Search USGS⌕ Search
Home/Archive/USGS

USGS · 1014868

Biochemical and conjugation studies of romet-resistant strains of Aeromonas salmonicida from salmonid rearing facilities in the eastern United States

C. E. Starliper·R.K. Cooper
View original source ↗Export metadata ↓

Abstract

Strains of Aeromonas salmonicida ( n = 585) were collected from covertly infected and diseased salmonid hosts from 12 hatcheries in the eastern United States. Strains and sites were selected because of their potential for harboring antimicrobial resistance, in particular, to Romet™. Resistance to Romet was displayed by 315 strains (53.8%), which were isolated from all six host species sampled at 10 of 12 sites. Thirty of the resistant strains (9.5%) from five sites had no zone of inhibition, whereas the other strains had either confluent growth or resistant colonies within a zone of inhibition. Fifty-one resistant strains, representing each of the three resistance phenotypes, were selected for biochemical and antimicrobial comparisons with Romet-sensitive strains. All were confirmed to be A. salmonicida , and no characteristic biochemical phenotypes were found to be associated with resistance to Romet. Differential resistances between resistant and sensitive strains were detected to the antimicrobials oxytetracycline, tetracycline, sulfadiazine, sulfamethizole, trimethoprim, and SXT, a potentiated sulfonamide composed of trimethoprim and sulfamethoxazole. Plasmid DNA isolation and agarose gel electrophoresis were done for 25 Romet-resistant strains, and R-plasmids, not present in sensitive strains, were detected in 23 of these. Two different sizes of R-plasmids were detected, one about 55 kilobase pairs long and another about 50 kilobase pairs. Two strains isolated from New York brook trout Salvelinus fontinalis had reduced confluent growth within a zone of inhibition but contained no large plasmids. This may indicate chromosomally mediated resistance. Conjugational mating studies evaluated transfer of the R-plasmid DNA using eight Escherichia coli recipients. Successful R-plasmid transfer was accomplished with two donor strains (MI1 and MI2 from New Hampshire brook trout). Our results, in addition to those of other workers, illustrate the widespread resistance in A. salmonicida to approved antimicrobials and the capacity of this bacterium to become resistant in the fish culture environment.

Explore related subjects

Journal of Aquatic Animal HealthArticle

Keep this discovery

Explore connections, maps & timelines

BibTeXRIS

C. E. Starliper, R.K. Cooper. 1998. Biochemical and conjugation studies of romet-resistant strains of Aeromonas salmonicida from salmonid rearing facilities in the eastern United States. https://doi.org/10.1577/1548-8667(1998)010%3C0221%3Abacsor%3E2.0.co%3B2

Cite the original work for its findings. Save a collection to share your selection of sources.

KEEP EXPLORING

Related USGS reports

USGS2026

Effects of a low-lipid diet on the gut microbiome and head kidney transcriptome of juvenile Chinook Salmon

Objective Pacific salmon Oncorhynchus spp. reared in production hatcheries are typically fed high-lipid, energy-dense diets to achieve large size and high body condition prior to release. In contrast, juveniles in natural environments tend to consume low-lipid, high-protein diets, and fish reared for research or conservation purposes are sometimes fed diets that are formulated to mimic natural diets and promote wild-like phenotypes. Understanding how these alternative diets affect fish health beyond growth and body condition could ultimately contribute to improving hatchery fish fitness. Methods In this work, we evaluated changes in the fecal microbiome and gene expression of juvenile Chinook Salmon O. tshawytscha on a standard high-lipid hatchery diet versus a low-lipid diet formulated to mimic the nutrition profile of natural-origin fish. To evaluate the time scale at which diet alters the fecal microbiome, we collected longitudinal samples over a 12-week period and switched the diets of a subset of fish twice during the experiment. We used 16S ribosomal RNA gene amplicon sequencing to characterize fecal microbiome differences between fish on the two diets as well as hatchery-reared fish at a production hatchery, hatchery fish that had been captured after release into a stream, and natural-origin, stream-reared fish of similar ages. Additionally, we conducted RNA sequencing on head kidney samples from laboratory-reared fish to evaluate changes in gene expression in this important immune organ. Results We found that the low-lipid diet and the hatchery diet resulted in microbiomes that differed from the microbiome of natural-origin fish and from each other and that diet-driven changes to the microbiome could occur in under 14 d. The low-lipid diet did not result in a microbiome that resembled the microbiome of naturally produced fish. Instead, the low-lipid diet resulted in a microbiome community that was distinct from those of fish reared on the hatchery diet and fish sampled from the wild. The RNA sequencing results indicated differential enrichment of pathways related to immunity, metabolism, and hormone synthesis between fish that were fed the two experimental diets. Conclusions The results suggest that additional environmental factors influence the microbiome more strongly than diet formulation or that the low-lipid diet has a smaller effect on the microbiome than a natural, ­invertebrate-based diet. Given that the gut microbiome and systemic immune function contribute significantly to disease resistance, our findings highlight the importance of understanding how diets fed to fish in captivity may affect fish health beyond growth and body condition metrics.

Journal of Aquatic Animal Health↗
USGS2026

Transcriptional changes in wild Yukon River Chinook Salmon associated with Ichthyophonus infections

Objective We compared differentially expressed genes in Chinook Salmon Oncorhynchus tshawytscha with three divergent Ichthyophonus statuses (undetected, subclinical infections, or clinical disease; n = 100) to investigate associated transcriptomic responses. Disease associated with the fish parasite Ichthyophonus sp. was first diagnosed in adult Chinook Salmon from the Yukon River in the late 1980s and has subsequently been implicated in premature host mortality. Methods Chinook Salmon tissue sample collections and Ichthyophonus infection data were leveraged from a multi-agency collaboration during summer 2022 at three locations along the main-stem Yukon River that spanned nearly 2,000 km of freshwater migration (lower, middle, and upper river). We sequenced the transcriptome and compared this to infection status based on routine diagnostic procedures. Results Among the 17,569 genes for which messenger RNA was detected, we identified a transcription signature in the skeletal muscle that was associated with Ichthyophonus infections and included 53 differentially expressed genes. The differentially expressed genes and their pathways included those known for involvement in immune functions, energy synthesis, cellular breakdown, and reproduction—all processes that are known to be influenced by senescence during spawning migrations. Conclusions Results demonstrate a clear transcriptional difference between diseased fish (clinical disease group) and those in which Ichthyophonus was undetected, including identifying candidate markers for infection in this population. These results provide a foundation for development of nonlethal biomarkers to evaluate potential Ichthyophonus infections in Chinook Salmon based on gene transcription, protein products, or gene variants (e.g., polymorphisms).

Journal of Aquatic Animal Health↗
USGS2025

Non-lethal detection of Renibacterium salmoninarum in Greenback Cutthroat Trout Oncorhynchus clarkii stomias comparing mucus, blood, and ovarian fluid samples to kidney tissues

Objective Renibacterium salmoninarum , the causative agent of bacterial kidney disease, poses a major threat to both wild and aquaculture salmonid populations. Traditional detection methods typically involve lethal sampling to collect kidney tissues but are often impractical for species of conservation concern. This study evaluates nonlethal sampling techniques for detecting R. salmoninarum in Greenback Cutthroat Trout Oncorhynchus clarkii stomias by comparing mucus, blood, and ovarian fluid samples to conventional kidney tissue. Methods During the 2019 spawning season, we collected samples from 781 adult fish and tested for R. salmoninarum via direct fluorescent antibody test (DFAT) and quantitative polymerase chain reaction (qPCR). Results A total of 25 and 256 kidney tissues were positive by DFAT and qPCR, respectively. Of the three nonlethal samples tested, mucus swabs showed the highest percent correlation for detection with positive kidney tissues (DFAT = 47.6%, qPCR = 41.7%). Blood and ovarian fluid samples showed a lower percent correlation with positive kidney tissues (blood: DFAT = 12.0%, qPCR = 1.2%; ovarian fluid: DFAT = 12.5%, qPCR = 21.4%). Conclusions Our results suggest that nonlethal mucus swabbing could serve as a practical alternative for monitoring R. salmoninarum, especially in conservation efforts where minimizing fish mortality is critical.

Colorado↗