Eureka, today the results from the fermentation tests I did with two yeast strains I isolated from a Girardin Gueuze and a 3 Fonteinen Gueze. The isolation for the Girardin strains can be found here, and the one for the 3 Fonteinen is here. I included the microscopy pictures which I already posted in older posts to get you an idea what kind of yeast I used (Fig 1/2).
Fig 1: Isolated yeast from 3 Fonteinen Geuze
Both yeast strains have a different morphology than typical Saccharomyces cerevisiae. I therefore assumed that these two yeast strains are from a different family like Brettanomyces, Kloeckera, Pichia etc.
Fig 2: Isolated yeast from Girardin Gueuze
Since I do not own the fancy techniques for determining the kind of yeast I have here, I used a different approach. To test if these strains are for example Brettanomyces, I inoculated two starters with these two isolated strains and waited…
02/29/2012: The starters were made with dried malt extract (20 g to 200 mL) and sterilized in a pressure cooker for approximately 15 min. Then let the starters cool down and inoculated them with a colony of each strain.
03/04/2012: Four days after inoculation. See what happened:
Fig 1: 3 Fonteinen bug after four days
Fig 2: Girardin bug after four days
There was a pellicle in the Girardin starter! And something similar in the 3 Fonteinen starter as well! Eureka! And the smell of the starters was just incredible. A lot of funky, sourness (no acetic acid) and cherry notes. Now I am pretty sure that these yeasts that I isolated from the dregs are a sort of souring bugs indeed. I assume that both strains are from the family of Brettanomyces.
03/09/2012: The pellicles are gone and there is a sediment in the starters.
02/24/2012: Tasting of the starters: Both starters smell just incredible. A lot of funk is going on. But no sourness detectable on the palate. I assume that both strains in the starter are indeed Brettanomyces. Both yeasts will ferment some sour beer in the future for sure.
Eureka Brewing 
Very interesting and exciting! Do you use those strains now?
> Very interesting and exciting!
Cheers 🙂
> Do you use those strains now?
Currently not. Focus more on local yeasts rather than isolates from the past.