15 
difficult to estimate. However, in calves fed on an 
jr:ti tic tic-supp lamented dvet 3 it has teen estimates that 
sue': trio or entail nonconfugative H plasmid transfer occurs 
at frequencies of no more than 10~ 10 to 10~' 2 per 24 hours 
rer cal f (30). In terms of considering other means for 
vlas'"id transmission in nature 3 it should be noted that 
transduction does operate in_ vivo for Start- loco ecus 
T.i^en s (32) and probably for E. coli as well. However, 
'io dr ta are available to indicate the frequencies of plasmid 
transfer in vivo by either transduction or transformation. 
These observations indicate the low probabilities for 
r ossicle dissemination of such plasmid vectors by accidental 
ingestion, which would probably involve only a few hundred 
or thousand bacteria provided that at least the standard 
practices (Section II -A above) are followed , particularly 
avoidance of mouth pipetting. The possibility of coloni- 
zation and ' r ance of transfer are increased 3 however, if the 
normal flora in the bowel is disrupted by, for example 3 anti- 
biotic therapy '33). For this reason 3 persons receiving such 
therapy should not work with DNA recombinants formed with any 
r. coli K-12 host-vector system during the therapy period and 
for seven days thereafter; similarly 3 persons who have achlorhydria 
or who hoove had surgical removal of part of the stomach or bowel 
should avoid such work 3 as should those who require large doses 
of antacids. 
[ 86 ] 
