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The transfer of a foreign DNA fragment from the initial K-12 host 
to other bacteria by means of bacteriophage vectors also requires con- 
sideration. 
Bacteriophage can escape from the laboratory either as mature 
infectious phdge particles or in bacterial host cells in which the phage 
DNA is carried either as a plasmid or inserted within the cell’s DNA 
(a well known example of natural recombination). The fate of K-12 
host cells carrying the bacteriophage DNA as a plasmid or within the 
cell's DNA is similar to that for plasmid -containing host cells as 
discussed above. The survival of bacteriophage DNAs when released 
as infectious particles depends on their stability in nature, their 
infectivity and on the probability of subsequent encounters with 
naturally occurring E_. coli strains sensitive to the bacteriophage. 
The probability of survival of those bacteriophage used in recombinant 
DNA experiments (called lambda) and their infection of resident 
intestinal E. coli in animals and humans is estimated to be small, given 
the high sensitivity of lambda to the acidity of the stomach, the 
insusceptibility of smooth type E. coli cells (the type that normally 
resides in the gut) to lambda infection, the infrequency of naturally 
occurring E. coli sensitive to lambda and the failure to detect infective 
11 
lambda particles in human feces after ingestion of up to 10 particles 
(15). Moreover, the particles are very sensitive to drying, as would 
occur if they escaped into the air. 
Establishment of lambda as a stable insertion into the host cell 
0 -1 
genome is, in certain cases, a frequent event (10 to 10 --i.e., 1 out 
of 1 to 1 out of 10) so that this mode of escape would be the preponderant 
laboratory hazard. However, most variants of lambda in use in recom- 
binant DNA experiments (16, 17, 18), and all those variants to be 
certified as EK2 and EK3, have greatly reduced ability to become so 
5 6 
inserted (probability of about 1 in 10 to 10 --100, 000 to 1, 000, 000) 
(19, 2 0, 21). The frequency for the conversion of lambda to a plasmid 
state is also only about 1 in a million (22). Moreover, the routine treat- 
ment of bacteriophage samples with chloroform (23) should eliminate 
all surviving bacteria. 
While not exact, the estimates for containment afforded by using 
bacteriophage host-vectors are sufficient to indicate that with currently 
employed systems the probability of transfer of a foreign DNA fragment 
from the original K-12 host to another bacteria is remote. 
A more detailed discussion of the relevant properties of the bacterio- 
phage lambda is found in Section III-B-1 of the NIH Guidelines. 
Additional information is in the following excerpt from reference 7. 
