limits to bacterial heat resistance?

[Kevin G. Korth]

Chris Michiels <chris.michiels at agr.kuleuven.ac.be> wrote:
>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
>
>

Though I can't recall the references off the top of my head (and I am 
too lazy to dig through my mountain of files just to respond to this 
posting), historically, many studies have been published addressing this 
very issue (especially with spores).

My personal opinion is that E.coli would only be able to develop heat 
resistance to a certain point without vastly altering its internal 
protein and membrane structure, which, I doubt could happen and still 
maintain an E.coli genome.

Spores, on the other hand have been boosted in heat resistance to 
phenominal levels.  I recall reading of a B.stearothermophilus strain 
that was capable of D-values in excess of 9 minutes at 121 C (autoclave 
temperature) [most B.stearothermophilus have D-values between 1 and 2 
minutes at that temp.]

If you are planning a study, make sure you include a large complement of 
appropriate controls.  There's nothing worse than making a phenominal 
claim, only to get it blasted by poor experimental design.

One more thing.  Never assume that a pathogen is crippled just because it 
barely survived some treatment.  The FDA would never buy such an 
argument.  To the FDA a pathogen is a pathogen, regardless of its 
history.

kkorth at kcc.com