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.

Anguilla sp. diseases diagnoses and treatments: The ideal methods at the crossroads of conservation and aquaculture purposes

Anguilla anguilla, A. japonica and A. rostrata are the most fished and consumed eel species. However, these species are Critically Endangered, Endangered and Endangered, respectively. A combination of factors is thought to be responsible for their decline including fisheries, climate change, habitat destruction, barriers to migration, pollution and pathogens. Among them, viruses, bacteria and parasites are causing weakening of wild eels and serious economic losses for fishermen and eel farmers. Early detection of pathogens is essential to provide appropriate responses both for conservation reasons and to limit economic losses. Classic diagnosis approaches are time consuming and invasive and usual treatments, for example, antipathogenic substances are becoming obsolete because of pathogen resistance and environmental impact problems. The need for early and non-invasive diagnostic methods as well as effective and environmentally friendly treatments has increased. Vaccine development and diet supplementation have known a growing interest since their use could allow prevention of diseases. In this review, we summarize the main pathogens-viruses, bacteria and parasites-of the three northern temperate eel species, the methods used to detect these pathogens and the different treatments used. We discussed and highlighted the need for non-invasive, rapid and efficient detection methods, as well as effective and environmentally friendly treatments for both conservation and aquaculture purposes.

Dietary Seleno-l-methionine Alters the Microbial Communities and Causes Damage in the Gastrointestinal Tract of Japanese Medaka Oryzias latipes

Excess dietary seleno-l-methionine (Se-Met) induces various adverse effects in fish inhabiting the Se-contaminated environments. However, there is an extreme paucity of data on the effects of excess dietary Se-Met on the microbiota in the gastrointestinal (GI) tract in fish. In this study, Japanese medaka Oryzias latipes (three months old) were fed the Se-Met enriched diets at environmentally relevant concentrations: 2.90 (Control: (C), 6.69 (L), 11.89 (M), and 27.05 (H) μg Se/g dw) for 60 d. Histopathological, high throughput sequencing, and biochemical approaches were used to investigate the alterations in histology and microbial communities of the GI tract, enzymatic activity, and transcripts of closely related genes. The results showed that the fish weight was reduced at ∼13% from the L and H treatments. Decreased height and thickness of villus in the GI tract were observed in the H treatment. Meanwhile, the level of D-lactate and activity of diamine oxidase (DAO), protease, and lipase were inhibited in the H treatment. The transcripts of the genes related to the inflammation (i.e., IL-1β and IL-8) were elevated, while those of the genes related to the intestinal barrier (i.e., cdh1, ZO-1, ocln, and cldn7) were inhibited in the H treatment. In addition, alpha diversity at the genus level was higher in the L treatment than the control, and the composition of the microbial community was altered by dietary Se-Met. Furthermore, 5 genera (Rhodobacter, Cloacibacterium, Bdellovibrio, Shinella, and Aeromonas) exhibited the largest variation in abundance among treatments. This study has demonstrated that excess dietary Se-Met inhibits growth, causes hispathological damage to the GI tract, and alters the composition of the microbial community in Oryzias latipes.

Time-resolved RNA-seq provided a new understanding of intestinal immune response of European eel (Anguilla anguilla) following infection with Aeromonas hydrophila

No studies systematically examined the intestinal immune response for yellow stage of European eel (Anguilla anguilla) with Aeromonas hydrophila infection by time-resolved RNA-seq. Here, we examined transcriptional profiles of the intestines at three-time points following infection with A. hydrophila. Intraperitoneal injections caused mortalities within 48 h post-injection (hpi), with the survival rate 87.5% at 24 hpi and 83.9% at 48 hpi. The result from KEGG pathway enrichment analysis showed that the immune related "cytosolic DNA-sensing pathway" was significantly enriched at the first and second time points (6 hpi and 18 hpi), with the up-regulated expression of irf3, il1b, tnfaip3, cxcl8a, ap1-2, c-fos, polr3d, polr3g and polr3k both at 6 hpi and 18 hpi, but not at the third time point (36 hpi). According to the KEGG annotation, 326 immune and inflammation-related DEGs were found. The co-expression network of those 326 DEGs revealed the existence of three modules, and tlr1 was found to be in the center of the biggest module which contained massive DEGs from "signal transduction" and "transport and catabolism". The c3 isoforms showed different expression pattern among the three time points, indicating a unique activation of complement systems at 18 hpi. Furthermore, two cathelicidins (aaCATH_1 and aaCATH_2) were highly up-regulated at the first two time points, and the bacterial growth inhibition assay revealed their antibacterial properties against A. hydrophila. Our data indicated the important roles of cytosolic DNA-sensing pathway, as well as transcripts including tlr1, c3, polr and cathelicidins in the intestine of A. anguilla in response to A. hydrophila infection. The present study will provide leads for functional studies of host-pathogen interactions.

Understanding the pathogenesis of Flavobacterium psychrophilum using the rainbow trout monocyte/macrophage-like cell line, RTS11, as an infection model

The life cycle of Flavobacterium psychrophilum (Fp), the causative agent of bacterial coldwater disease (BCWD) and rainbow trout fry syndrome (RTFS), appears to involve interactions with spleen and head kidney macrophages. To develop an in vitro model for studying this, F. psychrophilum was incubated with a rainbow trout splenic monocyte/macrophage-like cell line (RTS11) and fundamental macrophage functions evaluated. The animal cell basal medium, L15, supplemented with bovine serum (FBS) supports RTS11 maintenance, and surprisingly, L15 with 2% FBS (L15/FBS) also supported F. psychrophilum growth. L15/FBS in which the bacteria had been grown is referred to as F. psychrophilum conditioned medium (FpCM). Adding FpCM to RTS11 cultures caused a small, yet significant, percentage of cells to die, many cells to become more diffuse, and phagocytosis to be temporarily reduced. FpCM also significantly stimulated transcript expression for pro-inflammatory cytokines (IL-1β, TNFα and IL-6) and the anti-inflammatory cytokine (IL-10) after one day of exposure but this upregulation rapidly declined over time. Adding live F. psychrophilum to RTS11 cultures also altered the cellular morphology and stimulated cytokine expression more profoundly than FpCM. Additionally, the phagocytic activity of RTS11 was also significantly impaired by live F. psychrophilum, but not to the same extent as when exposed to FpCM. Adding heat-killed bacteria to RTS11 cultures elicited few changes. These bacteria/RTS11 co-cultures should be useful for gaining a deeper understanding of the pathogenesis of F. psychrophilum and may aid in the development of effective measures to prevent infection and spread of this troublesome disease.

Identification of Two Pathogenic Aeromonas Species Isolated from Juvenile Burbot during Production-Related Epizootics

In response to population declines of North American Burbot Lota lota maculosa (hereafter, Burbot), conservation aquaculture methods have been developed for this species. In general, Burbot are relatively resistant to many salmonid pathogens; however, cultured juvenile Burbot have experienced periodic epizootic disease outbreaks during production. A series of trials was conducted to determine the virulence of select bacteria isolated from juvenile Burbot after outbreaks that occurred in 2012 and 2013 at the University of Idaho's Aquaculture Research Institute. Initial clinical diagnostics and sampling resulted in the isolation of numerous putative bacterial pathogens. To determine which bacteria were the most likely causative agents contributing to these epizootics, juvenile Burbot received intraperitoneal (IP) injections of select bacteria in log-phase growth. Mortality associated with specific isolates was recorded, and more comprehensive challenges followed this initial screening. These challenges used side-by-side IP and immersion methods to expose Burbot to potential pathogens. The challenges resulted in significantly higher mortalities in fish after IP injection with two Aeromonas sp. isolates compared to controls, but no significant difference in mortality for immersion-challenged groups was observed. Results demonstrate that two Aeromonas sp. isolates cultured from the epizootics are virulent to Burbot.

Characteristics of disease spectrum in relation to species, serogroups, and adhesion ability of motile aeromonads in fish

An attempt was made to delineate the relationship between of Aeromonas species and/or serogroups and specific disease symptoms in common carp Cyprinus carpio L. and rainbow trout Oncorhynchus mykiss Walbaum. The adhesion of Aeromonas strains to various tissues in relation to disease spectrum was also tested. All strains of A. hydrophila caused skin ulcers as well as septicaemia in both carp and trout while the other strains were able to cause only skin ulcers or some specific internal lesions with or without septicaemia depending on which species and/or serogroup they represented. Disease symptoms depended also on fish species. It was found that adhesion intensity of Aeromonas strains tested was significantly higher to tissues, which were susceptible to infection with these strains. The results indicate that adhesion to various cells of the fish organism is principal marker to detect virulent Aeromonas strains. The findings presented in this study may be helpful in the appraisal of aeromonads disease risk and kind of the infection in particular fish farms by epizootiological studies or/and during routine fish examinations. They will also be useful to improve and facilitate diagnosis of bacterial fish disease.

Complex evolutionary history of the Aeromonas veronii group revealed by host interaction and DNA sequence data

Aeromonas veronii biovar sobria, Aeromonas veronii biovar veronii, and Aeromonas allosaccharophila are a closely related group of organisms, the Aeromonas veronii Group, that inhabit a wide range of host animals as a symbiont or pathogen. In this study, the ability of various strains to colonize the medicinal leech as a model for beneficial symbiosis and to kill wax worm larvae as a model for virulence was determined. Isolates cultured from the leech out-competed other strains in the leech model, while most strains were virulent in the wax worms. Three housekeeping genes, recA, dnaJ and gyrB, the gene encoding chitinase, chiA, and four loci associated with the type three secretion system, ascV, ascFG, aexT, and aexU were sequenced. The phylogenetic reconstruction failed to produce one consensus tree that was compatible with most of the individual genes. The Approximately Unbiased test and the Genetic Algorithm for Recombination Detection both provided further support for differing evolutionary histories among this group of genes. Two contrasting tests detected recombination within aexU, ascFG, ascV, dnaJ, and gyrB but not in aexT or chiA. Quartet decomposition analysis indicated a complex recent evolutionary history for these strains with a high frequency of horizontal gene transfer between several but not among all strains. In this study we demonstrate that at least for some strains, horizontal gene transfer occurs at a sufficient frequency to blur the signal from vertically inherited genes, despite strains being adapted to distinct niches. Simply increasing the number of genes included in the analysis is unlikely to overcome this challenge in organisms that occupy multiple niches and can exchange DNA between strains specialized to different niches. Instead, the detection of genes critical in the adaptation to specific niches may help to reveal the physiological specialization of these strains.

Phylogenetic analysis of intestinal bacteria and their adhesive capability in relation to the intestinal mucus of carp

AIMS

The aims of the present study are to characterize the intestinal microbial community displaying a high-adhesive capability in fish, and to evaluate the relationship between mucosal adhesion of intestinal bacteria and fish health and disease.

METHODS AND RESULTS

A total of 707 aerobic bacteria isolated from carp intestine that were maintained under either feeding (feeding group) or no-feeding (no-feeding group) conditions and were performed adhesive assay. Isolates were divided into three categories on the basis of adhesive capability: high-, medium-, and low- adhesive capabilities. The average proportions of isolates with high-adhesive capability in the feeding and no-feeding groups were 30% and 32%, respectively. A phylogenetic analysis using a partial 16S rRNA gene demonstrated that most isolates with high-adhesive capability in both groups were classified as belonging to an Aeromonas group, and populations of isolates within high- and low-adhesive categories were markedly different.

CONCLUSIONS

Intestinal bacteria with a high-adhesive capability in relation to intestinal mucous always colonize on the surface of intestinal mucosa and grow in the intestinal tract of feeding carp. The adhesive capability of intestinal bacteria is essential for colonization and growth in the intestinal tract of fish.

SIGNIFICANCE AND IMPACT OF THE STUDY

Our results indicate that members of the Aeromonas group with adhesive capability always colonize on the surface of intestinal mucosa.

Dominant pathogenic species of mesophilic aeromonads isolated from diseased and healthy fish cultured in Poland

Aeromonas isolates were collected from cultured fish, characterized phenotypically and identified to species using 16S rDNA. The pathogenicity of all isolates was assayed on the basis of haemolytic and proteolytic activity and challenge tests were performed for isolates from healthy fish. A total of 131 Aeromonas isolates were obtained and identified as follows: A. hydrophila (13), A. bestiarum (23), A. salmonicida (motile biogroup) (19), A. caviae (2), A. sobria (18), A. veronii bt. sobria (42), A. jandaei (1), A. encheleia (11) and A. allosaccharophila (2). All isolates of A. hydrophila and A. bestiarum and most isolates of A. salmonicida and A. veronii were classified as pathogenic. Aeromonas hydrophila was isolated only from diseased trout except for one isolate obtained from carp fry. The other potentially pathogenic Aeromonas species were present in diseased as well as healthy fish. The pathogenicity of isolates from healthy fish was correlated with their enzymatic activity and was also tested by challenge experiments. The dominant pathogenic species were A. veronii bt. sobria, A. bestiarum and A. salmonicida in common carp and A. hydrophila in rainbow trout.

Secretion of haemolysins and proteases by Aeromonas hydrophila EO63: separation and characterization of the serine protease (caseinase) and the metalloprotease (elastase)

AIMS

To determine the haemolysins and proteases excreted by the virulent strain EO63 of Aeromonas hydrophila grown in complex media and to then fractionate and characterize them, in particular those with elastolytic activity.

METHODS AND RESULTS

The amount of haemolytic and proteolytic activity in EO63 culture supernatants was dependent on the culture media used. In all media, haemolysins appeared during the phase of active growth and haemolytic activity decreased quickly thereafter, as previously described for aerolysin. In contrast, proteases were mainly released during the stationary phase. Serine protease activity in EO63 culture supernatants was four times greater than that caused by metalloproteases. Two main proteases were partially purified from EO63 culture supernatants by isoelectrophoresis: a serine protease (68 kDa) active against casein; a mixture of different protein bands (60, 44 and 31 kDa) representing a thermostable metalloprotease active against elastin and casein. This metallo-elastase was also inhibited by dithiothreitol and showed a pH optimum of 8.0. Both exoenzymes were toxic for eels at LD50 doses of 1.1 and 3.5 microg (g fish)(-1), respectively.

CONCLUSIONS

A serine caseinase and a metallo-elastase may play a role in the pathogenicity of EO63 for eels. These toxins are excreted in vitro by EO63 in the ratio of 4:1 during the stationary phase of growth. Strain EO63 also produced beta-haemolysins in vitro which could correspond to aerolysin.

SIGNIFICANCE AND IMPACT OF THE STUDY

This is the first report on the purification of a metallo-elastase excreted by a wild-type A. hydrophila strain.

Typing of clinical and environmental Aeromonas sp. strains by random amplified polymorphic DNA PCR, repetitive extragenic palindromic PCR, and enterobacterial repetitive intergenic consensus sequence PCR

A collection of 120 strains isolated from stool specimens collected from humans suffering from gastroenteritis and from environmental samples were analyzed by random amplified polymorphic DNA PCR (RAPD), repetitive extragenic palindromic PCR (REP-PCR), and enterobacterial repetitive intergenic consensus sequence PCR (ERIC-PCR). Species of Aeromonoas hydrophila, A. bestiarum, A. salmonicida, A. caviae, A. media, and A. veronii revealed clonal structure. There was no dominant clone causing gastroenteritis in humans. Moreover, there was no genetic similarity between clinical and environmental strains of Aeromonas sp. isolated from different geographical areas as well as from the same geographical area. Some clones colonized specific ecosystems, e.g., drinking water distribution systems. RAPD and ERIC-PCR methods had the same discriminatory power and proved to be useful for epidemiological investigation and population genetic analysis of Aeromonas spp., whereas REP-PCR was less effective for differentiating the isolates of Aeromonas spp.

Phenotypic characteristics and pathogenicity of Aeromonas genomospecies isolated from common carp (Cyprinus carpio L.)

AIMS

To evaluate the relationship between the genomospecies, phenotypic profile and pathogenicity for carp of 37 motile Aeromonas strains.

METHODS AND RESULTS

Aeromonas strains were identified to genomospecies level by the 16S rDNA restriction fragment length polymorphism (RFLP) method and characterized phenotypically by the API 20E and API Zym systems and by conventional tube or plate methods. 16S rDNA RFLP analysis showed that the strains belonged to five species, Aeromonas bestiarum (5), Aerom. salmonicida (13), Aerom. veronii (11), Aerom. sobria (6) and Aerom. encheleia (2). Most strains of Aerom. bestiarum (80%) and Aerom. salmonicida (85%) could be separated by growth at 4 and 42 degrees C, autoagglutination after boiling, reaction for lipase (C14) and naphthol-AS-BI-phosphohydrolase. All strains of Aerom. veronii corresponded to Aerom. veronii biotype sobria and could be separated from Aerom. sobria by citrate utilization, growth at 37 and 42 degrees C, amygdalin and cellobiose fermentation. All strains of Aerom. bestiarum and most strains of Aerom. salmonicida (76.9%) and Aerom. veronii (63.6%) were pathogenic for carp.

CONCLUSIONS

The biochemical identification of carp Aeromonas strains is not entirely clear. Some association between Aeromonas species, phenotypic profile and specific disease signs was observed.

SIGNIFICANCE AND IMPACT OF THE STUDY

The results will be useful for ichthyopathology laboratories in the diagnosis of motile aeromonad septicaemia in carp.

Virulence markers in Aeromonas hydrophila and Aeromonas veronii biovar sobria isolates from freshwater fish and from a diarrhoea case

AIMS

To evaluate the public health significance of representative strains of two Aeromonas spp., mainly from freshwater fish, on the basis of production of virulence-associated factors and presence of the haemolytic genes aerA and hlyA.

METHODS AND RESULTS

Eleven strains of Aer. hydrophila, three strains of Aer. veronii biovar sobria (all from freshwater fish) and one strain of Aer. hydrophila from human diarrhoea were tested for potential virulence traits and for the presence of the haemolytic genes aerA and hlyA. Ten Aer. hydrophila isolates were aerA(+)hlyA(+) and two aerA(+)hlyA(-). Aeromonas veronii biovar sobria isolates were aerA(-)hlyA(-). Strains from the three genotypes showed enterotoxic activity in the suckling mouse assay. At 28 degrees C, four Aer. hydrophila fish strains could be considered as potentially virulent (possessing at least two of these characteristics: haemolytic, cytotoxic and enterotoxic). One Aer. veronii biovar sobria strain and the clinical isolate were cytotoxic on Vero cells. When grown at 4 degrees C, these six isolates fulfilled virulence criterion, but at 37 degrees C, only one fish strain, an Aer. hydrophila, did.

CONCLUSIONS

The potential health risk derived from the presence of Aer. hydrophila and Aer. veronii biovar sobria in ice-stored freshwater fish should not be underestimated.

SIGNIFICANCE AND IMPACT OF THE STUDY

Expression of virulence factors is affected by temperature incubation and not always related to the presence of haemolytic genes.

Virulence factors of clinical isolates of Aeromonas caviae

The mechanism of pathogenicity of 13 Aeromonas caviae strains isolated from fecal specimens of children with diarrhea was examined. Isolates possessed several virulence agents. Six of 13 isolates produced cholera-like toxin, all strains exhibited adhesive ability and secreted catecholate siderophores. Six isolates were able to grow in heat-inactivated serum. Production of hemolysins was not detected. The isolates had relatively high values of LD50 in a mouse model, which suggests that there is no linkage between pathogenicity to mice and the diarrheagenic ability of the isolates.

Neurotoxic effect on two fish species and a PC12 cell line of the supernate of Vibrio alginolyticus and Vibrio anguillarum

The biological effects of supernates obtained from different strains of Vibrio alginolyticus and Vibrio anguillarum isolated from diseased fish have been studied by inoculation on two fish species, eel and rainbow trout, and two fish cell lines. These supernates possess neuroexcitatory properties, and so, when they are injected into both fish species, they trigger convulsions, wriggling, contortive swimming and respiratory arrest coupled with increased respiratory reflex. Furthermore, after the application of the supernates on cultures of noradrenergic pheochromocytoma PC12 cells, an increase of acetylcholine, released from the cells was obtained. The amount of released acetylcholine depends on the source of assayed supernates and on the dose applied to the cells. On the basis of the results obtained with PC12 cells, we suggest that the supernates from pathogenic Vibrio strains injected into fish may elicit an increased release of acetylcholine in the motor endplate of some muscles related to locomotion and ventilation of the inoculated fish.