Stimulation of genetic instability and associated large genomic rearrangements in Streptomyces ambofaciens by three fluoroquinolones

Abstract

In Streptomyces ambofaciens NSA2002, pigmented wild-type colonies spontaneously give rise to pigment-negative (Pig-) mutants at a frequency of about 0.5%. This genetic instability is related to large deletions which can be associated with amplifications of DNA sequences. The influence of three fluoroquinolones (ciprofloxacin, enoxacin, and norfloxacin) on this property was investigated. At a survival rate higher than 60%, most colonies showed a patchwork phenotype consisting of phenotypically heterogeneous colonies harboring numerous mutant sectors. Moreover, the frequency of Pig- mutants rose to more than 90% at survival rates equal to or higher than 10%. Induced Pig- mutants showed the same phenotypical features as did spontaneous mutants. Most of them also harbored deletions, associated in some cases with DNA amplifications, in two loci of the large unstable region, AUD6 and AUD90 (derived from amplifiable unit of DNA). The size of deletions in induced mutants could rise to 1.5 Mb. These results show that ciprofloxacin, enoxacin, and norfloxacin greatly stimulate genetic instability and the occurrence of DNA rearrangements in S. ambofaciens. Moreover, these three fluoroquinolones had the same rank order for both toxic (i.e., antibacterial) and genotoxic activities. If the antibacterial effect of fluoroquinolones in S. ambofaciens is due to their interference with DNA gyrase, as shown for some other organisms, the genotoxic effect observed could be due to their interaction with this type II topoisomerase. This suggests that DNA gyrase is involved in the process of genetic instability in S. ambofaciens.