In several bacteria (or other microbes such as yeast) all or most genes have been deleted individually to determine which genes are essential for survival.
This was a traditional brewing process; top-fermenting yeasts and a taste of fruity ester were usual and there was no emphasis on long term storability.
They were selected on the basis of their phylogenetic positions and their specific interest as human pathogens, or as industrially or environmentally important yeasts.
