I don't believe it's bacteria... nor do I believe it 'proves' pure AC... but these spots are a curiosity to me and something I believe that is connected to the fibromelanism of the chicks...
When I first saw the shell that was posted earlier, I said I hadn't seen it, but I had never really looked for them... I had several staggered batches of AC's going as I only had 2 pullets producing eggs and was setting weekly... I started checking shells after that, just for curiosity's sake...
I found these with spots... and then I decided to set some of my black Araucanas in one of my sets, so I figured that would be a good test comparison as well... from those sets, I got these...
This is from a very freshly hatched Cemani...
This shell was from one that had hatched about 2 hrs earlier...
And this is from a fresh hatched black Araucana... no spots in it at all...
If it was bacteria, then it would make sense that other eggs stored together, incubated together and hatched together would have at least one affected... not one Araucana egg shell hatched out with a single spot inside the shell... every single one of those was clean... but every Cemani egg hatched with varying degrees of amounts of spots, but very visible odd black spots... and there was not a hint of bacterial infection in any one of those chicks... all of those chicks, every single one of the Cemanis and Araucanas, is alive now, healthy and thriving... not even a hint of a weak chick in the whole bunch...
Please be advised, I was seriously skeptical of these when I saw the shell posted originally as well... and I still put no stock in the claims that spots in the egg shells certify a pure AC... I think it is just something that is connected with the deep expression of the fibro causing it... a curiosity for sure, but not something that says anyone else's eggs that hatch without them aren't pure...
Take this for what it's worth, it's just my own experiences with this breed so far...