Tamsin,
A difficult one, but you could try using a bacteriophage to kill the E.
coli. Here is one potential scheme, assuming you are conjugating in liquid
medium and not using a plate mating technique.
After conjugation has been accomplished, add Mg (as magnesium sulphate) to a
concentration of 10mM (to allow phage adsorption).
Add a large number of phage (10^11 or more). The ideal phage is lambda
carrying the _vir_ mutation - this will prevent bacteriophage lysogeny in
the E. coli.
Incubate for an hour or so at 37 C with shaking, and plate out.
You will get two populations of bacteria growing on the plates; the
Salmonella and a small number of phage-resistant E. coli. If you plate on an
X-gal/IPTG plate and select the white colonies this should allow you to
purify just the Salmonella (assuming the Salmonella is non-lactose
fermenting; a reasonable supposition).
Of course, when you come to the practicalities of the experiment you may
need to add some refinements, but in theory it would work.
Vince (former bacterial geneticist)
Vincent Mulholland,
Senior Plant Pathologist,
Diagnostics & Molecular Biology Section,
Scottish Agricultural Science Agency,
East Craigs,
Edinburgh EH12 8NJ, U.K.
URL: http://www.sasa.gov.uk/
E-Mail: Vince.Mulholland at sasa.gsi.gov.uk
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