III. EXPERIMENTS IN MAN WITH ORAL DOSES OF 
ESCHERICHIA COLI HYBRIDS 
Escherichia coli is especially useful in the study of hybrids because it 
can be changed in a great many ways by genetic transfer of plasmid-specified 
information from other microorganisms (391). The K-12 strain commonly used in 
recombinant studies was reported (463) as not surviving in the human intestines 
of nine volunteers for more than a few days. Two K-12 hybrid experimental 
vaccines in oral doses of 10 6 to 10^ cells and 5 x 10^ cells have been tested 
safely in 45 and 57 men, respectively (26). A noninvasive toxigenic K-12 
hybrid in oral doses of 10 6 to 10^ organisms was well tolerated by 85 of 86 
men (538). Fortunately, man-to-man transfer of £. coli for all practical 
purposes is entirely oral. To change this basic characteristic may be difficult 
The reported human infective dose of intestinal microorganisms is high (Tables 9 
and 10) (539), although the low ID of $>. Flexneri 2a (Table 9), and a report 
that 10 S. shigae were infective for 1 of 10 men (538), warns that the ID might 
be lowered if deliberately sought for by genetic manipulation. Although in 
selected instances extensive biohazard precautions would be necessary for 
research with £. col i hybrids, it is probable that the most important of all 
precautions would be the hardest to enforce, namely (1) use mechanical 
pipettors, (2) avoid hand contamination, (3) keep hands out of the mouth and 
off the face, (4) no smoking in the laboratory, (5) no candy, food, drinks, 
or gum in the laboratory. In this connection, the consensus of an inter- 
national meeting at Asilomar in February 1975 (464) included a recommendation 
for the precautionary utilization of only those organisms unable to survive in 
the human intestines or outside of the laboratory. In December 1975 additional 
guidelines were developed (536). 
In concluding this analysis of the potential biohazard of hybrid viruses 
it is apparent that, although transmission to family and public is only a remote 
possibility, protective measures should be taken in the laboratory in anticipa- 
tion of possible unexpected development of disease potential absent in the 
parental strains. For example, the temperature-sensi tive mutant of measles virus 
can induce, in the newborn hamster, an hydrocephal us that is not produced by the 
parental strain or by other measles virus mutants (448). 
[ 402 ] 
