Abstract: Approximately 2.7% of a collection of Salmonella enterica var. Typhimurium promoter-lux reporter strains showed altered transcriptional patterns when exposed to low concentrations of nine different fluoroquinolone antimicrobials.  Even at the subinhibitory concentrations employed, all nine fluoroquinolones up-regulated genes involved in the SOS response, umuD, lexA, sbmC and dinP.  Using the Ames test with Salmonella strain TA102, increased mutagenicity was demonstrated in response to all the fluoroquinolones tested: ciprofloxacin, moxifloxacin, levofloxacin and gatifloxacin.  Such responses are consistent with the primary mechanism of action of this class of inhibitor, namely, the introduction of DNA damage.  This work provides support for the notion that small molecules can have functions other than growth inhibition that may affect the establishment and maintenance of community dynamics in complex environments.