Larry Farrell farrlarr at
Wed Sep 29 09:20:09 EST 1999

ecoli at wrote:

> > From: "AKB" <abansal at>
> >
> > suppose you have a bacterial population with resistance to two
> antibiotics
> > on two different plasmids.  the bacteria are irradiated at low doses
> to kill
> > or inactive these plasmids.  how would you isolate and collect the new
> > bacterial population that is susceptible to at least one of these
> > antibiotics and how could you prove their susceptibility while at the
> same
> > time not killing them and being sure to recover a pure susceptible
> > population?
> >
> >
> Simple,
> You leave out the initial selective isolation medium. Isolate on a
> non-antibiotic agar - keep cultures and test what grows.
> Sorry-sorry-sorry but it takes a lot more work and isolations and time
> but that is how it crumbles.

Even worse is the fact that the plasmids are *much* smaller targets than the
bacterial chromosome and, therefore, will likely not be even hit by the
irradiation, let alone inactivated, until long after the chromosome has
suffered multiple hits.  This would reduce the probability of recovering
viable cells carrying inactivated plasmids to the point that I would not bet
much on your odds of doing so.  The best procedure would be acridine orange
curing of the host cells.

Larry D. Farrell, Ph.D.
Professor of Microbiology
Idaho State University

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