Citation:

Cross, K. AND D. DeMarini. Analysis of chemical structures and mutations detected by Salmonella TA98 and TA100. Mutation Research/Genetic Toxicology and Environmental Mutagenesis . Elsevier Science Ltd, New York, NY, 8227:na, (2023). https://doi.org/10.1016/j.mrfmmm.2023.111838

Impact/Purpose:

The Salmonella (Ames) mutagenicity assay is required by for registration of all pesticides by the U.S. EPA and for all drugs by the U.S. FDA.  It will also play a role in the implementation of TSCA for mutagens and carcinogens by the U.S. EPA (Patlewicz et al., Computational Toxicology 20:1-100185, 2021).  Thus, understanding what proportion of mutagenic chemicals this assay detects and chemical classes it detects, is critical to its continued requirement by the U.S. EPA.  Leadscope (Columbus, OH), for which the U.S. EPA has a site license, has created a searchable dataset of the data for >10,000 compounds tested in the Salmonella assay.  Using this dataset, we determined that just two Salmonella strains (TA98 and TA100) among the 5 strains recommended by the OECD for mutagenicity testing, detect 94% of all the known mutagens in the dataset.  (The remaining 6% cause chromosomal mutation rather than gene mutation and are not detected by bacterial assays, but they are detected by mammalian cell assays, such as the comet or micronucleus assays.)  We also found that 1/3rd of the known mutagens in the dataset require metabolic activation (S9) to be detected as mutagenic, something not known for the 50 years since the development of S9.  Finally, we determined the classes of compounds these strains detect:  TA100 detects hydrazines, azides, various halides, nitrosamines, epoxides, aziridines, difurans, and half-mustards, whereas TA98 detects aromatic amines, PAHs, triazines, and dibenzo-furans.  This information provides guidance to the U.S. EPA regarding the use of the Salmonella mutagenicity assay, showing that general screening using just strains TA98 and TA98 with and without S9 detects a broad range of chemical classes.

Description:

As part of an analysis performed under the auspices of the International Workshop on Genotoxicity Testing (IWGT) in 2017, we and others showed that there was redundancy among the bacterial strains specified in the Organization for Economic Cooperation and Development (OECD) Test Guideline TG471 for genotoxicity testing.  Salmonella frameshift strain TA98 and base-substitution strain TA100 together detected 93% of the mutagens detected by all the bacterial strains recommended by TG471 (Williams et al., Mutation Res. 848:503081, 2019).  We have extended this analysis by identifying the numbers and chemical classes of chemicals detected by these two strains either alone or in combination, as well as the role of S9.  Using the Leadscope 2021 SAR Genetox database containing >21,900 compounds, we assembled a dataset containing 7,170 compounds tested in both TA98 and TA100.  Together, TA98 and TA100 detected 94% (3,733/3,981) of the mutagens detected using all the TG471-recommended bacterial strains; 39% were mutagenic in one or both strains.  TA100 detected 77% of all of these mutagens and TA98 70%.  Considering the overlap of detection by both strains, 12% of these mutagens were detected only by TA98 and 19% only by TA100.  In the absence of S9, sensitivity dropped by 31% for TA98 and 29% for TA100. Overall, 32% of the mutagens required S9 for detection by either strain.  Using the 2021 Leadscope Genetox Expert Alerts, we found that TA100 detected 18 mutagenic alerting chemical classes with better sensitivity than TA98, whereas TA98 detected 10 classes better than TA100.  TA100 detected more chemical classes than did TA98, especially hydrazines, azides, various di- and tri-halides, various nitrosamines, epoxides, aziridines, difurans, and half-mustards; TA98 especially detected polycyclic primary amines, various aromatic amines, polycyclic aromatic hydrocarbons (PAHs), triazines, and dibenzo-furans.  Both TA98 and TA100 with and without S9 are needed for adequate mutagenicity screening with the Salmonella (Ames) assay.

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