There was a thread on this in rec.food.sourdough about a year ago.
A desireable yeast + bacteria combination that you can, but not always, get
is apparently Saccharomyces exiguus and a lactobaccillus whose name escapes
me at the moment.
Sanford Silverman >Opinions expressed here are my own<
silvermans at pt.cyanamid.com "Yeast is Best"