absence of selection and X1776 will not colonize germ-free 
mice, both strains survive passage through the intestinal 
tract of animals. (F: 624 and n. 13) . Colonization improves 
the likelihood of conjugational transfer in the gut but it is 
not a prerequisite. (F: 686-687). Dr. Smith noted at the 
Falmouth conference that conjugational transfer has been 
observed in animals when the donor and recipient strains are 
good colonizers of the alimentary tract and plasmid 
transmission is also efficient in vitro . (F: 711-712) . 
Transfer was observed between Salmonella and E. coli and 
between different strains of E. coli . Transfer to E. coli K12 
has been observed in the rumen of fasting sheep. (F: 711-712) . 
The gut of an animal or person is not the only environment 
in which transfer might occur. (F: 712). 
Dougan, Crosa and Falkow presented a paper in Falmouth 
on the mobilization of the plasmid ColEl and its derivatives. 
(F: 676-680). Mobilization of nonconjugative plasmids, such 
as ColEl of E. coli , can be mediated with high efficiency by 
coresident conjugative plasmids, such as R or F factors. This 
process requires a triparental cross. Mobilization is also 
dependent in part on genes located on the nonconjugative 
plasmids, for example, on the mob locus on ColEl. (F: 676-680 
and n. 14) . The mob- phenotype reduces the frequency of 
13/ C. L. Wells et al. , Nature , 274 (1978), 397. 
14 / G. Warren and D. J. Sherratt, Molecular and General 
Genetics , 151 (1977), 191-194. 
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