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The laboratory variants of K-12 to be used in recombinant DNA 
experiments have never been reported to cause disease, even in 
laboratory workers. K-12 has been grown in very large quantities (up to 
12 
hundreds of liters containing up to 10 --that is, 1, 000, 000, 000, 000-- 
bacteria per liter at a time) in hundreds of laboratories all over the 
world over a period of 30 years and under containment conditions lower 
than those specified as Pi in the Guidelines. Furthermore, K-12 has 
none of the many properties generally associated with pathogenic bacteria 
(2, 3, 4, 5, 6, 7, 8, 9). K-12 does not-- 
• survive and multiply readily in natural environments, and is 
thus unable to spread from animal to animal or plant to plant; 
• multiply readily on body surfaces or intestines and lungs; 
• penetrate animal cells; 
• spread throughout animal bodies; 
• produce a toxin or otherwise alter other living things to cause 
symptoms associated with disease 
• resist normal body defense mechanisms. 
Experiments (4, 5, 6, 8) have shown that even after normal humans 
10 
have ingested up to 10 (10, 000, 000, 000) K-12 cells, only transient 
multiplications of the bacteria in the intestines can be observed, and 
that after a time no K-12 can be detected in the feces. Thus, K-12 
does not establish itself as a permanent resident of normal human beings. 
It might be pointed out that animals, including humans, ingest large 
numbers of bacteria of many species daily. Most of these do not take 
up long-term residency. For example, a normal portion of yogurt 
may contain ten billion cells of the bacteria Lactobacillus vulgaris; 
in spite of daily consumption, the Lactobacillus quickly disappears from 
the human bowel. Similar data are available for certain laboratory 
animals (7, 8). 
K-12 can reside in certain abnormal environments, such as 
the intestines of laboratory -bred germ -free animals (8), and may 
establish itself in other abnormal conditions such as during antibiotic 
therapy (10). The Guidelines therefore prohibit investigators from 
carrying out recombinant DNA experiments when such abnormal 
conditions exist. 
