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1 _ Biological containment criteria using E. col i K-12 host-vectors 
EK1 host vectors - These are host-vector systems that can be esti- 
mated to already provide a moderate level of containment, and include 
most of the presently available systems. The host is always E_. co 1 i K-12, 
and the vectors include nonconjugative plasmids [e.g., pSClOl, ColEl or 
derivatives thereof (17-24)1 and variants of bacteriophage X (25-27). 
The E. coli K-12 nonconjugative -plasmid system is taken 
as an example to illustrate the approximate level of contain- 
ment referred to here. The available data from experiments 
involving the feeding of bacteria to humans and calves (28-30) 
indicate that E_. coli K-12 did not usually colonize the normal 
bowel 3 and exhibited little 3 if any 3 multiplication while 
passing through the alimentary tract even after feeding high 
doses (i.e. 3 10 ^ to 10 ^ bacteria per human or calf). However 3 
general extrapolation of these results may not be warranted 
because the implantation of bacteria into the intestinal tract 
depends on a number of parameters 3 such as the nature of the in- 
testinal flora present in a given invidivual and the physiological 
state of the inoculum. Moreover 3 since viable E. coli K-12 can 
be found in the feces after humans are fed 10 ? bacteria in 
broth (28) or 3 x 10^ bacteria protected by suspension in milk 
(29 ) j transductional and conjugational transfer of the plasmid 
vectors from E_. coli K-12 to resident bacteria in the fecal 
matter before and after excretion must also be considered. 
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