Association of Salmonella mutants with germ-free rats: characterization of the reverse mutational response to 2-nitrofluorene.

Salmonella typhimurium strain TA1538 described by Ames, in association with otherwise germ-free rats, colonizes the gastrointestinal tract. The revertants enumerated in the feces of each of these rats varies in a coordinated manner in relation to the day on which the measurement is made. In response to the oral ingestion of a single dose of 2-nitrofluorene, the concentration of revertants in the feces increases and then returns essentially to control values within 6 days. When these rats are challenged repeatedly with a similar oral dose of 2-nitrofluorene the revertent concentration in the feces remains elevated for a progressively longer time. A change in phenotype of strain TA1538 during prolonged association rather than a change in the biology of the rat seems to explain this phenomenon. Firstly, germ-free rats exposed repeatedly to 2-nitrofluorene and then associated with strain TA1538 do not have the prolonged response characteristic of multiple challenges with 2-nitrofluorene. Secondly, strain TA1538 reisolated after several weeks of association with the otherwise germ-free rat shows other alterations such as decreased sensitivity to 2-nitrofluorene in the pour plate assay and a decreased sensitivity to crystal violet. In spite of limitations imposed by these alterations in strain TA1538, it is possible to demonstrate a dose-response relationship between the amount of 2-nitrofluorene ingested and the concentration of revertants in the feces of exposed rats.