of the air pocket on a sample basis and egg production regulations limit the size that is acceptable for sale.

So the bottom line is buoyancy depends on the volume of the air pocket (relative to the volume of the egg).  An egg gets more buoyant as it loses moisture which generally happens as it gets older though storage conditions like humidity and temperature will have a big impact on this too.  The size of the air pocket is not consistent even with fresh eggs.  All these factors make the float test fairly unreliable as a measure of bacteria content despite what Delia Smith might tell you.
HF