Relationships between plasmids and phenotypes of presumptive strains of Vibrio anguillarum isolated from different fish species

Comparative pathogenicity of Vibrio spp., Photobacterium damselae ssp. damselae and five isolates of Aeromonas salmonicida ssp. achromogenes in juvenile Atlantic halibut (Hippoglossus hippoglossus)

Juvenile Atlantic halibut (~100 mg, Hippoglossus hippoglossus) were exposed to Vibrio proteolyticus, a Vibrio spp. isolate, Photobacterium damselae ssp. damselae and five different isolates of Aeromonas salmonicida ssp. achromogenes via an hour-long bath immersion to ascertain their variation in pathogenicity to this fish species. Results were analysed using Kaplan-Meier survival analysis. Analysis of the data from challenges using A. salmonicida ssp. achromogenes revealed three survival values of zero and a spread of values from 0 to 28.43. Challenges using a Vibrio spp isolate, V. proteolyticus and P. damselae resulted in Kaplan-Meier survival estimates of 31.21, 50.41 and 57.21, respectively. As all bacterial species tested could induce juvenile halibut mortalities, they must all be considered as potential pathogens. However, the degree of pathogenicity of A. salmonicida is isolate dependent.

Reactivation of the vanchrobactin siderophore system of Vibrio anguillarum by removal of a chromosomal insertion sequence originated in plasmid pJM1 encoding the anguibactin siderophore system

A chromosomal gene cluster encoding vanchrobactin biosynthesis and transport genes was identified in the Vibrio anguillarum serotype O1 strain, 775(pJM1), harbouring the anguibactin biosynthetic genes in the pJM1 plasmid. In this strain only anguibactin is produced as the vanchrobactin chromosome cluster has a RS1 transposition insertion into vabF, one of the vanchrobactin biosynthesis genes. Removal of this RS1 generating 775(pJM1)Delta tnp, still resulted in the detection of only anguibactin in specific bioassays. Surprisingly, when the pJM1 plasmid was not present as in the plasmidless strain H775-3, removal of the RS1 resulted in the detection of only vanchrobactin. These results thus can be interpreted as if presence of the pJM1 plasmid or of anguibactin itself is associated with the lack of detection of the vanchrobactin siderophore in bioassays. As high-performance liquid chromatography (HPLC) and mass spectrometry analysis demonstrated that both vanchrobactin and anguibactin were indeed produced in 775(pJM1)Delta tnp, it is clear that the pJM1-encoded anguibactin siderophore has higher affinity for iron than the vanchrobactin system in strains in which both systems are expressed at the same time. Our results underscore the importance of the anguibactin system in the survival of V. anguillarum 775 under conditions of iron limitation.

RpoN of the fish pathogen Vibrio (Listonella) anguillarum is essential for flagellum production and virulence by the water-borne but not intraperitoneal route of inoculation

To investigate the involvement of RpoN in flagellum production and pathogenicity of Vibrio (Listonella) anguillarum, the rpoN gene was cloned and sequenced. The deduced product of the rpoN gene displayed strong homology to the alternative sigma 54 factor (RpoN) of numerous species of bacteria. In addition, partial sequencing of rpoN-linked ORFs revealed a marked resemblance to similarly located ORFs in other bacterial species. A polar insertion or an in-frame deletion in the coding region of rpoN abolished expression of the flagellin subunits and resulted in loss of motility. Introduction of the rpoN gene of V. anguillarum or Pseudomonas putida into the rpoN mutants restored flagellation and motility. The rpoN mutants were proficient in the expression of other proposed virulence determinants of V. anguillarum, such as ability to grow under low available iron conditions, and expression of the LPS O-antigen and of haemolytic and proteolytic extracellular products. The infectivity of the rpoN mutants with respect to the wild-type strain was unaffected following intraperitoneal injection of fish but was reduced significantly when fish were immersed in bacteria-containing water. Thus, RpoN does not appear to regulate any factors required for virulence subsequent to penetration of the fish epithelium, but is important in the infection of fish by water-borne V. anguillarum.

Plasmid profiles, restriction fragment length polymorphisms and O-serotypes among Vibrio anguillarum isolates

A total of 279 Vibrio anguillarum strains were serotyped and examined for plasmid content. Plasmids were subjected to digestion with restriction enzymes. Most strains belonged to serogroup O1 (39%) and O2 (16%). In total 164 strains (53%) carried plasmids. Of the O1 and O2 isolates, 92% and 30%, respectively, carried one or more plasmids. Restriction fragment length polymorphism (RFLP) analysis of plasmid DNA indicated that plasmids belonged to several groups. Each group seemed to be restricted to a single O-serovar. The largest group was the pJM1-like plasmids among most serovar O1 strains. Most of these plasmids were about 67 kb like the pJM1 plasmid, but various derivatives ranged from 26-77 kb. RFLP studies of the 67 kb plasmids revealed 17 different restriction patterns. Some patterns were dominant among European strains whereas others were dominant among North American strains. The results confirmed the applicability of O-serotyping together with plasmid profile and restriction analysis of plasmids for typing of V. anguillarum. They also indicated that plasmids among strains which belonged to the traditional fish pathogenic serogroups, O1 and O2, showed more homology than did strains from most other serogroups, that were usually non-pathogenic, environmental bacteria.

Characterization of Vibrio anguillarum strains isolated from diseased fish in Finland

Characterization of V. anguillarum strains (n = 109) isolated from diseased salmonids was performed. Eight O serovars were found among the strains. Serovar O1 was predominant (90%), while serovars O2, O3, O5, O8, O9, and a new serovar Va NT2, were represented by 1 or 2 strains. Two strains remained non-typeable. One of these was cross-reactive with several antisera, but had a LPS profile identical to that of serovar O8. All serovars showed specific LPS profiles. All but 1 of the O1 strains had a plasmid comparable in size to the pJM1 virulence plasmid, while plasmids of different sizes were found in O2, Va NT2 and the non-typeable strains. Apart from a single strain resistant to tetracycline, all the strains were sensitive to oxolinic acid, tetracycline, and trimethoprim-sulfonamides. By their biochemical and antigenic properties strains causing vibriosis among salmonids in Finland closely resemble Scandinavian strains. Predominance of the serovars O1 and O2 suggests that commercial vaccines containing these serovars should afford sufficient protection against vibriosis in Finland.

Evidence for the existence of distinct populations of Vibrio anguillarum serogroup O1 based on plasmid contents and ribotypes

A total of 103 Vibrio anguillarum serogroup O1 strains displaying 15 different plasmid profiles were characterized with respect to biochemical properties and ribotypes. The results confirmed that V. anguillarum O1 is a biochemically homogeneous group. The 103 strains could be allocated to three main clusters with high similarity coefficients. None of the biochemical properties were connected with the presence of plasmids. In total, 12 different ribotypes were demonstrated, with HindIII being used as the restriction enzyme. Forty of the strains were isolated from the same Danish fish farm, some from the kidneys of diseased fish and some from the environment, and some strains were isolated from the mucus, gills, and feces of healthy fish. Nineteen of these isolates possessed the 67-kb virulence plasmid alone or in combination with other plasmids, while 21 had no plasmids. All strains isolated from the kidneys of diseased fish on this farm had plasmids. Irrespective of their origin (kidneys, gills, or mucus), all 19 strains carrying the 67-kb virulence plasmid had the same ribotype, profile 1, while isolates without plasmids belonged to five different profiles, all different from profile 1. These results suggest that pathogenic V. anguillarum O1 strains possessing a virulence plasmid and nonpathogenic strains without plasmids from a small geographical area and even from the same fish may constitute two essentially distinct populations. Thus, it may be suggested that an exchange of virulence plasmids among strains is unlikely to occur in vivo.

Characterization of Vibrio anguillarum strains isolated from diseased fish in Finland

Characterization of V. anguillarum strains (n = 109) isolated from diseased salmonids was performed. Eight O serovars were found among the strains. Serovar O1 was predominant (90%), while serovars O2, O3, O5, O8, O9, and a new serovar Va NT2, were represented by 1 or 2 strains. Two strains remained non-typeable. One of these was cross-reactive with several antisera, but had a LPS profile identical to that of serovar O8. All serovars showed specific LPS profiles. All but 1 of the O1 strains had a plasmid comparable in size to the pJM1 virulence plasmid, while plasmids of different sizes were found in O2, Va NT2 and the non-typeable strains. Apart from a single strain resistant to tetracycline, all the strains were sensitive to oxolinic acid, tetracycline, and trimethoprim-sulfonamides. By their biochemical and antigenic properties strains causing vibriosis among salmonids in Finland closely resemble Scandinavian strains. Predominance of the serovars O1 and O2 suggests that commercial vaccines containing these serovars should afford sufficient protection against vibriosis in Finland.

Ribotypes and plasmid contents of Vibrio anguillarum strains in relation to serovar

Eighty-six strains of the 10 major agglutination types of Vibrio anguillarum (serovars O1 to O10) and 6 nontypeable strains of V. anguillarum have been characterized by ribotyping with a probe complementary to 16S and 23S rRNA of Escherichia coli and by plasmid profile analysis. Forty-four different ribotypes were observed with the restriction enzyme HindIII. Ribotype similarity was compared by using the Dice coefficient (Sd), and three significantly different levels of homogeneity within the V. anguillarum serovars were observed (serovars O1, O3A, O7, and O9, Sds of > 90%; serovars O2B, O4, and O10, Sds of 80 to 90%; serovars O2A, O3B, O5, and O8, Sds between 46 and 70%). None of the ribotype patterns of V. anguillarum strains were observed among 20 other Vibrio strains typed for comparison. By cluster analysis, the V. anguillarum strains were divided into a main cluster containing 83 strains, while all strains of serovar O3B, one strain (each) of serovars O2A, O5, and O8, and a nontypeable strain were separated from this cluster by at least 15% difference in similarity coefficients. Plasmids were demonstrated in only six strains other than serovar O1. In serovar O1, a 67- to 70-kilobase-pair (kb) plasmid molecule was present in 17 of 19 strains tested; of the two remaining strains, one strain harbored two plasmids (45 and 6.5 kb) and one strain had no plasmids.

Distribution of plasmid- and chromosome-mediated iron uptake systems in Vibrio anguillarum strains of different origins

We investigated the incidence of plasmid-mediated and chromosome-mediated iron uptake systems in strains of Vibrio anguillarum that belong to serotypes O1 and O2 and were isolated from different fish species and in different geographic areas. All of the strains gave positive reactions in CAS agar medium and in the Arnow test, which indicated that catechol types of siderophores were produced. The majority of V. anguillarum serotype O1 strains harbored a 65-kb plasmid similar to plasmid pJM1 from strain 775, which encodes the siderophore anguibactin and its outer membrane receptor, protein OM2. All of the isolates harboring this plasmid promoted the growth of an anguibactin-deficient receptor-proficient mutant derived from strain 775, but none of these isolates promoted the growth of mutants lacking receptor OM2. Furthermore, under iron-limiting conditions all of these strains induced outer membrane proteins that were identical in size to protein OM2 of strain 775. In contrast, none of the serotype O2 strains contained a high-molecular-weight plasmid, but all of them induced the growth of mutants defective in the anguibactin-mediated system regardless of the presence or absence of receptor OM2. The serotype O2 strains, but not the plasmid-bearing serotype O1 strains, also induced the growth of Salmonella typhimurium enb-1 which utilizes only enterobactin as a siderophore.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of Vibrio anguillarum and closely related species isolated from farmed fish in Norway

A total of 264 bacterial strains tentatively or definitely classified as Vibrio anguillarum were examined. The strains were isolated from diseased or healthy Norwegian fish after routine autopsy. With the exception of five isolates from wild saithe (Pollachius virens), the strains originated from nine different species of farmed fish. The bacteria were subjected to morphological, physiological, and biochemical studies, numerical taxonomical analyses, serotyping by slide agglutination and enzyme-linked immunosorbent assay, DNA-plasmid profiling, and in vitro antimicrobial drug susceptibility testing. The results of the microbiological studies were correlated to anamnestic information. The bacterial strains were identified as V. anguillarum serovar O1 (n = 132), serovar O2 (n = 89), serovar O4 (n = 2), serovar O8 (n = 1), and not typeable (n = 1) as well as Vibrio splendidus biovar I (n = 36) and biovar II (n = 1), Vibrio tubiashii (n = 1), and Vibrio fischerii (n = 1). V. anguillarum serovar O1 or O2 was isolated in 176 out of 179 cases of clinical vibriosis in Atlantic salmon (Salmo salar). V. anguillarum serovar O1 was the only serovar isolated from salmonid fish species other than Atlantic salmon, while V. anguillarum serovar O2 was isolated from all marine fish suffering from vibriosis. A 48-Mda plasmid was isolated from all V. anguillarum serovar O1 isolates examined. Serovar O2 isolates did not harbor any plasmids. Resistance against commonly used antibiotic compounds was not demonstrated among V. anguillarum isolates. Neither V. splendidus biovar I nor other V. anguillarum-related species appeared to be of clinical importance among salmonid fish.(ABSTRACT TRUNCATED AT 250 WORDS)

Occurrence of plasmids in Danish isolates of Vibrio anguillarum serovars O1 and O2 and association of plasmids with phenotypic characteristics

Two hundred and twenty-eight isolates of Vibrio anguillarum serovar O1 (125 isolates) and serovar O2 (103 isolates) have been characterized with regard to plasmid contents, biochemical properties, and in vitro hemagglutination and hydrophobic properties. Among 74 V. anguillarum isolates from diseased fish, 63 carried only a 67-kb plasmid (pJM1), 9 carried an additional 98-kb plasmid, and 1 isolate carried only the 98-kb plasmid. Only one isolate was without plasmids. In V. anguillarum serovar O1 from nondiseased fish (mucus and gills), plasmids of the same sizes were present in 29 isolates (58%), whereas 21 isolates (42%) were plasmid free. Based on hemagglutination and biochemical properties, V. anguillarum serovar O1 isolates were divided into eight biovars. The plasmid-carrying strains (102 isolates) all fell within biovars 1 and 2, whereas the 23 strains of biovars 3 to 8 were without plasmids. It was tentatively concluded there are two populations of V. anguillarum serovar O1. One population contains plasmid(s), is hemagglutination negative and trehalose negative, and does not form pellicles in broth cultures, whereas the other population is plasmid free and has the opposite characteristics. The former group is the one related to disease in fish. All 20 V. anguillarum serovar O2 isolates from the environment were without plasmids, whereas 54 (65%) of the isolates from fish (trout and cod) carried plasmids. The biochemical diversity within serovar O2 was pronounced; 13 different biovars were demonstrated. No correlation between the presence of plasmids and biochemical properties was observed.

Polypeptides p40, pOM2, and pAngR are required for iron uptake and for virulence of the marine fish pathogen Vibrio anguillarum 775

Insertions were created in three iron uptake genes in plasmid pJM1 of Vibrio anguillarum 775 to assess their in vivo effects on virulence in fish. Insertions that blocked p40, pOM2, and pAngR expression resulted in iron uptake-negative strains and in 4.2 x 10(5)-, 8.8 x 10(5)-, and 2.5 x 10(5)-fold attenuations in virulence, respectively. A strain with an insertion in the pAngR coding region still synthesized significant constitutive levels of the outer membrane protein pOM2 and persisted in fish for at least 14 days postinjection. The results demonstrate a direct relationship between virulence and three pJM1-encoded gene products and also the feasibility of constructing live attenuated strains of V. anguillarum that might be useful in future vaccines.

Restriction fragment length polymorphism of the Vibrio anguillarum serovar O1 virulence plasmid

Seventy-eight strains of Vibrio anguillarum serovar O1, all harboring one 65- to 70-kilobase plasmid, were typed according to restriction fragment length polymorphism of the plasmid. Six types, three of which comprised 96% of the strains examined, were produced with the restriction endonuclease BamHI. The fragment length polymorphism type did not correlate to any of 12 different phenotypic properties tested.

Survival of Vibrio anguillarum and Vibrio salmonicida at different salinities

The fish pathogenic bacteria Vibrio anguillarum and V. salmonicida showed the capacity to survive for more than 50 and 14 months, respectively, in seawater microcosms. A salinity of 5% proved lethal to V. anguillarum harvested in the late-exponential growth phase, whereas a salinity of 9% was lethal to the bacterium after it had been starved at a salinity of 30% for 67 days. The lethal salinity for V. salmonicida harvested in the late-exponential growth phase was probably in the vicinity of 10%. V. anguillarum and V. salmonicida were very sensitive to nalidixic acid. Direct determination of viable cells after incubation with nalidixic acid was not possible, since the cells did not elongate. Samples of V. salmonicida were double stained with fluorescein isothiocyanate-labeled antibodies and 4',6-diamidino-2-phenylindole. After 3 or 4 days of starvation, there was a discrepancy between the total numbers of cells as determined by immunofluorescence versus by staining with 4',6-diamidino-2-phenylindole. The immunofluorescence counts remained high, which indicated the presence of intact cell envelopes but leakage of DNA and other cytoplasm components. After 2 weeks of starvation, for some of the cells, the region stained with 4',6-diamidino-2-phenylindole (i.e., DNA) was markedly smaller than the cell envelope. I attributed this to a shrinkage of the cytoplasm or a confined nucleoid or both. V. anguillarum lost its exoproteolytic activity before 11 days of starvation.

Virulence studies based on plasmid profiles of the fish pathogen Vibrio salmonicida

Strains of Vibrio salmonicida isolated from Atlantic salmon (Salmo salar) and rainbow trout (Salmo gairdneri) suffering from cold-water vibriosis could be divided on the basis of plasmid profiles into four different categories. Of 32 strains, 19% harbored three plasmids of 24, 3.4, and 26 megadaltons (MDa), 69% harbored the 24- and 3.4-MDa plasmids but not the 2.6-MDA plasmid, and 9% harbored only the 24-MDA plasmid. The fourth category, which consisted of only one strain, harbored a plasmid of 10 MDa. In spite of different plasmid patterns, the strains of V. salmonicida were very similar with respect to biochemical reactions. The one-third of the V. salmonicida strains which were serotyped were of the same type. The 50% lethal doses, which were determined by intraperitoneal injection, ranged from 4 x 106 to 1 x 108 CFU per fish.