DAO 80:173-180 (2008)  -  DOI: https://doi.org/10.3354/dao01959

Characterization of Renibacterium salmoninarum with reduced susceptibility to macrolide antibiotics by a standardized antibiotic susceptibility test

Linda D. Rhodes1,*, Oanh T. Nguyen1, Rebecca K. Deinhard2, Teresa M. White3, Lee W. Harrell1, Marilyn C. Roberts3

1Northwest Fisheries Science Center, National Marine Fisheries Service, 2725 Montlake Blvd. East, Seattle, Washington 98112, USA
2Pacific States Marine Fisheries Commission, 205 SE Spokane Street, Suite 100, Portland, Oregon 97202, USA
3Department of Environmental & Occupational Health Sciences, Box 357234, School of Public Health and Community Medicine, University of Washington, Seattle, Washington 98195, USA

ABSTRACT: Three cohorts of juvenile and subadult Chinook salmon Oncorhynchus tshawytscha received multiple treatments with macrolide antibiotics for bacterial kidney disease (BKD) during rearing in a captive broodstock program. A total of 77 mortalities among the cohorts were screened for Renibacterium salmoninarum, the etiologic agent of BKD, by agar culture from kidney, and isolates from 7 fish were suitable for growth testing in the presence of macrolide antibiotics. The minimum inhibitory concentration (MIC) of erythromycin and azithromycin was determined by a modification of the standardized broth assay using defined medium. The American Type Culture Collection (ATCC) type strain 33209 exhibited a MIC of 0.008 µg ml–1 to either erythromycin or azithromycin. Isolates from 3 fish displayed MICs identical to the MICs for the ATCC type strain 33209. In contrast, isolates from 4 fish exhibited higher MICs, ranging between 0.125 and 0.250 µg ml–1 for erythromycin and between 0.016 and 0.031 µg ml–1 for azithromycin. Sequence analysis of the mutational hotspots for macrolide resistance in the 23S rDNA gene and the open reading frames of ribosomal proteins L4 and L22 found identical sequences among all isolates, indicating that the phenotype was not due to mutations associated with the drug-binding site of 23S rRNA. These results are the first report of R. salmoninarum with reduced susceptibility to macrolide antibiotics isolated from fish receiving multiple antibiotic treatments.


KEY WORDS: Antimicrobial agent susceptibility testing · Macrolide antibiotic · Reduced susceptibility · Renibacterium salmoninarum · Bacterial kidney disease


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Cite this article as: Rhodes LD, Nguyen OT, Deinhard RK, White TM, Harrell LW, Roberts MC (2008) Characterization of Renibacterium salmoninarum with reduced susceptibility to macrolide antibiotics by a standardized antibiotic susceptibility test. Dis Aquat Org 80:173-180. https://doi.org/10.3354/dao01959

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