128 
DR. HELINSKI: I will try to answer your second question first. I 
don't think Dr. John Nutter is here, but there is a contract program that 
is supported by the National Institutes of Health to conduct these very 
experiments--that is, the natural environment experiments. 
Now, the point of the transduct ional mode of transfer of plasmid DNA, 
we have clearly considered this again in our estimates, and I could go into 
detail, but I think this is not the time. In our estimates the probability, 
again, is very low for a number of different reasons — size of the plasmid, 
what we know about the extent, the presence of the proper bacterial viruses 
for carrying out this transfer, the fact that a commonly-used transducing 
virus like PI would have great difficulty in attaching and inserting DNA 
into a smooth common intestinal E. coli cell. Considerations like this 
have been made. 
DR. FREDRICKSON: Any other comments or questions? Ms. Simring, do you 
have a question? 
MS. SIMRING: Yes, I do. 
DR. FREDRICKSON: Will you stand, please, so that we may all hear you? 
MS. SIMRING: It may be a naive question because I am not a scientist, 
but it would seem to me that if in nature this exchange that you describe 
takes place or could take place in the gut, for example, because calcium 
chloride might be present, and there might be an equivalent heat shock that 
could engender this event, has it been found or has it not been found fre- 
quently, never, rarely, or whatever, that human fragments of DNA have been 
found in intestinal E. coli ? 
DR. HELINSKI: I don't know of anyone who has systematically set out to 
carry out such experiments. I can give you a dozen reasons why it is ex- 
tremely difficult to carry out. 
MS. SIMRING: You don't need to give a dozen, but a few. 
DR. HELINSKI: One of them is that it is a question of on what level 
you would be looking at it. We are talking about very low probability 
events. Transformation, for example, occurs with one microgram of DNA — 
something in the order of one in 10 4 or one in 10 5 cells, and the techniques 
that you would use to pick up a small insert of mammalian DNA with what we 
call hybridization techniques, probably are not really sensitive enough to 
scan very large amounts of bacteria to say yes or no in a definitive kind 
of way that indeed E. coli cells take up mammalian DNA. It may be that 
someone will get very clever and do an experiment along the lines of Chang 
and Cohen to test this with markers that are very sensitive to pick up. 
But so far that hasn't been done. 
MS. SIMRING: May I ask one other question of you? Would you say that 
it would occur rarely, naturally? 
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