Streptococcus agalactiae Multilocus sequence type 261 is associated with mortalities in the emerging Ghanaian tilapia industry

Serotype distribution, virulence and antibiotic resistance of Streptococcus agalactiae isolated from cultured tilapia Oreochromis niloticus in Lake Volta, Ghana

Streptococcus agalactiae infection is one of the major factors limiting the expansion of tilapia farming globally. In this study, we investigated the serotype distribution, virulence and antimicrobial resistance of S. agalactiae isolates from tilapia farmed in Lake Volta, Ghana. Isolates from 300 moribund fish were characterised by Gram staining, MALDI-TOF/MS and 16S rRNA sequencing. Serotype identification was based on multiplex polymerase chain reaction (PCR) amplification of the capsular polysaccharide genes. Detection of virulence genes (cfb, fbsA and cspA) and histopathology were used to infer the pathogenicity of the isolates. The susceptibility of isolates to antibiotics was tested using the Kirby-Bauer disk diffusion assay. All 32 isolates identified as S. agalactiae were of serotype Ia. This was notably different from isolates previously collected from the farms in 2017, which belonged to serotype Ib, suggesting a possible serotype replacement. The prevalence of the pathogen was related to the scale of farm operation, with large-scale farms showing higher S. agalactiae positivity. Data from histopathological analysis and PCR amplification of targeted virulence genes confirmed the virulence potential and ability of the isolates to cause systemic infection in tilapia. Except for gentamicin, the majority of the isolates were less resistant to the tested antibiotics. All isolates were fully sensitive to oxytetracycline, erythromycin, florfenicol, enrofloxacin, ampicillin and amoxicillin. This study has improved our understanding of the specific S. agalactiae serotypes circulating in Lake Volta and demonstrates the need for continuous monitoring to guide the use of antimicrobials and vaccines against streptococcal infections in Ghanaian aquaculture systems.

A nanocarrier immersion vaccine encoding surface immunogenic protein confers cross-immunoprotection against Streptococcus agalactiae and Streptococcus iniae infection in tilapia

Streptococcosis is a highly contagious aquatic bacterial disease that poses a significant threat to tilapia. Vaccination is a well-known effective measure to prevent and control fish bacterial diseases. Among the various immunization methods, immersion vaccination is simple and can be widely used in aquaculture. Besides, nanocarrier delivery technology has been reported as an effective solution to improve the immune effect of immersion vaccine. In this study, the surface immunogenic protein (Sip) was proved to be conserved and potential to provide cross-immunoprotection for both Streptococcus agalactiae (S. agalactiae) and Streptococcus iniae (S. iniae) by multiple sequences alignment and Western blotting analysis. On this basis, we expressed and obtained the recombinant protein rSip and connected it with functionalized carbon nanotubes (CNT) to construct the nanocarrier vaccine system CNT-rSip. After immersion immunization, the immune effect of CNT-rSip against above two streptococcus infections was evaluated in tilapia based on some aspects including the serum specific antibody level, non-specific enzyme activities, immune-related genes expression and relative percent survival (RPS) after bacteria challenge. The results showed that compared with control group, CNT-rSip significantly (P < 0.05) increased the serum antibody levels, related enzyme activities including acid phosphatase, alkaline phosphatase, lysozyme and total antioxidant capacity activities, as well as the expression levels of immune-related genes from 2 to 4 weeks post immunization (wpi), and all these indexes peaked at 3 wpi. Besides, the above indexes of CNT-rSip were higher than those of rSip group with different extend during the experiment. Furthermore, the challenge test indicated that CNT-rSip provided cross-immunoprotection against S. agalactiae and S. iniae infection with RPS of 75 % and 72.41 %, respectively, which were much higher than those of other groups. Our study indicated that the nanocarrier immersion vaccine CNT-rSip could significantly improve the antibody titer and confer cross-immuneprotection against S. agalactiae and S. iniae infection in tilapia.

Stress-associated β -glucan administration stimulates the TLR - MYD88 - NFKB1 signaling pathway in Nile tilapia (Oreochromis niloticus)

There is evidence that the administration of β-glucan can effectively activate several defense mechanisms, such as the Tlr-Myd88-Nfkb1 pathway that induces the expression of immune cytokines. Thus, the objective of this work was to evaluate whether β-glucan acts on the mechanisms of gene transcription via the Tlr-Myd88-Nfkb1 pathway in Nile tilapia under stress after challenge with Streptococcus agalactiae. Therefore, we evaluated the expression of immune system genes such as toll-like receptors 1 (tlr1), toll-like receptors 2 (tlr2), primary myeloid differentiation response gene (myd88) and nuclear factor kappa B1 (nfkb1). A total of 408 fish were distributed in 24 polyethylene boxes and randomly divided into eight groups with 3 replications each: C15: Tilapias received a control diet (free of β-glucan) for 15 days and were sampled after the 15th day of the experiment; C15D: Tilapias received a control diet (free of β-glucan) for 15 days, were challenged on the 14th day and were sampled at the 15th day of the experiment; β15: Tilapias received experimental diet (1g kg-1 of β-glucan) for 15 days and were sampled after 15 days; β15D: Tilapias received an experimental diet (1g kg-1 of β-glucan) for 15 days, were challenged on the 14th day and were sampled at the 15th day of the experiment; C30: Tilapias received a control diet (free of β-glucan) for 30 days and were sampled on the 30th day of the experiment; C30D: Tilapias received a control diet (free of β-glucan) for 30 days, were challenged on the 29th day and were sampled at the 30th day of the experiment; β30: Tilapias received experimental diet (1g kg-1 of β-glucan) for 30 days and were sampled after 30 days and β30D: Tilapias received experimental diet (1g kg-1 of β-glucan) for 30 days, were challenged on the 29th day and were sampled at 30 of the experiment. In the fish sampled at 15 and 30 days of the experiment, after being anesthetized and killed by brain section, cranial kidney and spleen were collected for gene expression analysis. The analyzes showed that the association of β-glucan and stressful management modulated the immune system, using the Tlr-Myd88-Nfkb1 signaling pathway, indicating that this compound can be used to promote early defense and protect fish against diseases.

Exploring the Immunoprotective Potential of a Nanocarrier Immersion Vaccine Encoding Sip against Streptococcus Infection in Tilapia (Oreochromis niloticus)

Tilapia, as one of the fish widely cultured around the world, is suffering severe impact from the streptococcus disease with the deterioration of the breeding environment and the increasing of breeding density, which brings serious economic loss to tilapia farming. In this study, the surface immunogenic protein (Sip) of Streptococcus agalactiae (S. agalactiae) was selected as the potential candidate antigen and connected with bacterial nano cellulose (BNC) to construct the nanocarrier subunit vaccine (BNC-rSip), and the immersion immune effects against S. agalactiae and Streptococcus iniae (S. iniae) in Nile tilapia were evaluated on the basis of the serum antibody level, non-specific enzyme activity, the immune-related gene expression and relative percent survival (RPS). The results indicated that Sip possessed the expected immunogenicity according to the immunoinformatic analysis. Compared with the rSip group, BNC-rSip significantly induced serum antibody production and improved the innate immunity level of tilapia. After challenge, the RPS of BNC-rSip groups were 78.95% (S. agalactiae) and 67.86% (S. iniae), which were both higher than those of rSip groups,31.58% (S. agalactiae) and 35.71% (S. iniae), respectively. Our study indicated that BNC-rSip can induce protective immunity for tilapia through immersion immunization and may be an ideal candidate vaccine for controlling tilapia streptococcal disease.

A Multiplexed, Tiled PCR Method for Rapid Whole-Genome Sequencing of Infectious Spleen and Kidney Necrosis Virus (ISKNV) in Tilapia

Tilapia farming is one of the most important sectors in aquaculture worldwide and of major importance to global food security. Infectious spleen and kidney necrosis virus (ISKNV) has been identified as an agent of high morbidity and mortality, threatening tilapia aquaculture. ISKNV was detected in Lake Volta, Ghana, in September 2018 and spread rapidly, with mortality rates between 60 and 90% and losses of more than 10 tonnes of fish per day. Understanding the spread and evolution of viral pathogens is important for control strategies. Here, we developed a tiled-PCR sequencing approach for the whole-genome sequencing of ISKNV, using long read sequencing to enable field-based, real-time genomic surveillance. This work represents the first use of tiled-PCR for whole genome recovery of viruses in aquaculture, with the longest genome target (>110 kb dsDNA) to date. Our protocol was applied to field samples collected from the ISKNV outbreaks from four intensive tilapia cage culture systems across Lake Volta, between October 2018 and May 2022. Despite the low mutation rate of dsDNA viruses, 20 single nucleotide polymorphisms accumulated during the sampling period. Droplet digital PCR identified a minimum requirement of template in a sample to recover 50% of an ISKNV genome at 275 femtograms (2410 viral templates per 5 µL sequencing reaction). Overall, tiled-PCR sequencing of ISKNV provides an informative tool to assist in disease control in aquaculture.

Geographical, Temporal and Host-Species Distribution of Potentially Human-Pathogenic Group B Streptococcus in Aquaculture Species in Southeast Asia

Group B Streptococcus (GBS) is a major pathogen of humans and aquatic species. Fish have recently been recognized as the source of severe invasive foodborne GBS disease, caused by sequence type (ST) 283, in otherwise healthy adults in Southeast Asia. Thailand and Vietnam are among the major aquaculture producers in Southeast Asia, with GBS disease reported in fish as well as frogs in both countries. Still, the distribution of potentially human-pathogenic GBS in aquaculture species is poorly known. Using 35 GBS isolates from aquatic species in Thailand collected from 2007 to 2019 and 43 isolates from tilapia collected in Vietnam in 2018 and 2019, we have demonstrated that the temporal, geographical, and host-species distribution of GBS ST283 is broader than previously known, whereas the distribution of ST7 and the poikilothermic lineage of GBS are geographically restricted. The gene encoding the human GBS virulence factor C5a peptidase, scpB, was detected in aquatic ST283 from Thailand but not in ST283 from Vietnam or in ST7 from either country, mirroring current reports of GBS strains associated with human sepsis. The observed distribution of strains and virulence genes is likely to reflect a combination of spill-over, host adaptation through the gain and loss of mobile genetic elements, and current biosecurity practices. The plastic nature of the GBS genome and its importance as a human, aquatic, and potentially foodborne pathogen suggests that active surveillance of GBS presence and its evolution in aquaculture systems may be justified.

Isolation and Characterization of Aeromonas jandaei from Nile Tilapia in Lake Volta, Ghana, and Its Response to Antibiotics and Herbal Extracts

Production of Nile Tilapia Oreochromis niloticus contributes to economic growth in many countries. However, there has been a decline in its production over the years due to the influx of bacterial infections, with Aeromonas jandaei as an emerging threat. In this study, we identified and characterized A. jandaei from cage-cultured Nile Tilapia in Akosombo Stratum II of Lake Volta in Ghana and evaluated its response to commonly used antibiotics using the disc diffusion and agar well diffusion methods for herbal extracts at various concentrations (10, 30, 50, 70, and 100 mg/mL). The herbs considered included guava Psidium guajava leaf, bitter leaf Vernonia amygdalina, neem Azadirachta indica leaf, and their cocktail (GBNL in the ratio of 1:1:1). The bacterium was isolated from swab samples from the head kidneys of 27 moribund Nile Tilapia collected from nine fish farms. Samples were screened for A. jandaei by culturing and identification using morphological and molecular techniques. The bacterium isolate from fish in the study, identified as A. jandaei GH-AS II, had 92-93% identity to A. jandaei reference strains. Infection of healthy Nile Tilapia (n = 210) with the bacterium isolate showed that 1.0 × 10⁵  CFU/mL was the lethal dose causing 50% mortality. Antibiotic susceptibility testing showed that A. jandaei GH-AS II was resistant to tetracycline and ampicillin. Herbal extracts at the various concentrations inhibited the growth of the bacterium isolate, with a significant increment in the zones of inhibition with increasing concentrations of leaf extracts. However, GBNL showed prominence compared to the other extracts only at 100 mg/mL. Management of A. jandaei GH-AS II by using herbal extracts at Nile Tilapia farms in Lake Volta may be recommended since the use of antibiotics, such as tetracycline and ampicillin, may not yield the needed result.

Acanthopanax senticosus Promotes Survival of Tilapia Infected With Streptococcus iniae by Regulating the PI3K/AKT and Fatty Acid Metabolism Signaling Pathway

Streptococcus has greatly restricted the development of healthy tilapia aquaculture. As a green and efficient feed addition, Acanthopanax senticosus (APS) has been increasingly used in culture, but it is unclear whether it represents a disease-resistant feed. Genetically improved farmed tilapia (GIFT, Oreochromis niloticus) was fed with a feed supplemented with 0 (control), 0.5, 1, 2, 4, and 8‰ APS for 56 days, after which fish were injected with 5.9 × 10⁶ CFU/ml Streptococcus iniae into the abdominal cavity. At 96 h after infection, the cumulative survival of GIFT in control and 0.5‰ APS treatments was significantly lower than in other treatments; at APS supplementation rates of 1 and 2‰, serum glucose, triglycerides, and cholesterol contents were all significantly lower than in control treatment fish. Hepatic glycogen and triglyceride contents of 1‰ APS treatment fish were significantly higher than those in fish in control treatment. Transcription levels of peroxisome proliferator activated receptor α (PPAR), fatty acid synthase (FAS), and lipoprotein Lipase (LPL) genes were upregulated, and their expression levels in fish in 1, 2, and 4‰ treatments were significantly higher than those in fish in control treatment at 96 h after S. iniae infection. After 96 h of infection, the red blood cells, hemoglobin, hematocrit, and white blood cells of fish in 1‰ APS treatment were significantly lower than those of fish in 4 and 8‰ treatments; hepatic catalase activity was activated at 48 h, superoxide dismutase activity was also significantly upregulated at 96 h, and the malondialdehyde content significantly decreased. It is noted that 0.5–2‰ APS treatments significantly activated the expression of PI3K and AKT in the liver, while inhibiting the expression of Caspase-9. Therefore, feed with 1‰ APS can promote hepatic glycogen and lipid metabolism in GIFT after infection with S. iniae, which is beneficial to alleviating oxidative stress damage and cell apoptosis in liver tissue.

First detection of infectious spleen and kidney necrosis virus (ISKNV) associated with massive mortalities in farmed tilapia in Africa

In late 2018, unusual patterns of very high mortality (>50% production) were reported in intensive tilapia cage culture systems across Lake Volta in Ghana. Samples of fish and fry were collected and analysed from two affected farms between October 2018 and February 2019. Affected fish showed darkening, erratic swimming and abdominal distension with associated ascites. Histopathological observations of tissues taken from moribund fish at different farms revealed lesions indicative of viral infection. These included haematopoietic cell nuclear and cytoplasmic pleomorphism with marginalization of chromatin and fine granulation. Transmission electron microscopy showed cells containing conspicuous virions with typical iridovirus morphology, that is enveloped, with icosahedral and/or polyhedral geometries and with a diameter c.160 nm. PCR confirmation and DNA sequencing identified the virions as infectious spleen and kidney necrosis virus (ISKNV). Samples of fry and older animals were all strongly positive for the presence of the virus by qPCR. All samples tested negative for TiLV and nodavirus by qPCR. All samples collected from farms prior to the mortality event were negative for ISKNV. Follow‐up testing of fish and fry sampled from 5 additional sites in July 2019 showed all farms had fish that were PCR‐positive for ISKNV, whether there was active disease on the farm or not, demonstrating the disease was endemic to farms all over Lake Volta by that point. The results suggest that ISKNV was the cause of disease on the investigated farms and likely had a primary role in the mortality events. A common observation of coinfections with Streptococcus agalactiae and other tilapia bacterial pathogens further suggests that these may interact to cause severe pathology, particularly in larger fish. Results demonstrate that there are a range of potential threats to the sustainability of tilapia aquaculture that need to be guarded against.

Genome-Wide DNA Methylation and RNA Analysis Reveal Potential Mechanism of Resistance to Streptococcus agalactiae in GIFT Strain of Nile Tilapia (Oreochromis niloticus )

Streptococcus agalactiae is an important pathogenic bacterium causing great economic loss in Nile tilapia (Oreochromis niloticus) culture. Resistant and susceptible groups sharing the same genome showed significantly different resistance to S. agalactiae in the genetically improved farmed tilapia strain of Nile tilapia. The resistance mechanism is unclear. We determined genome-wide DNA methylation profiles in spleen of resistant and susceptible O. niloticus at 5 h postinfection with S. agalactiae using whole-genome bisulfite sequencing. The methylation status was higher in the spleen samples from resistant fish than in the susceptible group. A total of 10,177 differentially methylated regions were identified in the two groups, including 3725 differentially methylated genes (DMGs) (3129 hyper-DMGs and 596 hypo-DMGs). The RNA sequencing showed 2374 differentially expressed genes (DEGs), including 1483 upregulated and 891 downregulated. Integrated analysis showed 337 overlapping DEGs and DMGs and 82 overlapping DEGs and differentially methylated region promoters. By integrating promoter DNA methylation with gene expression, we revealed four immune-related genes (Arnt2, Nhr38, Pcdh10, and Ccdc158) as key factors in epigenetic mechanisms contributing to pathogen resistance. Our study provided systematic methylome maps to explore the epigenetic mechanism and reveal the methylation loci of pathogen resistance and identified methylation-regulated genes that are potentially involved in defense against pathogens.

Galleria mellonella as an infection model for the multi-host pathogen Streptococcus agalactiae reflects hypervirulence of strains associated with human invasive disease

Streptococcus agalactiae, or group B Streptococcus (GBS), infects diverse hosts including humans and economically important species such as cattle and fishes. In the context of human health, GBS is a major cause of neonatal infections and an emerging cause of invasive disease in adults and of foodborne disease in Southeast Asia. Here we show that GBS is able to establish a systemic infection in Galleria mellonella larvae that is associated with extensive bacterial replication and dose-dependent larval survival. This infection model is suitable for use with GBS isolates from both homeothermic and poikilothermic hosts. Hypervirulent sequence types (ST) associated with invasive human disease in neonates (ST17) or adults (ST283) show increased virulence in this model, indicating it may be useful in studying GBS virulence determinants, albeit with limitations for some host-specific virulence factors. In addition, we demonstrate that larval survival can be afforded by antibiotic treatment and so the model may also be useful in the development of novel anti-GBS strategies. The use of G. mellonella in GBS research has the potential to provide a low-cost infection model that could reduce the number of vertebrates used in the study of GBS infection.

Delineation of the pan-proteome of fish-pathogenic Streptococcus agalactiae strains using a label-free shotgun approach

BACKGROUND

Streptococcus agalactiae (GBS) is a major pathogen of Nile tilapia, a global commodity of the aquaculture sector. The aims of this study were to evaluate protein expression in the main genotypes of GBS isolated from diseased fishes in Brazil using a label-free shotgun nano-liquid chromatography-ultra definition mass spectrometry (nanoLC-UDMS^E) approach and to compare the differential abundance of proteins identified in strains isolated from GBS-infected fishes and humans.

RESULTS

A total of 1070 protein clusters were identified by nanoLC-UDMS^E in 5 fish-adapted GBS strains belonging to sequence types ST-260 and ST-927 and the non-typeable (NT) lineage and 1 human GBS strain (ST-23). A total of 1065 protein clusters corresponded to the pan-proteome of fish-adapted GBS strains; 989 of these were identified in all fish-adapted GBS strains (core proteome), and 62 were shared by at least two strains (accessory proteome). Proteins involved in the stress response and in the regulation of gene expression, metabolism and virulence were detected, reflecting the adaptive ability of fish-adapted GBS strains in response to stressor factors that affect bacterial survival in the aquatic environment and bacterial survival and multiplication inside the host cell. Measurement of protein abundance among different hosts showed that 5 and 26 proteins were exclusively found in the human- and fish-adapted GBS strains, respectively; the proteins exclusively identified in fish isolates were mainly related to virulence factors. Furthermore, 215 and 269 proteins were up- and down-regulated, respectively, in the fish-adapted GBS strains in comparison to the human isolate.

CONCLUSIONS

Our study showed that the core proteome of fish-adapted GBS strains is conserved and demonstrated high similarity of the proteins expressed by fish-adapted strains to the proteome of the human GBS strain. This high degree of proteome conservation of different STs suggests that, a monovalent vaccine may be effective against these variants.

GroEL, a novel vaccine candidate of piscine Streptococcus agalactiae identified by immunoproteome

Streptococcus agalactiae is the major etiological agent of streptococcosis, which is responsible for huge economic losses in fishery, particularly in tilapia (Oreochromis niloticus) aquaculture. A research priority to control streptococcosis is to develop vaccines, so we sought to figure out the immunogenic proteins of S. agalactiae and screen the vaccine candidates for streptococcosis in the present study. Immunoproteomics, a technique involving two-dimensional gel electrophoresis (2-DE) followed by immunoblotting and mass spectrometry (MS), was employed to investigate the immunogenic proteins of S. agalactiae THN0901. Whole-cell soluble proteins were separated using 2-DE, and the immunogenic proteins were detected by western blotting using rabbit anti-S. agalactiae sera. A total of 17 immunoreactive spots on the soluble protein profile, corresponding to 15 different proteins, were identified by MALDI-TOF/TOF MS. Among the immunogenic proteins, GroEL attracted our attention as it was demonstrated to be immunogenic and protective against other streptococci. Nevertheless, to date, there have been no published reports on the immunogenicity and protective efficacy of GroEL against piscine S. agalactiae. Therefore, recombinant GroEL (rGroEL) was expressed in Escherichia coli BL21 (DE3) and purified by affinity chromatography. Immunization of tilapia with rGroEL resulted in an increase in antibody titers and conferred protection against S. agalactiae, with the relative percentage survival of 68.61 ± 7.39%. The immunoproteome in the present study narrows the scope of vaccine candidates, and the evaluation of GroEL immunogenicity and protective efficacy shows that GroEL forms an ideal candidate molecule in subunit vaccine against S. agalactiae.

Aeromonas shuberti as a cause of multi-organ necrosis in internal organs of Nile tilapia, Oreochromis niloticus

A disease with white spots in internal organs of Nile tilapia occurred in Zhanjiang, southern China. Multiple, white nodules, 0.8-2.2 mm in diameter, were scattered throughout the liver, spleen and kidney of diseased fish. Signs of nodules reproduced after artificial infection with the isolated strain. Isolated bacteria were Gram-negative, facultative anaerobic, motile, short rod-shaped, with a length of 1.2-2.2 μm. Morphological and biochemical tests, as well as phylogenetic analysis, all strongly indicated that the isolate from tilapia is identical to Aeromonas schubertii (A. schubertii) which temporary named LF1708 strain. Antibiotic sensitivity assays showed the LF1708 is sensitive to 24 of 27 tested antibiotics. Pathogenicity test revealed that the isolate at the dose of 3.75 × 10⁶ CFU/g killed 100% of experimental tilapia within 2 days and the dose of 1 × 10⁷ CFU/g killed 100% of experimental zebrafish within 1 day. Histopathology of diseased tilapia infected with A. schubertii showed numerous necrotic lesions widely distributed in spleen, liver and kidney, and infiltration with a large number of bacteria. To our knowledge, this was the first report that associated A. schubertii with mortality in tilapia.

Microevolution of Streptococcus agalactiae ST-261 from Australia Indicates Dissemination via Imported Tilapia and Ongoing Adaptation to Marine Hosts or Environment

Streptococcus agalactiae (group B Streptococcus [GBS]) causes disease in a wide range of animals. The serotype Ib lineage is highly adapted to aquatic hosts, exhibiting substantial genome reduction compared with terrestrial conspecifics. Here, we sequence genomes from 40 GBS isolates, including 25 isolates from wild fish and captive stingrays in Australia, six local veterinary or human clinical isolates, and nine isolates from farmed tilapia in Honduras, and compared them with 42 genomes from public databases. Phylogenetic analysis based on nonrecombinant core-genome single nucleotide polymorphisms (SNPs) indicated that aquatic serotype Ib isolates from Queensland were distantly related to local veterinary and human clinical isolates. In contrast, Australian aquatic isolates are most closely related to a tilapia isolate from Israel, differing by only 63 core-genome SNPs. A consensus minimum spanning tree based on core-genome SNPs indicates the dissemination of sequence type 261 (ST-261) from an ancestral tilapia strain, which is congruent with several introductions of tilapia into Australia from Israel during the 1970s and 1980s. Pangenome analysis identified 1,440 genes as core, with the majority being dispensable or strain specific, with non-protein-coding intergenic regions (IGRs) divided among core and strain-specific genes. Aquatic serotype Ib strains have lost many virulence factors during adaptation, but six adhesins were well conserved across the aquatic isolates and might be critical for virulence in fish and for targets in vaccine development. The close relationship among recent ST-261 isolates from Ghana, the United States, and China with the Israeli tilapia isolate from 1988 implicates the global trade in tilapia seed for aquaculture in the widespread dissemination of serotype Ib fish-adapted GBS.IMPORTANCEStreptococcus agalactiae (GBS) is a significant pathogen of humans and animals. Some lineages have become adapted to particular hosts, and serotype Ib is highly specialized to fish. Here, we show that this lineage is likely to have been distributed widely by the global trade in tilapia for aquaculture, with probable introduction into Australia in the 1970s and subsequent dissemination in wild fish populations. We report here the variability in the polysaccharide capsule among this lineage but identify a cohort of common surface proteins that may be a focus of future vaccine development to reduce the biosecurity risk in international fish trade.