16 
The observations on the fate of E_. coli Y.-12 in the 
human alimentary tract one o.Xso re levant to the conto.ivment 
of recombinant DNA formed dth bacteriophage X variants. 
Bacteriophage can escape from the laboratory either a.s 
ma.ture infectious phouge par-tides or in baeteria.l host cells 
in which the phage genome is carried, ojs a. plasmid or prcrphouge . 
The fate of E. co li K-12 host cells carrying thus phage genome 
as a plasmid, or prophage is similar to that for plasmid- 
containing host cells as discussed, above. Thus survival of 
the X phage genome when released, as infectious particles 
depenudjs on their stabilitry in nature 3 their infectivitry and. 
on the probability of subsequent encounters with naturally 
occurring \-sensitive E. coli strains. Although thus pro- 
habilitry of survival of X and its infection of resident intestine.! 
E. coli in animals and humans has not been measured 3 it is 
estimated, to be small given the high sensitivity of X to the 
low pH of the stomach 3 the insusceptibility to X infection of 
smooth E_. coli cells (the hype that normally resides in the 
gut) 3 the infrequency of naturally occurring X-serusitive 
E_. coli (24) and the failure to detect infective X par-tides 
in human feces after ingestion of up to 10- - X particles d5). 
Moreover 3 X par-tides are very sensitive to dessicaticrr. . 
Establishment of X as a stable lysogen is a. frequent 
event (10® to 10~~ ) for the att + int + cd phage so that this 
mode of escape would be the preponderant laboratory hazard; 
[ 87 ] 
