limits to bacterial heat resistance?

[Chris Michiels]

How heat resistant can I make an E. coli (or other bacterial) culture by 
consecutive rounds of selection, i.e. regrowing survivors of a partial 
heat kill?
What about bacterial spores? Can their heat resistance be boosted? 
What is the probability and the risk of such selections taking place 
during (mild) thermal food processing, and of more heat resistant 
pathogens entering the food chain? Or would such variants necessarily be 
cripple and have attenuated virulence?

Opinions and references to relevant studies in the literature, please? 
Thank you.

Chris Michiels
KULeuven
Lab Food Microbiology
Kard. Mercierlaan 92
B-3001 Heverlee