The main objective of the work is to evaluate various mutant and recombinant microorganisms and/or their metabolites and identify any adverse effects they could have on human health. One of the primary focuses is to develop new methods and to improve existing methods for monitoring exposure to engineered bacterial strains and/or genotoxic metabolites generated by these organisms. Several areas of research are being pursued. They include (1) competition and survival of mutant microorganisms, (2) the identification and effects of toxic metabolites produced by mutant or recombinant microorganism biodegradation products and the associated toxicity, (4) genetic transfer capabilities and mechanisms of interest to the microbial gut inhabitants, and (5) expression within intestinal flora of genes transferred from the mutant or recombinant microorganisms of interest. In general, the Pseudomonas spp. were poor competitors in the mouse intestinal tract. Even though their numbers decreased significantly over the test period, some of the strains were still recoverable 14 days after dosing. In vitro results are similar.