Environmental and Molecular Mutagenesis. DOI 10.1002/em 408 Lemieux et al. Fig. 1. Direct-acting (i.e., 2S9) and indirect-acting (i.e., 1S9) mutagenic potencies of the nonpolar neutral and semipolar aromatic soil fractions measured during day 0, 3, 7, 24, and 29 of the bioremediation treat- ment in Salmonella strains TA98 (n), YG1041 (&), and YG1042 ( ). Note that TA100 did not detect any mutagenic activity in any samples on any days. of S9 was expected because this fraction contains PAHs that are known to be indirect-acting mutagens that require metabolic transformation to electrophilic intermediates to react with DNA. Cytochrome P450 isozymes (e.g., Cyp1A1 and 1A2) present in the S9 fraction of rat liver transform PAHs into reactive metabolites that form DNA adducts, and contribute to the formation of stable muta- tions detected by the Salmonella mutation assay [Xue and Warshawsky, 2005]. The results for the nonpolar neutral fractions indicate that positive responses are more frequently observed with the Salmonella strains that detect frameshift mutations (i.e., TA98 and YG1041), in comparison with those that detect base-pair substitutions (i.e., TA100 and YG1042). Moreover, the highest levels of mutagenic activity without S9 were detected with these frameshift strains. In fact, TA100 did not detect any signiﬁcant levels of mutagenic activity, with or without metabolic activation, in any sam- ple. This was unexpected because previous studies have demonstrated strong base-pair substitution activity for PAHs and related compounds [McCann et al., 1975]. However, it should be noted that Salmonella strain YG1042 detected high mutagenic activities in the nonpo- lar neutral fractions with the addition of S9. The relative change in base-pair substitution activity between YG1042 and TA100 may suggest the presence of aromatic amines in this fraction; these compounds require metabolic acti- vation by S9 and O-acetyltransferase to induce a muta- genic response. However, their presence in this fraction would be surprising, because validation of the fractiona- tion procedure by Lundstedt et al. has demonstrated that aromatic amines elute in the semipolar aromatic fraction [Lundstedt et al., 2003]. Alternatively, the S-heterocyclic compounds present in the nonpolar neutral fractions of the bioslurry extracts (e.g., benzothiophene and diben- zo[b]naphtha[2,1-d]thiophene) may be inducing a larger response on the metabolically enhanced Salmonella strain. Limited information is currently available about the path- ways involved in metabolism and activation of mutagenic S-heterocyclic compounds and it is not known if the mutagenicity of these compounds can be modulated by Cnr and/or OAT. Overall, a general trend toward increasing mutagenic activity in the nonpolar neutral fractions was observed as the treatment progressed, and this was particularly appa- rent with the metabolically enhanced strains (i.e., YG1041 and YG1042). This increase in mutagenicity was surpris- ing because many of the compounds in this fraction decreased to 10% of their initial concentration over the course of the remediation [Lundstedt et al., 2003]. More- over, it is commonly assumed that decreases in the con- centration of priority substances are correlated with