I think there is no such resistance develope,adverse metabolic conditions may hamper the growth .All the organism use as a probiotics act as opportunistic helpful organisms.
Probiotics act to exclude bad or harmfull bacteria. They will provide some stress protection for your birds. In areas of high gastro intestinal problems (SE for example) the "good" bacteria (probiotics) given in large doses as provide will reduce the space available for the harmful bacteria providing "protection from the harmful bacteria. There is no direct action on the bacteria so ressistance is not possible.