Abstract
Quinolone‐resistant strains of the fish‐pathogenic bacterium, Photobacterium damselae subsp. piscicida are distributed widely in cultured yellowtail, Seriola quinqueradiata (Temminck & Schlegel), in Japan. The quinolone resistance‐determining region (QRDR) was amplified with degenerate primers, followed by cassette ligation‐mediated PCR. Open reading frames encoding proteins of 875 and 755 amino acid residues were detected in the gyr A and par C genes, respectively. Resistant strains of P. damselae subsp. piscicida carried a point mutation only in the gyr A QRDR leading to a Ser‐to‐Ile substitution at residue position 83. No amino acid alterations were discovered in the ParC sequence. A mutation in the gyr A gene was also detected in nalidixic acid‐resistant mutants of strain SP96002 obtained from agar medium containing increased levels of quinolone. These results suggest that GyrA, as in other Gram‐negative bacteria, is a target of quinolone in P. damselae subsp. piscicida . Furthermore, we attempted to detect a point mutation using targeting‐induced local lesions in genomes (TILLING), which is a general strategy used for the detection of a variety of induced point mutations and naturally occurring polymorphisms. We developed a new detection method for the rapid and large‐scale identification of quinolone‐resistant strains of P. damselae subsp. piscicida using TILLING.