APPENDIX E 
j for foreign DNA. Bacteriophage vectors could be spread either as mature 
: phage, or in cells either lysogenic for the phage or carrying the phage as a 
Ij plasmid. The bacteriophage lambda can be used to illustrate relevant considera- 
1 tions since this widely studied bacteriophage is most likely to be used for recom- 
I binant experiments at the present time. 
,|i Considering first escape as a phage particle, lambda is sensitive to the 
' acidity of the stomach and is likely to be destroyed there. Normal intestinal 
strains of co I i are usually not susceptible to infection by lambda and in 
fact, susceptible strains are rare in nature. Further, in at least one case, 
1 1 
injestion of 10 lambda particles yielded no detectable lambda in resulting feces. 
j 
Lambda is also readily destroyed by drying in air. Dissemination of lambda 
recombinants through I ysogen formation, a frequent event with susceptible 
I E. col i strains, can be minimized by use of mutant varieties of lambda which 
! 
lack genes necessary for I ysogen formation: with such ohage the freauency of 
-5 ' -6 
integration into the host chromosome is reduced to 1 0 or 1 0 . Finally, 
conversion of lambda DNA to a stable plasmid is also a relatively unlikely event, 
-6 
occur ing at a frequency of about 10 
Considering then the properties of E. col i K-12, as well as those of the 
existing plasmid and bacteriophage vectors, the proposed guidelines conclude 
that, using such host-vector systems, recombinant DNAs are unlikely to be 
spread by the ingestion or dissemination of the few hundred or thousand bacteria, 
such as might be involved in laboratory accidents, given standard microbiological 
practice. Therefore, these existing systems, and analogous combinations of E. col i 
K-12 with other vectors and bacteriophage are judged to offer a moderate level of 
biological containment and are defined as EK1 , the lowest level of biological 
Appendix E — 11 
