Iron uptake by Pasteurella piscicida and its role in pathogenicity for fish

We evaluated the iron uptake mechanisms in Pasteurella piscicida strains as well as the effect of iron overload on the virulence of these strains for fish. With this aim, the capacity of the strains to obtain iron from transferrin and heme compounds as well as their ability to overcome the inhibitory activity of fish serum was analyzed. All the P. piscicida strains grew in the presence of the iron chelator ethylene-diamine-di (O-hydroxyphenyl acetic acid) or of human transferrin, which was used by a siderophore-mediated mechanism. The chemical tests and cross-feeding assays showed that P. piscicida produced a siderophore which was neither a phenolate nor a hydroxamate. Cross-feeding assays as well as preliminary chromatographic analysis suggest that this siderophore may be chemically related to multocidin. All the P. piscicida isolates utilized hemin and hemoglobin as an iron source, since the virulence of the strains increased when the fish were preinoculated with these compounds. This effect was stronger in the avirulent strains (50% lethal dose was reduced by 4 logs when fish were pretreated with hemin or hemoglobin). Only the pathogenic P. piscicida isolates were resistant to the bactericidal action of the fresh fish serum. The nonpathogenic strains grew in fish serum only when it was heat-inactivated or when it was supplemented with ferric ammonium citrate, hemin, or hemoglobin. In all the strains, at least three iron-regulated outer membrane proteins (IROMPs) (105, 118, and 145 kDa) were increased when the strains were cultured in iron-restricted medium.(ABSTRACT TRUNCATED AT 250 WORDS)

In situ detection of hepatitis A virus in cell cultures and shellfish tissues

An in situ transcription method was developed to detect hepatitis A virus RNA in both cell cultures and shellfish tissues. Radiolabeled cDNA copies were synthesized in situ by reverse transcriptase-directed transcription after annealing with a specific primer to the viral RNA. Both tritium (3H) and 35S were useful in the in situ transcription reaction, but the use of 3H resulted in a lower background and finer detail in the localization of viral particles. Application of the method to different organs of oysters which had bioaccumulated hepatitis A virus allowed the first in situ localization of the virus, specifically in stomach and hepatopancreatic tissues.

Evidence of a dormant but infective state of the fish pathogen Pasteurella piscicida in seawater and sediment

The stability of Pasteurella piscicida strains in seawater and sediment microcosms at different temperatures (6 and 20 degrees C) was investigated during a 1-month period. Three strains of P. piscicida showed similar survival kinetics. By a standard plate count method they survived in water and sediment for only 6 to 12 days, depending on the strain and type of microcosm. During this starvation period, the metabolic activity of the cells was reduced by more than 80%. Culturable cells of each P. piscicida strain persisted better in sediment than in water, as well as at 20 degrees C compared to 6 degrees C. However, in all the microcosms, the acridine orange direct counts remained at about 10(5) cells per ml during the experimental period, which demonstrated that P. piscicida possesses a capacity to enter a viable but not culturable state. Moreover, dormant cells were always resuscitated by the addition of fresh medium to the microcosms, since we recovered numbers of culturable cells similar to the acridine orange direct counts. These resuscitated cells exhibited the same respiration rate as that seen prior to the start of the experiments. Although the biochemical, physiological, and serological characteristics; lipopolysaccharides; membrane proteins; and plasmid content of P. piscicida strains were unaffected during the starvation conditions, the dormant cells were smaller (dwarf cells) and had increased surface hydrophobicity. The starved cells maintained their infectivity and pathogenic potential for fish, with 50% lethal doses similar to those of the original strains.

Pathological activities of Yersinia ruckeri, the enteric redmouth (ERM) bacterium

The adherence and invasive capacities as well as the pathobiological activities exhibited by Yersinia ruckeri were examined. Although adhesive ability was dependent on the cell-line employed, all the strains showed moderate adhesion and invasiveness in the salmon cell-line CHSE-214. With regard to the extracellular products (ECP) all of them were strongly toxic for fish with LD50 ranging from 2 to 9.12 micrograms protein per g fish. In addition, all the ECP samples showed caseinase, gelatinase, amylase, lipase and phospholipase activities, hydrolysed esculin and displayed hemolytic activities for trout, salmon, sheep and human erythrocytes. Heat treatment (100 degrees C for 10 min) caused the loss of all these biological activities except the hydrolysis of gelatin. On the other hand, SDS-PAGE analysis of the LPS and protein components of the ECP revealed variations among strains depending on the serotype. The lack of lethal effects of the LPS present in the ECP was also demonstrated.

Pathogenic activities of live cells and extracellular products of the fish pathogen Pasteurella piscicida

The pathobiological activities in vivo and in vitro of live cells and extracellular products (ECP) of eleven Pasteurella piscicida strains of different origin were examined. Infectivity trials showed that P. piscicida did not possess strict host specificity since the majority of the isolates were virulent for gilthead seabream, rainbow trout and turbot, with LD50 values ranging between 10(3) and 10(6) live cells. However, none of the strains tested were pathogenic for mice (LD50 > 10(8) cells)). In addition, the ECP were strongly toxic for fish (LD50 ranging from 1.0 to 4.6 micrograms protein per g fish), which clearly demonstrates their important role in the pathogenesis of pasteurellosis. All the ECP samples were cytotoxic for fish and homoiothermic cell lines, possessed notable phospholipase activity and displayed haemolytic activity for sheep, salmon and turbot erythrocytes (but not for trout erythrocytes). However, the production of proteolytic enzymes differed among the P. piscicida strains. Although no strain displayed elastase activity, five isolates (the Japanese and Italian strains) hydrolysed casein and gelatin. All these biological activities in vivo and in vitro were lost after heat treatment (100 degrees C for 10 min). The general enzymic patterns of both live cells and ECP evaluated by the API-ZYM system also revealed some variation among the P. piscicida isolates. Generally, whole cells showed a wider range of enzymic activities than ECP. The results presented here are important for the selection of strains in the development of effective polyvalent pasteurellosis vaccines containing both whole cells and ECP.

Use of pulsed field gel electrophoresis to size the chromosome of the bacterial fish pathogen Yersinia ruckeri

Field inversion gel electrophoresis (FIGE) and contour-clamped homogeneous field (CHEF) electrophoresis were used to analyse the chromosome of Yersinia ruckeri. The 8 base-pair recognition endonucleases, NotI and SfiI, generated less than 47 DNA fragments whose size and distribution were appropriate for pulsed field separation. Each isolate displayed a characteristic restriction pattern, with about 20% of bands in common. Depending on the strain used, the estimated genome size for this bacterial fish pathogen ranged from 4460 to 4770 kilobase pairs.

Evidence that Yersinia ruckeri possesses a high affinity iron uptake system

This work represents the first evidence of the presence of an iron uptake system siderophore mediated in the bacterial fish pathogen Yersinia ruckeri. A group of 20 strains representative of this species, with different serotype and origin were examined. All of them were able to grow at high concentrations (from 0.7 to 1.1 mM) of the iron chelator EDDA. Although the Y. ruckeri isolates failed to cross-feed the indicator strains for enterobactin and aerobactin production, the chemical tests revealed the presence in the culture supernatants of phenolate siderophores. At least three outer membrane proteins were induced in iron limiting conditions. All the strains showed a similar pattern of induced membrane proteins regardless their serotype or origin, which suggests a similarity in the iron uptake system. This system could have an important role in the pathogeneicity of Y. ruckeri for fish.

Vibrios associated with red tides caused by Mesodinium rubrum

Vibrios were isolated from red tides caused by Mesodinium rubrum and also throughout the year in the Ria de Pontevedra, Spain. The isolates were grouped into 14 phena by numerical toxonomy. Strains associated with red tides were restricted to four phena: phena I and II were Vibrio alginolyticus, and phena III and IV were Vibrio tubiashii and Vibrio anguillarum, respectively. V. anguillarum-like strains (phena V through XI) predominated throughout the year outside the red tide areas. Cytotoxicity assays conducted in different poikilothermic and homoiothermic cell lines showed that cytotoxin production was not necessarily associated with the species selected during the red tides.

Changes in bacterial populations during red tides caused by Mesodinium rubrum and Gymnodinium catenatum in North West Coast of Spain

Heterotrophic bacterial communities associated with four red tides caused by Mesodinium rubrum and Gymnodinium catenatum in two Galician Rias (North West Spain) were examined. Three of these were produced by the Mesodinium rubrum and the causative organism of a toxic bloom was Gymnodinium catenatum. In early stages of all the blooms, the diversity decreased but the total marine bacterial counts increased by one or two logs. Vibrio numbers were also incremented by two logs in two blooms of M. rubrum, while in the other bloom of this organism and in the red tide caused by G. catenatum a decrease in number of these bacteria was observed. A total of 116 bacterial strains were identified at the genus level and grouped into 12 phena. During the decomposition processes of two blooms of M. rubrum a zooplanktonic-type bacterial succession was observed (Vibrio, pseudomonads and Moraxella-Acinetobacter). On the other hand, during decomposition of the other red tide of M. rubrum and the bloom of G. catenatum, a typical phytoplanktonic-type succession occurred, as Pseudomonas and Moraxella groups became dominant for all the process. These results support the conflicting taxonomical position of M. rubrum. After the blooms, the changes in the community point towards the restablishment of the normal bacterial flora of the estuary (increase in diversity and decreases of bacterial numbers). Only the Vibrio strains, isolated from the non-toxic first and second red tides, displayed cytotoxic activities. A relationship among bacterial cytotoxicity and toxic effects of blooms cannot therefore be established.