selective pressure on E. coli stored on LB plate in lab fridge

Chris=Michiels%LMM%AGR at Chris=Michiels%LMM%AGR at
Fri Jul 7 18:12:24 EST 1995

From: Chris=Michiels%LMM%AGR at
Subject: selective pressure on E.coli stored on LB plate in lab fridge
It is common practice in research labs to keep bacterial strains on plate with a regular
transfer when viability decreases. For example, molecular geneticists keep E. coli
on LB plates at 4°C and transfer to fresh medium every month.
When these E.coli LB plates are kept for 2 or more months, viability decreases. It is our
experience that the surviving cells show poorer growth, not only a longer lag, but a slower
exponential growth rate. This suggests that a selection has taken place during cold
storage (starvation), and the most resistant (surviving) cells appear to be slower growing.
-Should this kind of starvation necessarily select for the slower-growing mutants,
and if yes, why?
-What kind of mutations are selected in this way, and what other phenotypes do these
mutations affect (besides growth rate)?
Thanks for any useful input.

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

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