the intentional creation of such drug-resistant, pathogenic 
organisms. Therefore, the prohibitions should apply to 
such recombinations regardless of whether they are 
naturally occurring or not. 
C. Exemptions 
1. The exemption in § l-E-4 for experiments which 
would combine DNAs from natural exchangers of chromosomal 
markers is not advisable at the present time. (A list of 
exchangers is provided in Appendix A, 43 FR 33089.) 
Several problems are inherent in this proposed exemption. 
Data on exchange between some organisms on the list is 
flimsy or non-existent. For example, exchange has not been 
documented between Hafnia and Edwarsiella or between 
Salmonella and Pseudomonas . Only four species of Pseudomonas 
are known to exchange with E. coli . Similarly, exchange 
has not been demonstrated to occur between several of the 
plant pathogens listed. 
Many of the species included in Appendix A are known 
pathogens. Salmonella , Shigella , Klebsiella , and 
enteropathogenic E. coli are Class 2 etiologic agents; 
Pseudomonas pseudomallei is a Class 3 agent. The standards 
for handling these organisims should be applied to the 
handling of bacteria containing DNA from these organisms , 
particularly since § I-E-4 places no restrictions on the 
use of readily transmissible vectors or healthy host strains. 
The inclusion of several plant pathogens in Appendix A 
does not provide sufficient protection for plants. Caution 
[A-188] 
