Nocardiosis in freshwater reared Chinook salmon (Oncorhynchus tshawytscha)

Species distribution and susceptibility of Nocardia isolates in New Zealand 2002-2021

The aim was to record the distribution and susceptibility of Nocardia species in New Zealand. Local and referred isolates were identified by an evolving approach over the study period including conventional phenotypic methods, susceptibility profiles, matrix-assisted laser desorption ionisation-time of flight mass spectrometry (MALDI-TOF) and molecular sequencing. Isolates previously identified as a Nocardia sp. or part of the N. asteroides complex were reidentified by MALDI-TOF and/or molecular methods. Antimicrobial susceptibility to eight antibiotics was performed by standard microbroth dilution. The site of isolation, susceptibility profiles and species distribution were analysed. A total of 383 isolates were tested: N. brasiliensis 23 (6%), N. cyriacigeorgica 42 (11%), N. farcinica 41 (11%), N. nova complex 226 (59%), and 51 (13%) other species/complexes. The respiratory tract was the most common site of infection (244, 64%), with skin and soft tissue the second most common site (104, 27%). All 23 N. brasiliensis isolates were from skin and soft tissue specimens. Almost all isolates (≥98%) were susceptible to amikacin, linezolid and trimethoprim-sulfamethoxazole; 35% and 77% were resistant to clarithromycin and quinolones, respectively. The expected susceptibility profiles of the four common species and complex were observed for most agent-organism parings. Multi-drug resistance was uncommon (3.4%). The spectrum of Nocardia species in New Zealand is similar to overseas reports and our most common group is the N. nova complex. While amikacin, linezolid and trimethoprim-sulfamethoxazole remain good empiric treatment choices, other agents should have their activity confirmed before use.

Applications of Actinobacteria in aquaculture: prospects and challenges

Disease outbreaks due to improper culture management, poor water quality, and climate change are major concerns in aquaculture. Most of the aquatic pathogens are opportunistic and any imbalance in the host-pathogen-environment triad will result in a disease outbreak. The indiscriminate use of chemotherapeutics such as antibiotics to prevent diseases in aquaculture will lead to antimicrobial resistance in aquaculture. Hence, the demand for natural microbial strains which can be used as beneficial probiotics and bioaugmentors in fish farming systems has increased to ensure one health in aquaculture. Studies have proved the probiotic and bioremediation potential of several Actinobacterial species that can be applied in aquaculture. Actinobacteria, especially Streptomyces, can be applied in aquaculture for disease prevention, treatment, and bioremediation of organic and inorganic waste in the culture systems. The growth, immunity, and resistance towards aquatic pathogens in cultured organisms also get enhanced through their capability to release potent antimicrobial compounds, bioactive molecules, and novel enzymes. Their broad-spectrum antimicrobial and quorum quenching activity can be well exploited against quorum sensing biofilm forming aquatic pathogens. Even though they impart specific adverse effects like the production of off-flavour compounds, this could be controlled through proper management strategies. This review discusses the applications, challenges, and prospects of Actinobacteria in aquaculture. Research gaps are also highlighted, which may shed light on the existing complexities and should pave the way for their better understanding and utilisation in aquaculture.

Genomic characterization of Nocardia seriolae strains isolated from diseased fish

Members of the genus Nocardia are widespread in diverse environments; a wide range of Nocardia species are known to cause nocardiosis in several animals, including cat, dog, fish, and humans. Of the pathogenic Nocardia species, N. seriolae is known to cause disease in cultured fish, resulting in major economic loss. We isolated two N. seriolae strains, CK‐14008 and EM15050, from diseased fish and sequenced their genomes using the PacBio sequencing platform. To identify their genomic features, we compared their genomes with those of other Nocardia species. Phylogenetic analysis showed that N. seriolae shares a common ancestor with a putative human pathogenic Nocardia species. Moreover, N. seriolae strains were phylogenetically divided into four clusters according to host fish families. Through genome comparison, we observed that the putative pathogenic Nocardia strains had additional genes for iron acquisition. Dozens of antibiotic resistance genes were detected in the genomes of N. seriolae strains; most of the antibiotics were involved in the inhibition of the biosynthesis of proteins or cell walls. Our results demonstrated the virulence features and antibiotic resistance of fish pathogenic N. seriolae strains at the genomic level. These results may be useful to develop strategies for the prevention of fish nocardiosis.

Current knowledge of nocardiosis in teleost fish

Nocardia sp. is the causative agent of nocardiosis, a lethal granulomatous disease of the skin, muscle, and various inner tissues affecting various teleost and shellfish. Four species of Nocardia have been isolated from diseased fish and shellfish, namely Nocardia asteroides, Nocardia seriolae, Nocardia salmonicida and Nocardia crassostreae. Therefore, in fish aquaculture, nocardiosis has caused severe economic losses, especially in the Asian region. Considerable research has been performed, since the first report of identified Nocardia sp. in fish, to characterize Nocardia sp. and identify rapid detection techniques, immune response against infection and prophylactic approaches. In this review, the current state of knowledge about nocardiosis in fish has been presented, including the pathogenesis, diagnosis, host immune response and vaccine development.