44 
Slide 6 
Containment of E. coli Bacteriophage (X) 
As a bacteriophage 
1. X is sensitive to acidity of stomach. 
2. Normal gut strains of E. coli usually not susceptible to X infection. 
3. Susceptible strains of E. Coli are rare in nature. 
4. Ingestion of 10 11 X particles yielded no detectable X in human feces 
As a lysogen 
1. Frequent event (1 to 1C 1 ) 
2. X mutants lacking ability to lysogenize. 
(frequency 10 ' 5 to 10 ‘ 6 
As a plasmid 
1. Frequency about 10 ' 6 . 
The second mode for dissemination of genes recombined into lambda DNA 
is integration into the chromosome of the DNA as a lysogen. This is a 
frequent end result of infection of E^ coli by lambda. There are, however, 
known mutant varieties of lambda which lack the genes necessary for lysogen 
formation, and with such bacteriophage, the frequency of integration into 
the host chromosome is reduced to 1 in 1(P or 1 in 1C>6 — 106 being 1,000,000. 
The third mode, the conversion of lambda DNA to a stable plasmid, is 
also a relatively unlikely event occurring at a frequency of about one in a 
million. Considering then the properties of E_j_ coli K12 as well as those 
of the existing useful plasmid and bacteriophage vectors, the proposed 
guidelines conclude that using such host-vector systems, recombinant DNAs 
are unlikely to be spread to humans by the ingestion or dissemination of a 
few hundred or thousand bacteria such as might be involved in laboratory 
accidents, given the standard microbiological practices. Therefore, these 
existing systems, and combinations of E. coli K12 with other similar vec- 
tors, plasmid vectors, and bacteriophage, are judged to offer a moderate 
level of biological containment and are defined as the lowest level of 
biological containment for experiments with E. coli systems. This level is 
called EK1. 
As for the physical containment levels, increasing numbers specify 
increasing levels of containment for the E_j_ coli systems. The next level, 
called EK2 is summarized on the next slide (7). 
[185] 
