Genotypic diversity among Shewanella spp. collected from freshwater fish

Shewanella xiamenensis-associated ulcerative dermatitis in koi carp (Cyprinus rubrofuscus)

Ulcerative dermatitis (UD) is common in ornamental fish collections and is typically associated with a wide range of bacterial aetiologies. Clinical reports describing Shewanella xiamenensis-associated UD are limited, however, despite growing attention to pathogenic Shewanella species in fish. Two out of 95 koi carp with UD were presented for clinical assessment by a commercial collection (n = 3000 fish) and subsequently killed on welfare grounds for necropsy. Both specimens exhibited extensive cutaneous ulcers and coelomic fat necrosis with petechial haemorrhages on post-mortem examination. Shewanella xiamenensis was cultured from ulcerated skin tissues taken from both fish, with consistent intralesional gram-negative rod-like bacteria seen on skin scrape cytology. Histology also confirmed intralesional gram-negative rod-like bacteria within multiple ulcerative and erosive dermatitis lesions, plus myofibre necrosis and necrotising coelomic steatitis, in both specimens. Features associated with impaired generalised osmoregulation secondary to UD were detected within the striated muscle underlying the ulcers, the gills, and the caudal aspects of the kidneys. Additional histological features suggestive of sepsis were also seen in one of the fish. In the interim period, morbidity had increased from 3.2% to around 30% of the entire stock. Following culture results, increased pond water changes were implemented (q.2-3d) and the remaining stock was treated with florfenicol, resulting in complete resolution of UD in the collection (as per client). This article highlights the first description of S. xiamenensis-associated UD in koi carp/diseased ornamental fish in the United Kingdom.

Microbiological analysis of skin lesions of cod (Gadus morhua) from the southern part of the Baltic Sea

Introduction

Since the middle of the 1980s, severe skin disorders have been observed in Baltic cod (Gadus morhua) each year. Available data on the spectrum of bacteria isolated from the clinical cases being limited, and evaluation of the microbial background of fish skin lesions being useful, a bacteriological examination has been undertaken.

Material and Methods

A total of 1,381 cod were caught during two voyages of the Baltica research vessel in the Polish exclusive economic zone of the southern Baltic Sea. After an examination which found lesions in 164 of the fish, a microbiological analysis was performed to isolate bacteria from them. The collected strains were phenotyped and genotyped, and their antimicrobial resistance was analysed by minimum inhibitory concentration (MIC) techniques.

Results

Bacteriological examinations provided 850 isolates. The dominant microorganisms were mesophilic Aeromonas spp., Pseudomonas spp. and Shewanella baltica. Opportunistic bacteria potentially hazardous to human health were also isolated, e.g. Alcaligenes faecalis, Staphylococcus epidermidis, Stenotrophomonas maltophilia and Vibrio sp. The MIC analysis determined the highest number of bacteria to resist sulphamethoxazole and amoxicillin and clavulanic acid.

Conclusion

Most of the collected bacteria were opportunistic pathogens for fish, widespread in the aquatic environment, and potentially threatening to humans.

Phylogenetic analysis of Shewanella spp. isolated from fish

Different species of Shewanella spp. widely inhabit freshwater and marine environments. Some of them are opportunistic fish pathogens. The application of high-throughput sequencing enabled the characterization and taxonomic reclassification of many Shewanella spp. species. Still, some strains collected from fish need to be better recognized. The aim of the present study was to classify and determine the phylogenetic relationships of Shewanella spp. collected from fish. The complete genomes of 94 strains of Shewanella spp. from different fish species were sequenced using Illumina platform (MiSeq). The 16S rRNA gene, genomic features and whole-genome relationships of those bacteria were comprehensively analysed in comparison to reference strains. Whole-genome analysis showed that the tested Shewanella spp. strains were clustered into six groups similar to reference strains of S. xiamenensis, S. oneidensis, S. glacialipiscicola, S. hafniensis, S. baltica and S. oncorhynchi. Our study indicates that the whole-genome sequence analysis enabled taxonomic classification and assessment of the diversity of the Shewanella spp. strains, as opposed to recently the gold standard method of 16S rRNA amplicon sequencing. The high genetic diversity and low similarity to the reference genome of S. oneidensis indicate that the group of strains may be a subspecies or even new species. Furthermore, we showed that the most frequent Shewanella spp. species occurring in freshwater fish in our study is the recently described species S. oncorhynchi.

Dietary supplementation with microalgae enhances the zebrafish growth performance by modulating immune status and gut microbiota

Microalgae are known to be abundant in various habitats around the globe, and are rich in high value-added products such as fatty acids, polysaccharides, proteins, and pigments. Microalgae can be exploited as the basic and primitive food source of aquatic animals. We investigated the effects of dietary supplementation with Schizochytrium sp., Spirulina platensis, Chloroella sorokiniana, Chromochloris zofingiensis, and Dunaliella salina on the growth performance, immune status, and intestinal health of zebrafish (Danio rerio). The results showed that these five microalgae diets could improve the feed conversion rate (FCR), especially the D. salina (FCR = 1.02%) and Schizochytrium sp. (FCR = 1.20%) additive groups. Moreover, the microalgae diets decreased the gene expression level of the pro-inflammatory cytokines IL6, IL8, and IL1β at a normal physiological state of the intestine, especially the Schizochytrium sp., S. platensis, and D. salina dietary groups. The expression of neutrophil marker b7r was increased in the C. sorokiniana diet group; after, the zebrafish were challenged with Vibrio anguillarum, improving the ability to resist this disease. We also found that microalgae diets could regulate the gut microbiota of fish as well as increase the relative abundance of probiotics. To further explain, Cetobacterium was significantly enriched in the S. platensis additive group and Stenotrophomonas was higher in the Schizochytrium sp. additive group than in the other groups. Conversely, harmful bacteria Mycoplasma reduced in all tested microalgae diet groups. Our study indicated that these microalgae could serve as a food source supplement and benefit the health of fish. KEY POINTS: • Microalgae diets enhanced the growth performance of zebrafish. • Microalgae diets attenuated the intestinal inflammatory responses of zebrafish. • Microalgae diets modulated the gut microbiota composition to improve fish health.

Antibiotic Resistance of Gram-Negative Bacteria from Wild Captured Loggerhead Sea Turtles

Sea turtles have been proposed as health indicators of marine habitats and carriers of antibiotic-resistant bacterial strains, for their longevity and migratory lifestyle. Up to now, a few studies evaluated the antibacterial resistant flora of Mediterranean loggerhead sea turtles (Caretta caretta) and most of them were carried out on stranded or recovered animals. In this study, the isolation and the antibiotic resistance profile of 90 Gram negative bacteria from cloacal swabs of 33 Mediterranean wild captured loggerhead sea turtles are described. Among sea turtles found in their foraging sites, 23 were in good health and 10 needed recovery for different health problems (hereafter named weak). Isolated cloacal bacteria belonged mainly to Enterobacteriaceae (59%), Shewanellaceae (31%) and Vibrionaceae families (5%). Although slight differences in the bacterial composition, healthy and weak sea turtles shared antibiotic-resistant strains. In total, 74 strains were endowed with one or multi resistance (up to five different drugs) phenotypes, mainly towards ampicillin (~70%) or sulfamethoxazole/trimethoprim (more than 30%). Hence, our results confirmed the presence of antibiotic-resistant strains also in healthy marine animals and the role of the loggerhead sea turtles in spreading antibiotic-resistant bacteria.

Phenotypic Diversity and Potential Virulence Factors of the Shewanella Putrefaciens Group Isolated from Freshwater Fish

Introduction

The Shewanella putrefaciens group are ubiquitous microorganisms recently isolated from different freshwater fish species and causing serious health disorders. The purpose of the study was to characterise isolates of the S. putrefaciens group with special emphasis on elucidating serological diversity and determining putative virulence factors.

Material and Methods

Isolates collected from freshwater fish (n = 44) and reference strains were used. The identification of bacteria was carried out using biochemical kits and 16S rRNA sequencing. Polyclonal antibodies were prepared against the S. putrefaciens group. The bacterium’s susceptibility to antimicrobial agents, its enzymatic properties, and its adhesion ability to fish cell lines were also tested. Finally, selected isolates were used in challenge experiments in common carp and rainbow trout.

Results

Excluding six isolates undeterminable for species, the bacteria were classified to three species: S. putrefaciens, S. xiamenensis, and S. oneidensis, and showed some phenotypic diversity. Fourteen serological variants of the S. putrefaciens group were determined with the newly developed serotyping scheme.

Conclusion

Serodiversity may play an important role in the virulence of particular isolates. Further, S. putrefaciens group members adhere to epithelial cells and produce enzymes which may contribute to their virulence. Challenge tests confirmed the pathogenicity of the S. putrefaciens group for fish.