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"in order to obtain additional information on the likelihood 
of transmission of recombinant DNA, we have also commenced 
to collect a diversity of E. coli strains obtained from patients 
with bacteriemias, wouncT infections and urinary tract infections, 
from healthy individuals and from sewage. We have been exam- 
ining these strains for the presence of nonsense suppressor 
mutations that would allow for the replication and perpetuation 
of lambda or plasmid vectors that contain nonsense mutations 
and also for the ability of these strains to be infected by lambda 
DNA that is tagged by an antibiotic resistance marker. So far, 
in a test of some 100 strains, we have failed to detect any strain 
with a nonsense suppressor or that was infectable by lambda. 
R. Davis, P. Leder and their colleaques have also examined 
some 2000 E. coli strains for sensitivity to phage lambda and 
although they found a few strains that appeared partially sensitive 
to lambda, none would propagate the virus." 
One of the theoretical mechanisms by which organisms containing 
recombinant DNA might result in untoward effects is that the foreign 
DNA might be excised from the experimental or secondary host-vectors 
and recombined with cellular DNA of an animal harboring that host- 
vector [see Section IV-C-1 -b-(2)] . The last paragraph of Appendix O 
deals with the probability of such events (11). 
Thus far, the suitability of K-12 for recombinant DNA experiments 
has been considered in relation to the ability of the K-12 cells to cause 
any harm either directly or by transfer of a foreign DNA fragment to 
another bacterial cell. These properties will always depend on the 
ability of the K-12 to survive, multiply, and infect another living 
organism. The relative inability of K-12 to survive in natural environ- 
ments has already been described. If it should survive and multiply, 
it is still unlikely to infect living things. E. coli bacteria are primarily 
spread by ingestion of food and water containing them; such agents are 
6 9 
seldom spread by aerosols. It is estimated that about 10 to 10 
(1 million to 1 billion) cells of pathogenic E. coli are required to cause 
disease (7, 13). Therefore, were a K-12 to become pathogenic, or 
to transfer a foreign DNA fragment to another bacteria that was or 
became pathogenic, then at least a million bacteria would be required 
to cause disease in a single individual. 
The Guidelines emphasize protection of laboratory workers, as pre- 
viously stated, because the individuals most at risk are those conducting 
experiments and because laboratory workers are the most likely carriers 
of recombinant DNA out of the laboratory. Should a laboratory worker 
carry out such agents, it would still be highly unlikely that any others 
would be infected, and the risk of a resulting epidemic is virtually non- 
existent. There is abundant evidence for this assertion. We know from 
many years' experience that the separation of sewage from food and 
water supplies prevents epidemics of enteric bacteria such as E. coli. 
