Binding of haemin by the fish pathogen Photobacterium damselae subsp. piscicida

An ex vivo Approach in European Seabass Leucocytes Supports the in vitro Regulation by Postbiotics of Aip56 Gene Expression of Photobacterium damselae subsp. piscicida

Shewanella putrefaciens Pdp11 (SpPdp11) is a probiotic strain assayed in aquaculture; however, its postbiotic potential is unknown. Postbiotics are bacterial metabolites, including extracellular products (ECPs) that improve host physiology and immunity. Their production and composition can be affected by different factors such as the growing conditions of the probiotics. Photobacterium damselae subsp. piscicida strain Lg 41/01 (Phdp) is one of the most important pathogens in marine aquaculture. The major virulent factor of this bacterium is the exotoxin aip56, responsible for inducing apoptosis of fish leucocytes. Viable SpPdp11 cells have been reported to increase resistance to challenges with Phdp. This work aimed to evaluate the effect of two ECPs, T2348-ECP and FM1548-ECP, obtained from SpPdp11 grown under different culture conditions that previously demonstrated to exert different degradative and non-cytotoxic activities, as well as the effect on pathogens biofilm formation. These SpPdp11-ECPs were then analyzed by their effect on the viability, phagocytosis, respiratory burst and apoptogenic activity against European sea bass leucocytes infected or not with Phdp supernatant. Both ECPs, T2348-ECP and FM1548-ECP, were not cytotoxic against leucocytes and significantly reduced their apoptosis. Phagocytosis and respiratory burst of leucocytes were significantly reduced by incubation with Phdp supernatant, and not influenced by incubation with T2348-ECP or FM1548-ECP. However, both activities were significantly increased after leucocyte incubation with combined T2348-ECP and FM1548-ECP with Phdp supernatant, compared to those incubated only with Phdp supernatant. Finally, both T2348-ECP and FM1548-ECP significantly reduced the relative in vitro expression of the Phdp aip56 encoding gene.

Iron uptake mechanisms as key virulence factors in bacterial fish pathogens

This review summarizes the current knowledge about iron uptake systems in bacterial fish pathogens and their involvement in the infective process. Like most animal pathogens, fish pathogens have evolved sophisticated iron uptake mechanisms some of which are key virulence factors for colonization of the host. Among these systems, siderophore production and heme uptake systems are the best studied in fish pathogenic bacteria. Siderophores like anguibactin or piscibactin, have been described in Vibrio and Photobacterium pathogens as key virulence factors to cause disease in fish. In many other bacterial fish pathogens production of siderophores was demonstrated but the compounds were not yet chemically characterized and their role in virulence was not determined. The role of heme uptake in virulence was not yet clearly elucidated in fish pathogens although there exist evidence that these systems are expressed in fish tissues during infection. The relationship of other systems, like Fe(II) transporters or the use of citrate as iron carrier, with virulence is also unclear. Future trends of research on all these iron uptake mechanisms in bacterial fish pathogens are also discussed.

Genomic diversity in flavobacterial pathogens of aquatic origin

Flavobacterium species are considered important fish pathogens in wild and cultured fish throughout the world. They can cause acute, subacute, and chronic infections, which are mainly characterized by gill damage, skin lesions, and deep necrotic ulcerations. Primarily, three Flavobacterium species, F. branchiophilum, F. columnare, and F. psychrophilum, have been reported to cause substantial losses to freshwater fish. In this study, we evaluated genomes of 86 Flavobacterium species isolated from aquatic hosts (mainly fish) to identify their unique and shared genome features. Our results showed that F. columnare genomes cluster into four different genetic groups. In silico secretion system analysis identified that all genomes carry type I (T1SS) and type IX (T9SS) secretion systems, but the number of type I secretion system genes shows diversity between species. F. branchiophilum, F. araucananum, F. chilense, F. spartansii, and F. tructae genomes have full type VI secretion system (T6SS). F. columnare, F. hydatis, and F. plurextorum carry partial T6SS with some of the T6SS genes missing. F. columnare, F. araucananum, F. chilense, F. spartansii, F. araucananum, F. tructae, Flavobacterium sp., F. crassostreae, F. succinicans, F. hydatis, and F. plurextorum carry most of the type IV secretion system genes (T4SS). F. columnare genetic groups 1 and 2, Flavobacterium sp., and F. crassostreae encode the least number of antibiotic resistance elements. F. hydatis, F. chilense, and F. plurextorum encode the greatest number of antibiotic resistance genes. Additionally, F. spartansii, F. araucananum, and chilense encode the greatest number of virulence genes while Flavobacterium sp. and F. crassostreae encode the least number of virulence genes. In conclusion, comparative genomics of Flavobacterium species of aquatic origin will help our understanding of Flavobacterium pathogenesis.

Transcription of IVIAT and Virulence Genes in Photobacterium damselae Subsp. piscicida Infecting Solea senegalensis

Photobacterium damselae subsp. piscicida (Phdp) is responsible for disease outbreaks in marine aquaculture worldwide. Solea senegalensis, a valuable fish species for aquaculture in the south of Europe, is frequently affected by this pathogen. It is well established that bacteria respond to environmental signals and, in the case of pathogens, this ability may determine the outcome of their interaction with the host. Determination of gene expression under in vivo conditions constitutes a valuable tool in the assessment of microbial pathogenesis. Considering that different hosts may represent different environments for the pathogen, expression of Phdp virulence and in vivo induced antigen (IVIAT) genes during S. senegalensis infection has been determined in the present work. Increased transcription of genes encoding proteins involved in iron acquisition (Irp1, Irp2, HutB and HutD), oxidative stress defence (AhpC and Sod), adhesion (PDP_0080), toxins (AIP56) and metabolism (Impdh, Shmt and AlaRS) were detected in Phdp infecting S. senegalensis head kidney or liver. The highest increases corresponded to genes involved in survival under iron limiting conditions and oxidative stress, indicating their essential role during infection of sole. Results obtained give insight into Phdp virulence strategies and contribute to the identification of promising targets for the control of photobacteriosis.

Molecular and phenotypic characterization of Photobacterium damselae among some marine fishes in Lake Temsah

Photobacterium damselae species are one of the most devastating bacterial pathogens in mariculture worldwide. Some species of Photobacterium are pathogenic for marine animals and human. They are the causative agents of photobacteriosis, formerly known as pasteurellosis. A total of (202) marine fishes of three different species were represented as: seabass (Dicentrarchus labrax), seabream (Sparus aurata) and gray mullet (Mugil capitus) randomly collected from Lake Temsah at Ismailia governorate along the parallel Pelagic road to the lake in the governorate from August 2015 to July 2016. The clinical picture and gross lesions of the diseased fishes were recorded. Isolation and identification of suspected bacteria using traditional and molecular methods. Samples from affected organs were collected for studying the histopathological alterations of these pathogens. Fifty one fishes were found to be infected with Photobacterium damselae subsp. Piscicida. Seabass (Dicentrarchus labrax) was the most infected fish species (23), followed by seabream (Sparus aurata) (18) finally gray mullet (Mugil capitus) was (10). 91fishes were found to be infected with P. damselae subsp. damselae, seabass (Dicentrarchus labrax) was the most infected fish sp. (36), followed by seabream (Sparus aurata) (32), then gray mullet (Mugil capitus) (23). The results indicated that, the total prevalence of P. damselae subsp. piscicida in all examined species (25.24%), the highest seasonal prevalence was recorded in summer season (37.09%) followed by autumn (26%) then spring (20.37%) and winter (11.11%). On the other hand, the total prevalence of P. damselae subsp. damselae in all examined species (45.04%), the highest seasonal prevalence was recorded in summer season (67.74%) followed by autumn (52%) then spring (29.62%) and winter (19.44%). Molecular diagnosis with conventional PCR used to confirm the traditional isolation was applied by using specific primers of two genes (polycapsular saccharide gene and urease C gene). The histopathological studies of naturally infected marine fishes showed severe inflammatory reactions in different organs with accumulation of melanomacrophages and necrosis. The results confirm that P. damselae subspecies damsalea is the most prevalent pathogen between marine fishes, and seabass (Dicentrarchus labrax) was the highly affected marine fishes in this study.

Transcription of immune related genes in Solea senegalensis vaccinated against Photobacterium damselae subsp. piscicida. Identification of surrogates of protection

Solea senegalensis is a flatfish with a great potential for aquaculture, but infectious diseases restrict its production, being this fish species highly susceptible to Photobacterium damselae subsp. piscicida (Phdp) infections. A better understanding of the mechanisms related to fish immune response is crucial for the development of effective approaches in disease management. In the present work, transcriptional changes of immune related genes have been evaluated in farmed S. senegalensis specimens vaccinated against Phdp by intraperitoneal injection (IP) and immersion (IM). IP fish showed higher antibody levels and increased transcription of genes encoding lysozyme C1, complement factors involved in the classical pathway and components involved in the opsonization and the limitation of free iron availability, all of them facilitating the faster elimination of the pathogen and promoting higher RPS after the infection with Phdp. The results of this study seem to support a different intensity of the specimens immune response in the head kidney. Analysis of the immune response in 15 day post-challenged fish showed up-regulation of genes involved in all stages of S. senegalensis immune response, but especially those genes encoding proteins related to the innate response such as complement, lysozyme and iron homeostasis in the head kidney. On the other hand, liver transcription was higher for genes related to inflammation, apoptosis and cell mediated cytotoxicity (CMC). Furthermore, comparison of the differential response of S. senegalensis genes in vaccinated and unvaccinated fish to Phdp infection allowed the identification of a potential biosignature, consisting in 10 genes, as a surrogate of protection and therefore, as indicator of vaccine success against fotobacteriosis after IP vaccination. These results provide important insights into the S. senegalensis protection against Phdp induced by vaccination.

Two routes of infection with Photobacterium damselae subsp. piscicida are effective in the modulation of the transcription of immune related genes in Solea senegalensis

The marine fish pathogen Photobacterium damselae subsp. piscicida (Phdp) is responsible for important disease outbreaks affecting cultured fish species including the flatfish Solea senegalensis. In the present work, transcription of iron metabolism related genes (TF, FERR-M, HP-1 and HAMP-1) as well as innate immune system components such as complement proteins (C3 and C7), lysozyme (LYS-G), TNF family (TNFα, TRAF-3), NCCRP-1 and heat shock protein encoding genes (HSP70, HSP90AA, HSP90AB and GP96) has been determined in the liver and kidney of S. senegalensis specimens after Phdp infection. Intraperitoneal injection (IP) and immersion (IM) routes have been used for infection. Fish developed specific antibodies in both cases, higher levels being detected in IP infected specimens. Both infection routes resulted in increased relative transcript levels of FERR-M, HP-1 and HAMP-1 genes and TF decreased relative transcription, conducting to lower iron availability for the pathogen. This response can be considered as a strategy to limit iron availability for Phdp, a pathogen capable to obtain iron from transferrin. Relative transcription of genes encoding lysozyme and complement factors C3 and C7 were also increased regardless the infection route; the liver was the main organ involved in the initial stages and the kidney in later stages of the infection. TNFα and TRAF-3 relative gene transcription increased 24h post-infection. TRAF-3 gene induction was detected 30 d post-infection, whilst no changes in TNFα were observed 72h or 30 d post-infection. NCCRP-1 changes were observed after IP infection in the liver and kidney; however, IM infection resulted only in slight changes in the kidney of infected fish. This different response observed maybe related to a lower number of invaded cells by the pathogen. Finally, changes in HSP90AB and GP96 have been detected after infection by both routes. Different late modulation has been observed in assayed genes depending on the route of infection. Thus, only LYS-G, TF, NCCRP-1, GP96 and HSP90AB gene transcription was modulated 30 d post-infection in the kidney of IM infected specimens; however, IP infected fish showed modulation in a higher number of genes both in liver and kidney tissues. The implications of these responses in resistance to infection by Phdp need to be elucidated.

Fur-regulated iron uptake system of Edwardsiella ictaluri and its influence on pathogenesis and immunogenicity in the catfish host

The ability of bacterial pathogens to take up iron from the host during infection is necessary for their multiplication within the host. However, host high-affinity iron binding proteins limit levels of free iron in fluids and tissues. To overcome this deficiency of iron during infection, bacterial pathogens have developed iron uptake systems that are upregulated in the absence of iron, typically tightly controlled by the ferric uptake regulator (Fur) protein. The iron uptake system of Edwardsiella ictaluri, a host-restricted pathogen of channel catfish (Ictalurus punctatus) and the main pathogen of this fish in aquaculture, is unknown. Here we describe the E. ictaluri Fur protein, the iron uptake machinery controlled by Fur, and the effects of fur gene deletion on virulence and immunogenicity in the fish host. Analysis of the E. ictaluri Fur protein shows that it lacks the N-terminal region found in the majority of pathogen-encoded Fur proteins. However, it is fully functional in regulated genes encoding iron uptake proteins. E. ictaluri grown under iron-limited conditions upregulates an outer membrane protein (HemR) that shows heme-hemoglobin transport activity and is tightly regulated by Fur. In vivo studies showed that an E. ictaluri Δfur mutant is attenuated and immune protective in zebrafish (Danio rerio) and catfish (Ictalurus punctatus), triggering systemic immunity. We conclude that an E. ictaluri Δfur mutant could be an effective component of an immersion-oral vaccine for the catfish industry.

Transferrin and ferritin response to bacterial infection: the role of the liver and brain in fish

Iron is essential for growth and survival, but it is also toxic when in excess. Thus, there is a tight regulation of iron that is accomplished by the interaction of several genes including the iron transporter transferrin and iron storage protein ferritin. These genes are also known to be involved in response to infection. The aim of this study was to understand the role of transferrin and ferritin in infection and iron metabolism in fish. Thus, sea bass transferrin and ferritin H cDNAs were isolated from liver, cloned and characterized. Transferrin constitutive expression was found to be highest in the liver, but also with significant expression in the brain, particularly in the highly vascularized region connecting the inferior lobe of the hypothalamus and the saccus vasculosus. Ferritin, on the other hand, was expressed in all tested organs, but also significantly higher in the liver. Fish were subjected to either experimental bacterial infection or iron modulation and transferrin and ferritin mRNA expression levels were analyzed, along with several iron regulatory parameters. Transferrin expression was found to decrease in the liver and increase in the brain in response to infection and to increase in the liver in iron deficiency. Ferritin expression was found to inversely reflect transferrin in the liver, increasing in infection and iron overload and decreasing in iron deficiency, whereas in the brain, ferritin expression was also increased in infection. These findings demonstrate the evolutionary conservation of transferrin and ferritin dual functions in vertebrates, being involved in both the immune response and iron metabolism.

In vivo morphological and antigenic characteristics of Photobacterium damselae subsp. piscicida

The present study was conducted to examine the morphology and antigenicity of Photobacterium damselae subsp. piscicida by culturing the bacterium in vivo in the peritoneal cavity of sea bass (Dicentrarchus labrax) within dialysis bags with either a low molecular weight (LMW) cut-off of 25 kDa or a high molecular weight (HMW) cut-off of 300 kDa. Differences were observed in the growth rate between the bacteria cultured in vivo or in vitro. Bacteria cultured in vivo were smaller and produced a capsular layer, which was more prominent in bacteria cultured in the HMW bag. Antigenicity was examined by Western blot analysis using sera from sea bass injected with live Ph. d. subsp. piscicida. The sera recognised bands at 45 and 20 kDa in bacteria cultured in vivo in the LMW bag. Bacteria cultured in vivo in the HMW bag did not express the 45 kDa band when whole cell extracts were examined, although the antigen was present in their extracellular products. In addition, these bacteria had a band at 18 kDa rather than 20 kDa. Differences in glycoprotein were also evident between bacteria cultured in vitro and in vivo. Bacteria cultured in vitro in LMW and HMW bags displayed a single 26 kDa band. Bacteria cultured in the LMW bag in vivo displayed bands at 26 and 27 kDa, while bacteria cultured in vivo in the HMW bag possessed only the 27 kDa band. These bands may represent sialic acid. The significance of the changes observed in the bacterium's structure and antigenicity when cultured in vivo is discussed.

Variation in the molecular weight of Photobacterium damselae subsp. piscicida antigens when cultured under different conditions in vitro

The antigenicity of Photobacterium damselae (Ph. d.) subsp. piscicida, cultured in four different growth media [tryptone soya broth (TSB), glucose-rich medium (GRM), iron-depleted TSB (TSB + IR^-), and iron-depleted GRM (GRM + IR^-)] was compared by enzyme-linked immunosorbent assay (ELISA) and Western blot analysis using sera obtained from sea bass (Dicentrarchus labrax) raised against live or heat-killed Ph. d. subsp. piscicida. The antigenic expression of Ph. d. subsp. piscicida was found to differ depending on the culture medium used. A significantly higher antibody response was obtained with iron-depleted bacteria by ELISA compared with non-iron depleted bacteria obtained from the sera of sea bass raised against live Ph. d. subsp. piscicida. The sera from sea bass raised against live bacteria showed a band at 22 kDa in bacteria cultured in TSB + IR^- or GRM+ IR^- when bacteria that had been freshly isolated from fish were used for the screening, while bands at 24 and 47 kDa were observed with bacteria cultured in TSB or GRM. When bacteria were passaged several times on tryptic soya agar prior to culturing in the four different media, only bands at 24 and 47 kDa were recognized, regardless of the medium used to culture the bacteria. It would appear that the molecular weight of Ph. d. subsp. piscicida antigens change in the presence of iron restriction, and sera from sea bass infected with live bacteria are able to detect epitopes on the antigens after this shift in molecular weight.

Heme, an iron supply for vibrios pathogenic for fish

One of the main mechanisms present in gram-negative bacterial pathogens to obtain iron is the utilization of free heme or heme proteins from the host tissues. Vibrio anguillarum, the etiological agent of vibriosis in fish, and Photobacterium damselae subsp. piscicida, the causative agent of fish pasteurellosis, can acquire iron from free heme or heme-containing proteins present in the host tissues by a siderophore-independent mechanism. Similarly to other animal and human pathogens, the general mechanism for heme uptake in these two species consists in the presence of an outer membrane receptor that transport the heme molecule into the periplasm via a TonB-dependent process, and additional proteins that complete the transport of heme from the periplasm into the cell cytoplasm. Expression of heme uptake genes is iron-regulated at the transcriptional level by the repressor protein Fur. The heme uptake mechanisms are believed to contribute to virulence for fish. The existence of variability in the distribution of heme transport genes among strains suggests that gene inactivation and/or horizontal transfer might play a significant role in generating intraspecific genetic diversity.

Genetic variability of the heme uptake system among different strains of the fish pathogen Vibrio anguillarum: identification of a new heme receptor

The ability to utilize heme compounds as iron sources was investigated in Vibrio anguillarum strains belonging to serotypes O1 to O10. All strains, regardless of their serotype or isolation origin could utilize hemin and hemoglobin as sole iron sources. Similarly, all of the isolates could bind hemin and Congo red, and this binding was mediated by cell envelope proteins. PCR and Southern hybridization were used to assay the occurrence of heme transport genes huvABCD, which have been previously described in serotype O1. Of 23 strains studied, two serotype O3 isolates proved negative for all huvABCD genes, whereas nine strains included in serotypes O2, O3, O4, O6, O7, and O10 tested negative for the outer membrane heme receptor gene huvA. A gene coding for a novel outer membrane heme receptor was cloned and characterized in a V. anguillarum serotype O3 strain lacking huvA. The new heme receptor, named HuvS, showed significant similarity to other outer membrane heme receptors described in Vibrionaceae, but little homology (39%) to HuvA. This heme receptor was present in 9 out of 11 of the V. anguillarum strains that tested negative for HuvA. Furthermore, complementation experiments demonstrated that HuvS could substitute for the HuvA function in Escherichia coli and V. anguillarum mutants. The huvS and huvA sequences alignment, as well as the analysis of their respective upstream and downstream DNA sequences, suggest that horizontal transfer and recombination might be responsible for generating this genetic diversity.

Iron uptake mechanisms in the fish pathogen Tenacibaculum maritimum

We present here the first evidence of the presence of iron uptake mechanisms in the bacterial fish pathogen Tenacibaculum maritimum. Representative strains of this species, with different serotypes and origins, were examined. All of them were able to grow in the presence of the chelating agent ethylenediamine-di-(o-hydroxyphenyl acetic acid) (EDDHA) and also produced siderophores. Cross-feeding assays suggest that the siderophores produced are closely related. In addition, all T. maritimum strains utilized transferrin, hemin, hemoglobin, and ferric ammonic citrate as iron sources when added to iron-deficient media. Whole cells of all T. maritimum strains, grown under iron-supplemented or iron-restricted conditions, were able to bind hemin, indicating the existence of constitutive binding components located at the T. maritimum cell surface. This was confirmed by the observation that isolated total and outer membrane proteins from all of the strains, regardless of the iron levels of the media, were able to bind hemin, with the outer membranes showing the strongest binding. Proteinase K treatment of whole cells did not affect the hemin binding, indicating that, in addition to proteins, some protease-resistant components could also bind hemin. At least three outer membrane proteins were induced in iron-limiting conditions, and all strains, regardless of their serotype, showed a similar pattern of induced proteins. The results of the present study suggest that T. maritimum possesses at least two different systems of iron acquisition: one involving the synthesis of siderophores and another that allows the utilization of heme groups as iron sources by direct binding.

Heme uptake genes in human and fish isolates of Photobacterium damselae: existence of hutA pseudogenes

The marine bacterium Photobacterium damselae includes strains classified into two distinct subspecies, namely subsp. damselae and subsp. piscicida, which have been reported to cause disease in a variety of marine animals and in humans. P. damselae strains utilize heme compounds as sole iron sources. In the present study, ten potential heme uptake and utilization genes are described in P. damselae subsp. damselae and subsp. piscicida. One gene cluster includes the genes coding for putative proteins HutZ, HutX and HutW; TonB, ExbB and ExbD, the three components of the TonB system; HutB, the putative periplasmic binding protein; HutC, the putative inner membrane permease; and HutD, the putative ABC-transporter ATP-ase. A gene coding for HutA, the outer membrane heme receptor, has also been identified, but it is not linked to the rest of the heme transport genes. RT-PCR analyses showed that heme uptake genes are arranged in three iron-regulated transcriptional units. A plasmid carrying the gene for the heme receptor HutA in combination with a plasmid carrying tonBexbBDhutBCD genes conferred to Escherichia coli 101ESD (ent) the ability to use heme and hemoglobin as iron sources. The hutA gene was present in strains isolated from humans and a variety of fish species, but it was shown to be interrupted in some subsp. piscicida strains, constituting a pseudogene. This is the first description of a heme-uptake system in a Photobacterium species, and shows some structural and functional similarities to heme-uptake systems reported in other gram-negative bacteria.

Molecular cloning and functional analysis of Photobacterium damselae subsp. piscicida haem receptor gene

A haem receptor gene from Photobacterium damselae subsp. piscicida (formerly known as Pasteurella piscicida) has been cloned, sequenced and analysed for its function. The gene, designated as pph, has an open reading frame consisting of 2154 bp, a predicted 718 amino acid residues and exists as a single copy. It is homologous with the haem receptors of Vibrio anguillarum hupA, V. cholerae hutA, V. mimicus mhuA and V. vulnificus hupA at 32.7, 32.7, 45.6 and 30.9%, respectively, and is highly conserved, consisting of a Phe-Arg-Ala-Pro sequence (FRAP), an iron transport related molecule (TonB) and a Asn-Pron-Asn-Leu sequence (NPNL), binding motifs associated with haem receptors. As a single gene knockout mutant P. damselae subsp. piscicida was able to bind haem in the absence of pph, suggesting that other receptors may be involved in its iron transport system. This study shows that the P. damselae subsp. piscicida pph belongs to the haem receptor family, is conserved and that its iron-binding system may involve more than one receptor.

Isolation of mutants of Vibrio anguillarum defective in haeme utilisation and cloning of huvA, a gene coding for an outer membrane protein involved in the use of haeme as iron source

The isolation of Vibrio anguillarum mutants lacking the ability to use haemin and haemoglobin as the only iron sources, as well as the identification of a gene involved in haeme utilisation are described. One of the isolated mutants defective in haeme utilisation lacked an iron-regulated outer membrane protein of 79-kDa. Although growth on haeme as iron source was completely abolished, the haemin and haemoglobin binding activities remained intact in the mutant, suggesting that the absent protein is not the only one involved in haeme binding. The wild-type phenotype in this mutant was restored by transformation with a cosmid clone (pML1) containing a 21-kb DNA fragment isolated from a gene library derived from the parental strain of V. anguillarum. Sequence analysis of pML1 subclones led to the finding of an ORF, huvA, that codes for a 79-kDa protein (HuvA) and whose sequence shows high identity with haeme receptors from Vibrio choleare (HutA) and Vibrio vulnificus (HupA). The sequence of huvA from the V. anguillarum haeme-utilisation mutant revealed a single mutation, leading to the synthesis of a truncated HuvA protein of 70 kDa. The parental strain and the cosmid-complemented mutant showed a higher degree of virulence for fish than the mutant strain in experimental infections in which fish were previously overloaded with haemin. This finding suggests that haeme uptake plays an important role in V. anguillarum multiplication in fish tissues when free haeme is available.