darwinian medicine

Walter Ogston ogston at HOBBES.KZOO.EDU
Mon Oct 9 14:28:16 EST 1995

Scott Hoffman (schoffma at badlands.nodak.edu) writes:

> If a population of vancomycin resistant cells were selected for by 
> vancomycin treatment, and then the selective pressure of vancomycin was 
> removed, would it be resonable to believe that a population of vancomycin 
> sensitive cells would eventually outcompete and re-establish itself?  As 

I think this is a very good question.  As Scott says, if
unselected antibiotic resistance involves a metabolic cost, then
one would expect it to disappear from the population quite
quickly, but if the gene is repressed and presents essentially
no cost, then what?  

I think the key to this is the probability of (selectively neutral)
mutation in the gene during the period of no selection by
antibiotic.  Given an estimate of mutation rate, and a starting
population of resistant bacteria, their growth and death rate
etc. I guess it would be possible to calculate the likely 
distribution of bacteria remaining after time t with no, one, 
two etc.  mutations.  

My guess is that a population of cells carrying a single muation
would give rise to resistant clones almost immediately when
antibiotic selection is re-imposed, but there must be a number
of mutations for which this probability drops effectivley to
zero.  One should be able to determine this experimentally.
Maybe someone has already?  What do y'all think out there?  
Walter Ogston				ogston at hobbes.kzoo.edu
Department of Biology			Phone: (616)337-7010
Kalamazoo College			Fax:   (616)337-7251
Kalamazoo, MI 49006-3295

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