Insights into the virulence-related genes of Edwardsiella tarda isolated from turbot in Europe: genetic homogeneity and evidence for vibrioferrin production

Edwardsiella tarda has long been known as a pathogen that causes severe economic losses in aquaculture industry. Insights gained on E. tarda pathogenesis may prove useful in the development of new methods for the treatment of infections as well as preventive measures against future outbreaks. In this report, we have established the correlation between the presence of virulence genes, related with three aspects typically involved in bacterial pathogenesis (chondroitinase activity, quorum sensing and siderophore-mediated ferric uptake systems), in the genome of E. tarda strains isolated from turbot in Europe and their phenotypic traits. A total of 8 genes were tested by PCR for their presence in 73 E. tarda isolates. High homogeneity was observed in the presence/absence pattern of all the strains. Positive results in the amplification of virulence-related genes were correlated with the detection of chondroitinase activity in agar plates, in vivo AHL production during fish infection and determination of type of siderophore produced by E. tarda. To the best of our knowledge, this is the first study carried out with European strains on potential virulence factors. Furthermore, we demonstrated for the first time that E. tarda produces the siderophore vibrioferrin.

The ABC-transporter hutCD genes of Photobacterium damselae subsp. piscicida are essential for haem utilization as iron source and are expressed during infection in fish

The marine fish pathogen Photobacterium damselae subsp. piscicida utilizes haem compounds as the sole iron source. In a previous work, we characterized a gene cluster with ten potential haem uptake and utilization genes. Two of these genes, hutC and hutD, which are iron-regulated, conform a putative inner membrane haem ABC transporter. In this study, we constructed an insertional mutant, leading to the inactivation of hutCD genes. Reverse transcriptase-PCR analyses demonstrated that an insertion between the hutB and hutC genes abolished transcription of the downstream hutC and hutD genes. The hutCD mutant was unable to utilize haem as the sole iron source, demonstrating that the putative ABC-transporter proteins HutC and HutD are essential for haem utilization as an iron source in P. damselae subsp. piscicida. In addition, reverse transcriptase-PCR assays conducted with RNA samples isolated from experimentally infected fish revealed the presence of hutCD transcripts. The results demonstrate for the first time that haem uptake genes of a fish pathogen are expressed during the infective process in fish.

Evaluation of four polymerase chain reaction primer pairs for the detection of Edwardsiella tarda in turbot

Edwardsiella tarda is an important emergent pathogen in European aquaculture, causing several mortality events in turbot Scophthalmus maximus cultures in recent years. Here, we evaluated in parallel the specificity of 4 previously published pairs of primers, gyrBF1/gyrBR1, tardaF/ tardaR, etfA and etfD, for the detection of 53 E. tarda strains isolated from different sources, as well as 18 representatives of related and unrelated bacterial species. On the basis of the obtained results, we selected the pair of primers etfD, because it was the only one that recognized all E. tarda strains without false positive reactions. The sensitivity of this primer set showed detection limits of 2 cells per reaction tube in the case of pure cultures and 200 cells per reaction tube in mixed cultures. With regard to the sensitivity in seeded turbot tissues (kidney, liver and mucus), the detection limit was 3 x 10(2) E. tarda cells per reaction. In experimentally infected turbot, the etfD primer set was able to detect the pathogen in internal organs even 1 d post-infection, with a dose of 0.1 cells g(-1) of fish. In addition, this polymerase chain reaction protocol was useful for the detection of E. tarda in the field, and, based on the findings, we propose it as the most appropriate for accurate detection of E. tarda in routine diagnosis of edwardsiellosis in fish.

Evaluation of the AQUARAPID-Va, AQUAEIA-Va and dot-blot assays for the detection of Vibrio anguillarum in fish tissues

The comparative accuracy of the serological assays AQUARAPID-Va, AQUAEIA-Va (BIONOR AS), and dot-blot for a rapid diagnosis of vibriosis in fish was evaluated. Twenty-one Vibrio anguillarum strains, representative of pathogenic and environmental serotypes, and 13 strains of other fish pathogenic bacteria were used to assess the sensitivity and specificity of the detection methods. The serological assays tested detected all the strains of V. anguillarum serotypes O1 and O2. The dot-blot assay was the most specific and sensitive method, detecting almost all isolates from serotypes O1, O2 and O3, with an average sensitivity of 1 x 10(6) bacteria g(-1) of fish tissue. The AQUARAPID-Va and the AQUAEIA-Va systems were able to detect 5 x 10(6) and 5 x 10(7) bacteria g(-1) of fish tissue, respectively. The simplicity, effectiveness and speed of the AQUARAPID-Va system confirmed this method as the most suitable serological test for the detection of V. anguillarum in field analysis and small-scale laboratory studies.

Detection of Babesia caballi infection by enzyme-linked immunosorbent assay using recombinant 48-kDa merozoite rhoptry protein

The 48-kDa Babesia caballi merozoite rhoptry protein was expressed using a pGEX4T expression vector in Escherichia coli as glutathione S-transferase fusion protein (GST-BC48), and the expressed GST-BC48 was used in an ELISA to detect specific antibodies in serum samples. No cross-reaction was observed with sera from horses experimentally infected with Babesia equi. GST-BC48 ELISA was a highly sensitive and specific test when compared with the CFT. A total of 209 horse sera obtained from Central Mongolia were examined with the GST-BC48 ELISA and 46.4% (97/209) were found to be seropositive for B. caballi, suggesting that the GST-BC48 ELISA can be successfully used for both quarantine and epidemiological studies.

Multiplex PCR assay for ureC and 16S rRNA genes clearly discriminates between both subspecies of Photobacterium damselae

A multiplex-PCR approach, employing 2 primer pairs directed to internal regions of the 16S rRNA and ureC genes, was utilized to analyze a collection of Photobacterium damselae strains, including 25 isolates of subspecies piscicida and 15 isolates of subspecies damselae. With this procedure, all the P. damselae subsp. damselae strains yielded 2 amplification products, one of 267 bp and the other of 448 bp, corresponding to internal fragments of the 16S rRNA and ureC genes, respectively. However, P. damselae subsp. piscicida isolates only showed the PCR product of 267 bp (16S rRNA fragment), indicating the absence of the urease gene in its genome. We have constructed a DNA probe directed to an internal region of the ureC gene, and corroborated by dot blot hybridization that the P. damselae subsp. piscicida lacks this gene, whereas it is present in the subspecies damselae. This constitutes the first successful discrimination between both subspecies using a PCR procedure, which could become a useful tool for diagnosis of pasteurellosis in the field. In addition, since these 2 subspecies have been shown to share nearly the same rrn operon sequence, our results provided evidence that one of the steps in the P. damselae speciation proccess included gain/loss events associated with the ure operon.

Presence of phospholipase-D (dly) gene coding for damselysin production is not a pre-requisite for pathogenicity in Photobacterium damselae subsp. damselae

The presence of the phospholipase-D (dly) gene as pre-requisite for virulence of Photobacterium damselae subsp. damselae for poikilotherm and homoiotherm animals was investigated in a total of 17 strains isolated from fish, shellfish, mammals and seawater. With this aim, we developed two PCR protocols. A simple PCR using primers flanking the almost complete dly gene, and a multiplex-PCR using two sets of primers directed to internal fragments of the dly and 16S rRNA genes. Only six of the 17 Ph. damselae subsp. damselae strains studied harboured the dly gene regardless of their haemolytic activity against sheep or rabbit erythrocytes as well as their virulence for mammals and marine fish. In fact, all strains but one were pathogenic for one or both animals, with LD(50)values ranging from 1x10(3)and 3x10(5)bacteria for turbot, and 2x10(6)and 8x10(7)cells for mice. The PCR results were corroborated in dot blot hybridization experiments employing a DNA probe directed to an internal region of the dly gene. From the data obtained in this work, we can conclude that the presence of the dly gene is not an indicative of the pathogenicity of Ph. damselae subsp. damselae and, therefore, the role of damselysin as the main virulence factor of this marine bacterium for poikilotherm and homoiotherm hosts should be re-evaluated.

A region of the transmembrane regulatory protein ToxR that tethers the transcriptional activation domain to the cytoplasmic membrane displays wide divergence among Vibrio species

The virulence regulatory protein ToxR of Vibrio cholerae is unique in that it contains a cytoplasmic DNA-binding-transcriptional activation domain, a transmembrane domain, and a periplasmic domain. Although ToxR and other transmembrane transcriptional activators have been discovered in other bacteria, little is known about their mechanism of activation. Utilizing degenerate oligonucleotides and PCR, we have amplified internal toxR gene sequences from seven Vibrio and Photobacterium species and subspecies, demonstrating that toxR is an ancestral gene of the family Vibrionaceae. Sequence alignment of all available ToxR amino acid sequences revealed a region between the transcriptional activation and transmembrane domains that displays wide divergence among Vibrio species. We hypothesize that this region merely tethers the transcriptional activation domain to the cytoplasmic membrane and thus can tolerate wide divergence and multiple insertions and deletions. The divergence in the tether region at the nucleotide level may provide a useful tool for the distinction of Vibrio and Photobacterium species.

Assessment of a magnetic bead-EIA based kit for rapid diagnosis of fish pasteurellosis

The accuracy of the magnetic beads-EIA based BIONOR AQUAEIA-Pp kit for the rapid diagnosis of pasteurellosis was evaluated. The kit reacted with all the Photobacterium damselae subsp. piscicida strains included in this study, with a detection limit of 10(4) bacteria/ml. However, non-specific reactions were observed with isolates of Ph. damselae subsp. damselae or Ph. histaminum when the bacterial concentration was high (10(9)-10(10) bacteria/ml). Similar findings in specificity and sensitivity were observed when the kit was applied to experimentally infected fish tissues. However, since those bacterial species are not usually found in the fish species susceptible to pasteurellosis, the AQUAEIA kit appears applicable for a rapid screening of the disease. In addition, when the kit was utilized to analyze cultured populations of seabream, it allowed the detection of the pathogen, not only in individuals affected by the disease, but also in asymptomatic carrier fish. Furthermore, the positive detection of Ph. damselae subsp. piscicida in broodstock gonads, seminal, and ovaric fluids, and also in eggs indicated the possibility of vertical transmission of pasteurellosis.

16S rRNA gene sequence analysis of Photobacterium damselae and nested PCR method for rapid detection of the causative agent of fish pasteurellosis

The causative agent of fish pasteurellosis, the organism formerly known as Pasteurella piscicida, has been reclassified as Photobacterium damselae subsp. piscicida on the basis of 16S rRNA gene sequence comparisons and chromosomal DNA-DNA hybridization data; thus, this organism belongs to the same species as Photobacterium damselae subsp. damselae (formerly Vibrio damselae). Since reassignment of P. damselae subsp. piscicida was based on only two strains, one objective of the present work was to confirm the taxonomic position of this fish pathogen by sequencing the 16S rRNA genes of 26 strains having different geographic and host origins. In addition, a nested PCR protocol for detection of P. damselae based on 16S rRNA was developed. This PCR protocol was validated by testing 35 target and 24 nontarget pure cultures, and the detection limits obtained ranged from 1 pg to 10 fg of DNA (200 to 20 cells). A similar level of sensitivity was observed when the PCR protocol was applied to fish tissues spiked with bacteria. The PCR approach described in this paper allows detection of the pathogen in mixed plate cultures obtained from asymptomatic fish suspected to be carriers of P. damselae subsp. piscicida, in which growth of this bacterium cannot be visualized. Our results indicate that the selective primers which we designed represent a powerful tool for sensitive and specific detection of fish pasteurellosis.

Arthrobacter rhombi sp. nov., isolated from Greenland halibut (Reinhardtius hippoglossoides)

Two strains of a hitherto undescribed Gram-positive coryneform bacterium isolated from Greenland halibut (Reinhardtius hippoglossoides) were characterized by phenotypic and molecular taxonomic methods. Comparative 16S rRNA gene sequencing studies demonstrated that the unknown strains constitute a new line within the genus Arthrobacter. The nearest relatives of the bacterium from fish were members of the Arthrobacter nicotianael Arthrobacter sulfureus group. The unknown bacterium was readily distinguished from these species by phenotypic methods. Based on phylogenetic and phenotypic evidence, it is proposed that the unknown bacterium be classified as Arthrobacter rhombi sp. nov. The type strain of Arthrobacter rhombi is CCUG 38813T.

Corynebacterium confusum sp. nov., isolated from human clinical specimens

Three strains of a previously unknown coryneform bacterium were isolated from two patients with foot infections and from a blood culture of a third patient. The three non-lipophilic strains exhibited very slow fermentative acid production from glucose but not from maltose or sucrose, nitrate reductase activity, no tyrosinase activity and the presence of small amounts of tuberculostearic acid as the most significant phenotypic features. Differentiation of these strains from all other presently defined coryneform bacteria was readily achieved. Chemotaxonomic investigations revealed that the three strains unambiguously belonged to the genus Corynebacterium. Comparative 16S rRNA gene sequence analysis demonstrated that the isolates were almost identical and represented a new subline within the genus Corynebacterium, for which the designation Corynebacterium confusum sp. nov. is proposed. The type strain of Corynebacterium confusum is CCUG 38267T.