Characterization of Lactococcus garvieae and Streptococcus agalactiae in cultured red tilapia Oreochromis sp. in Thailand

OBJECTIVE

Acute mortality with clinical symptoms of streptococcal-like infections was observed in red tilapia Oreochromis sp. cultured in floating cages in Prachin Buri Province, Thailand, during May 2023. Herein, we identified an emerging pathogen, Lactococcus garvieae, as the etiological agent.

METHODS

After bacterial isolation from the brain and kidney of diseased fish, identification was performed using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and the VITEK 2 system. Sequencing of the 16S ribosomal RNA (rRNA) gene and phylogenetic analysis were applied to confirm bacterial species. Antimicrobial susceptibility testing was conducted. Histopathological findings in the brain, kidney, spleen, liver, and heart were evaluated.

RESULT

From 20 fish samples, L. garvieae (n = 18 isolates) and Streptococcus agalactiae (n = 2 isolates) were identified. A phylogenetic tree of the 16S rRNA gene revealed that Thai isolates of either L. garvieae or S. agalactiae clustered with reference piscine isolates from intercontinental locations. Our isolates showed resistance against quinolones while being susceptible to other antimicrobials. Histopathological changes demonstrated severe septicemic conditions, with more invasive lesions-especially in the heart and liver-being apparent in L. garvieae-infected fish compared to S. agalactiae-infected fish.

CONCLUSION

This study represents the first reported outbreak of L. garvieae with a concurrent S. agalactiae infection in farmed red tilapia in Thailand.

Development of Immersion and Oral Bivalent Nanovaccines for Streptococcosis and Columnaris Disease Prevention in Fry and Fingerling Asian Seabass (Lates calcarifer) Nursery Farms

In the present study, chitosan-based bivalent nanovaccines of S. iniae and F. covae were administered by immersion vaccination at 30 and 40 days after hatching (DAH), and the third vaccination was orally administered by feeding at 50 DAH. ELISA revealed that the levels of total IgM and specific IgM to S. iniae and F. covae were significantly elevated in all vaccinated groups at 10, 20, and 30 days after vaccination (DAV). A qRT-PCR analysis of immune-related genes revealed significantly higher IgT expression in the vaccinated groups compared to the control group, as revealed by 44-100-fold changes in the vaccinated groups compared to the control (p < 0.001) at every tested time point after vaccination. All vaccinated groups expressed IgM, MHCIIα, and TCRα at significantly higher levels than the control group at 10 and/or 20 DAV (p < 0.05). In the S. iniae challenge tests, the survival of vaccinated groups ranged from 62.15 ± 2.11 to 75.70 ± 3.36%, which significantly differed from that of the control group (44.44 ± 1.92%). Similarly, all vaccinated groups showed higher survival rates of 68.89 ± 3.85 to 77.78 ± 5.09% during F. covae challenge than the control groups (50.00 ± 3.33%) (p < 0.05).

Susceptibility Profile and Epidemiological Cut-Off Values Are Influenced by Serotype in Fish Pathogenic Streptococcus agalactiae

Streptococcus agalactiae is a major health concern in tilapia farming worldwide. In contrast to the availability of susceptibility profile results, interpretative criteria for disk diffusion assays and the influence of serotypes on resistance profiles are not available. To address this, sixty isolates (thirty of each serotype, Ib and III) were evaluated using the disk diffusion assay against six antibiotics, and the epidemiological cut-off value (ECV) was calculated. All the isolates were classified as non-wild type (NWT) for sulfamethoxazole (SUT) and norfloxacin (NOR). The inhibition zones for oxytetracycline (OXY) and doxycycline (DOX) were largely distinct; all serotype Ib and III isolates were classified as wild-type (WT) and NWT, respectively. The results for serotype III of fish group B Streptococcus (GBS) were comparable to the NWT tetracycline profile of human GBS available in EUCAST, suggesting the presence of resistance mechanisms in these fish isolates. The calculation of the cut-off wild type (COWT) values for OXY and DOX was appropriate for both serotypes. Differences between the distribution of florfenicol (FLO) and amoxicillin (AMO) were found, and we attribute this to the faster growth rate of serotype III, which promotes smaller inhibition zones. Therefore, using separate COWT for each serotype is necessary. In conclusion, the serotype of fish GBS affects its susceptibility profile, and it is recommended to use serotype-specific COWT values as interpretative criteria for disk diffusion assays against FLO and AMO.

Main Causes of Death in Piglets from Different Brazilian Nursery Farms Based on Clinical, Microbiological, and Pathological Aspects

Necropsies can reveal herd problems or comorbidities that can lead to management corrections, improvements in animal performance, and better decision making. Furthermore, the pattern and causes of mortality might differ when different systems are evaluated. The present study was conducted to establish the main causes of death in nursery pigs from different systems in Brazil, as well as the clinical, microbiological, and pathological aspects of these mortalities. Eighteen nurseries were analyzed (a total of 120,243 housed piglets), and 557 necropsies were performed. Streptococcus suis infection was the most prevalent cause of death (21.2%), followed by bacterial polyserositis (16.7%), chronic atrophic enteritis (13.5%), salmonellosis (8.8%), pneumonia (8.6%), and colibacillosis (6.1%). The increase in mortality rate in individual nurseries and, consequently, in the diagnoses was commonly associated with disease outbreaks. Infectious diseases constituted the largest portion of the diagnoses, making a great opportunity for improving production rates in herds. Moreover, the extensive range of observed diagnoses highlights the importance of conducting preliminary diagnostic investigations based on necropsy to determine the causes of death. This approach allows for the direction of complementary tests, which can diagnose agents with greater specificity. As a result, this allows for the implementation of more effective prevention and control strategies.

Improving the diagnosis of Streptococcus iniae using a novel probe-based qPCR assay combined with an enrichment step

Streptococcus iniae is a bacterial pathogen that causes streptococcosis, leading to significant losses in fish aquaculture globally. This study reported a newly developed probe-based quantitative polymerase chain reaction (qPCR) method for the detection of S. iniae. The primers and probes were designed to target the lactate oxidase gene. The optimized method demonstrated a detection limit of 20 copies per reaction and was specific to S. iniae, as evidenced by no cross-reactivity when assayed against genetic materials extracted from 23 known aquatic animal pathogens, and fish samples infected with Streptococcus agalactiae or Streptococcus dysgalactiae. To validate the newly developed qPCR protocol with field samples, fish specimens were systematically investigated following the Food and Agriculture Organization of the United Nations & Network of Aquaculture Centres in Asia-Pacific three diagnostic levels approach, which integrated basic and advanced techniques for disease diagnosis, including observation of gross signs (level I), bacterial isolation (level II), qPCR and 16S rDNA sequencing (level III). The result showed that 7/7 affected farms (three Asian seabass farms and four tilapia farms) experiencing clinical signs of streptococcosis were diagnosed positive for S. iniae. qPCR assays using DNA extracted directly from fish tissue detected S. iniae in 11 out of 36 fish samples (30.6%), while 24 out of 36 samples (66.7%) tested positive after an enrichment step, including apparently healthy fish from affected farms. Bacterial isolation of S. iniae was only successful in a proportion of clinically diseased fish but not in healthy-looking fish from the same farm. Overall, the newly developed qPCR protocol combined with enrichment would be a useful tool for the diagnosis and surveillance of S. iniae infections in fish populations, thereby aiding in the disease control and prevention.

Immunogenicity and Efficacy of a Feed-Based Bivalent Vaccine against Streptococcosis and Motile Aeromonad Septicemia in Red Hybrid Tilapia (Oreochromis sp.)

Streptococcosis and motile Aeromonad septicemia (MAS) are the main bacterial diseases in tilapia culture worldwide, causing significant economic losses. Vaccination is an effective method of preventing diseases and contributes to economic sustainability. This study investigated the immuno-protective efficacy of a newly developed feed-based bivalent vaccine against streptococcosis and MAS in red hybrid tilapia. The feed-based bivalent vaccine pellet was developed by incorporating the formalin-killed S. agalactiae and A. hydrophila antigens into a commercial feed pellet with palm oil as the adjuvant. The bivalent vaccine was subjected to feed quality analyses. For immunological analyses, 900 fish (12.94 ± 0.46 g) were divided into two treatment groups in triplicate. Fish in Group 1 were unvaccinated (control), while those in Group 2 were vaccinated with the bivalent vaccine. The bivalent vaccine was delivered orally at 5% of the fish's body weight for three consecutive days on week 0, followed by boosters on weeks 2 and 6. Lysozyme and enzyme-linked immunosorbent assays (ELISAs) on serum, gut lavage, and skin mucus were performed every week for 16 weeks. Lysozyme activity in vaccinated fish was significantly (p ≤ 0.05) higher than in unvaccinated fish following vaccination. Similarly, the IgM antibody levels of vaccinated fish were significantly (p ≤ 0.05) higher after vaccination. The bivalent vaccine provided high protective efficacy against S. agalactiae (80.00 ± 10.00%) and A. hydrophila (90.00 ± 10.00%) and partial cross-protective efficacy against S. iniae (63.33 ± 5.77%) and A. veronii (60.00 ± 10.00%). During the challenge test, fewer clinical and gross lesions were observed in vaccinated fish compared with unvaccinated fish. Histopathological assessment showed less severe pathological changes in selected organs than the unvaccinated fish. This study showed that vaccination with a feed-based bivalent vaccine improves immunological responses in red hybrid tilapia, and thus protects against streptococcosis and MAS.

Immune Activation Following Vaccination of Streptococcus iniae Bacterin in Asian Seabass (Lates calcarifer, Bloch 1790)

Juvenile Asian seabass (Lates calcarifer) (body weight 10 ± 0.7 g) were intraperitoneally injected with 10¹² CFU fish^-1 of formalin-killed Streptococcus iniae. The protective efficacy of the vaccine on survival and infection rate was assessed upon challenge at 4, 8, 12, 20, and 28 weeks post-vaccination. The results revealed that the challenged vaccinated fish showed no mortality at all time points, and the control fish presented 10-43.33% mortality. The infection rate at 2 weeks post-challenge was 0-13.33% in the vaccinated fish and 30-82.35% in the control group. At 8 weeks post-vaccination, the vaccinated fish showed comparable ELISA antibody levels with the control; however, the antibody levels of the vaccinated fish increased significantly after the challenge (p < 0.05), suggesting the presence of an adaptive response. Innate immune genes, including MHC I, MHC II, IL-1β, IL-4/13B, and IL-10, were significantly upregulated at 12 h post-challenge in the vaccinated fish but not in the control. In summary, vaccination with S. iniae bacterin provided substantial protection by stimulating the innate and specific immune responses of Asian seabass against S. iniae infection.

Suppression of Streptococcosis and Modulation of the Gut Bacteriome in Nile Tilapia (Oreochromis niloticus) by the Marine Sediment Bacteria Bacillus haynesii and Advenella mimigardefordensis

Streptococcosis is one of the major threats to Nile tilapia (Oreochromis niloticus) in most regions of the world. Recently, Enterococcus faecalis has been widely reported to be involved in streptococcosis in O. niloticus in Asia and Africa. This study aimed to isolate beneficial marine bacteria to evaluate their effects on growth, hematological parameters, nonspecific immunity, the gut bacteriome, and streptococcosis prevention efficacy in O. niloticus. A total of 36 marine soil bacteria were isolated, and in vitro screening was conducted to determine their antibacterial activities against fish pathogens. Two antagonistic bacteria were identified based on 16S rRNA gene sequencing, Bacillus haynesii CD223 and Advenella mimigardefordensis SM421. These bacteria were incorporated into fish feed and fed to O. niloticus for 90 days. The application of these strains via incorporation into fish feed significantly promoted growth, improved hematological parameters and immunoglobulin M (IgM) levels, modulated the gut bacteriome by reducing the load of pathogenic Enterococcus spp., and developed disease prevention efficacy in O. niloticus. Furthermore, in vivo assays revealed that the inclusion of extracellular products (ECPs) (at 250 μg mL-1) of CD223 and SM421 with feed significantly enhanced the rate of survival (100%) of O. niloticus from streptococcosis compared to the controls (only 30%). The ECPs of these bacteria also prevented 90 to 100% of fish from developing streptococcosis. These strains could be promising for safe use in O. niloticus farming to prevent and control the emergence of streptococcosis caused by E. faecalis. IMPORTANCE Nile tilapia (Oreochromis niloticus) is one of the most economically important cultured fish species throughout the world. Streptococcosis is a significant threat to global Nile tilapia farming. Enterococcus faecalis has recently emerged as an important pathogen of streptococcosis in Asia and Africa. The application of antibiotics and probiotics and vaccination are the major ways to combat streptococcosis. However, the extensive use of antibiotics leads to the development of antibiotic resistance in pathogenic as well as environmental bacteria, which is a great threat to public health. There is no study on preventing streptococcosis caused by E. faecalis using beneficial bacteria. For the first time, the present study demonstrated that two marine bacteria, Bacillus haynesii strain CD223 and Advenella mimigardefordensis strain SM421, have great potential for controlling streptococcosis in Nile tilapia. These bacteria also enhanced the growth, improved hematological parameters and IgM levels, and positively modulated the gut bacteriome of Nile tilapia.

Trial Evaluation of Protection and Immunogenicity of Piscine Bivalent Streptococcal Vaccine: From the Lab to the Farms

Streptococcosis is one of the major diseases that causes devastation to farmed fish, leading to significant economic losses all around the world. Currently, two serotypes of Streptococcus agalactiae, serotype Ia and III, have been identified as virulent strains and major causative agents of the disease in farmed Nile tilapia (Oreochromis niloticus Linn.) in Thailand. Upon inactivated vaccine development, monovalent inactivated whole-cell vaccines demonstrated high specific antibody production against homologous serotypes and limited production with heterologous serotypes. However, for higher efficacy, a bivalent streptococcal vaccine was designed to maximize protective immunity to both serotypes. Interestingly, our bivalent vaccine could successfully induce specific antibody production against both serotypes with similar levels, and the response could extend over the 8 weeks of the experimental period. Evaluation of vaccines in the laboratory scale revealed relative percent survival (RPS) of vaccinated tilapia to serotype Ia (81.2 ± 9.4%) and serotype III (72.2 ± 4.8%), respectively. The efficacy of the bivalent vaccine showed significant RPS higher than the monovalent vaccine (p < 0.05) at 30 days, and the protection of all those vaccines was reduced thereafter. Evaluation of the vaccine in a farm trial in different locations in Thailand revealed the efficacy of the bivalent vaccine in increasing the production yield by greater than 80% in all tested farms in 2015 and 2021. Taken together, this study affirms the efficacy of the bivalent streptococcal vaccine in the prevention of streptococcus disease in Nile tilapia, which could be used in different areas. This vaccine development could be effectively applied in the tilapia culture industry.

A Novel Efficient Piscine Oral Nano-Vaccine Delivery System: Modified Halloysite Nanotubes (HNTs) Preventing Streptococcosis Disease in Tilapia (Oreochromis sp.)

Generally, the injection method is recommended as the best efficient method for vaccine applications in fish. However, labor-intensive and difficult injection for certain fish sizes is always considered as a limitation to aquatic animals. To demonstrate the effectiveness of a novel oral delivery system for the piscine vaccine with nano-delivery made from nano clay, halloysite nanotubes (HNTs) and their modified forms were loaded with killed vaccines, and we determined the ability of the system in releasing vaccines in a mimic digestive system. The efficaciousness of the oral piscine vaccine nano-delivery system was evaluated for its level of antibody production and for the level of disease prevention in tilapia. Herein, unmodified HNTs (H) and modified HNTs [HNT-Chitosan (HC), HNT-APTES (HA) and HNT-APTES-Chitosan (HAC)] successfully harbored streptococcal bivalent vaccine with inactivated S. agalactiae, designated as HF, HAF, HCF and HACF. The releasing of the loading antigens in the mimic digestive tract demonstrated a diverse pattern of protein releasing depending on the types of HNTs. Remarkably, HCF could properly release loading antigens with relevance to the increasing pH buffer. The oral vaccines revealed the greatest elevation of specific antibodies to S. agalactiae serotype Ia in HCF orally administered fish and to some extent in serotype III. The efficacy of streptococcal disease protection was determined by continually feeding with HF-, HAF-, HCF- and HACF-coated feed pellets for 7 days in the 1st and 3rd week. HCF showed significant RPS (75.00 ± 10.83%) among the other tested groups. Interestingly, the HCF-treated group exhibited noticeable efficacy similar to the bivalent-vaccine-injected group (RPS 81.25 ± 0.00%). This novel nano-delivery system for the fish vaccine was successfully developed and exhibited appropriated immune stimulation and promised disease prevention through oral administration. This delivery system can greatly support animals’ immune stimulation, which conquers the limitation in vaccine applications in aquaculture systems. Moreover, this delivery system can be applied to carrying diverse types of biologics, including DNA, RNA and subunit protein vaccines.

Isolation and Genetic Characterization of Streptococcus iniae Virulence Factors in Adriatic Sturgeon (Acipenser naccarii)

The first case of infection of Streptococcus iniae in Adriatic sturgeon (Acipenser naccarii) was recently reported in a raceway system located in Northern Italy. A second episode of infection in sturgeons with absence of mortality and evident clinical signs, was registered in November 2020 in the same farm and is reported in this study. Histopathological changes observed in infected organs are described. The strains isolated in the two episodes were compared using molecular analysis based on PCR, phylogeny and virulence factors analysis. Not all the major virulence factors were detected for the two strains; in particular the strains 78697, isolated in November, lacks cpsD, compared to the strains 64844, isolated in September. Moreover, genetic variations were reported for lctO and pmg genes. These findings let us hypothesize a different virulence of the strains in accordance with clinical findings related to the sturgeons.

Research Advances on Tilapia Streptococcosis

Streptococcus agalactiae, often referred to as group B streptococci (GBS), is a severe pathogen that can infect humans as well as other animals, including tilapia, which is extremely popular in commercial aquaculture. This pathogen causes enormous pecuniary loss, and typical symptoms of streptococcosis—the disease caused by S. agalactiae—include abnormal behavior, exophthalmos, and meningitis, among others. Multiple studies have examined virulence factors associated with S. agalactiae infection, and vaccines were explored, including studies of subunit vaccines. Known virulence factors include capsular polysaccharide (CPS), hemolysin, Christie-Atkins-Munch-Peterson (CAMP) factor, hyaluronidase (HAase), superoxide dismutase (SOD), and serine-threonine protein kinase (STPK), and effective vaccine antigens reported to date include GapA, Sip, OCT, PGK, FbsA, and EF-Tu. In this review, I summarize findings from several studies about the etiology, pathology, virulence factors, and vaccine prospects for S. agalactiae. I end by considering which research areas are likely to yield success in the prevention and treatment of tilapia streptococcosis.

Vaccine Efficacy of a Newly Developed Feed-Based Whole-Cell Polyvalent Vaccine against Vibriosis, Streptococcosis and Motile Aeromonad Septicemia in Asian Seabass, Lates calcarifer

Multiple infections of several bacterial species are often observed under natural farm conditions. The infections would cause a much more significant loss compared to a single infectious agent. Vaccination is an essential strategy to prevent diseases in aquaculture, and oral vaccination has been proposed as a promising technique since it requires no handling of the fish and is easy to perform. This research attempts to develop and evaluate a potential feed-based polyvalent vaccine that can be used to treat multiple infections by Vibrios spp., Streptococcus agalactiae, and Aeromonas hydrophila, simultaneously. The oral polyvalent vaccine was prepared by mixing formalin-killed vaccine of V. harveyi, S. agalactiae, and A. hydrophila strains with commercial feed pellet, and palm oil as an adjuvant was added to improve their antigenicity. Thereafter, a vaccinated feed pellet was tested for feed quality analysis in terms of feed stability in water, proximate nutrient analysis, and palatability, safety, and growth performance using Asian seabass, Lates calcarifer as a fish host model. For immune response analysis, a total of 300 Asian seabass juveniles (15.8 ± 2.6 g) were divided into two groups in triplicate. Fish of group 1 were not vaccinated, while group 2 was vaccinated with the feed-based polyvalent vaccine. Vaccinations were carried out on days 0 and 14 with oral administration of the feed containing the bacterin at 5% body weight. Samples of serum for antibody and lysozyme study and the spleen and gut for gene expression analysis were collected at 7-day intervals for 6 weeks. Its efficacy in protecting fish was evaluated in aquarium challenge. Following vaccination by the polyvalent feed-based vaccine, IgM antibody levels showed a significant (p < 0.05) increase in serum against Vibrio harveyi, Aeromonas hydrophila, and Streptococcus agalactiae and reached the peak at week 3, 5, and 6, respectively. The high-stimulated antibody in the serum remained significantly higher than the control (p < 0.05) at the end of the 6 weeks vaccination trial. Not only that, but the serum lysozyme level was also increased significantly at week 4 (p < 0.05) as compared to the control treatment. The immune-related gene, dendritic cells, C3, Chemokine ligand 4 (CCL4), and major histocompatibility complex class I (MHC I) showed significantly higher expression (p < 0.05) after the fish were vaccinated with the oral vaccine. In the aquarium challenge, the vaccine provided a relative percentage survival of 75 ± 7.1%, 80 ± 0.0%, and 80 ± 0.0% after challenge with V. harveyi, A. hydrophila, and S. agalactiae, respectively. Combining our results demonstrate that the feed-based polyvalent vaccine could elicit significant innate and adaptive immunological responses, and this offers an opportunity for a comprehensive immunization against vibriosis, streptococcosis, and motile aeromonad septicemia in Asian seabass, Lates calcarifer. Nevertheless, this newly developed feed-based polyvalent vaccination can be a promising technique for effective and large-scale fish immunization in the aquaculture industry shortly.

Meningitis Caused by Streptococcus agalactiae in Nile Tilapia (Oreochromis niloticus): Infection and Inflammatory Response

Simple Summary

Streptococcus agalactiae (group B Streptococcus, GBS) is the main pathological agent in meningitis in Nile tilapia (Oreochromis niloticus). In this study, we describe the mechanism of infection and the immune response in the brain tissue of experimentally infected tilapia. This study understanding of the pathophysiology of meningitis in this species and bring the possibility of using tilapia as a study model for meningitis.

Abstract

Streptococcus agalactiae (Sta) of Lancefield group B is the primary etiological agent of bacterial meningitis in Nile tilapia and newborn humans. Thus, the study of this disease is of fundamental importance for aquaculture and human medicine. Additionally, elucidation of the mechanisms involved in the host–pathogenic response is important for the success of new therapies. In the present study, we elucidated important aspects of the innate immune response in the brain tissue of Nile tilapia (Oreochromis niloticus) infected by Sta. The neuroinflammatory process in the meninges started with the migration of MHC class II and CD68 + cells, production of TNF-alpha, and the effective immune response to Sta was mediated by the increased iNOs+. In conclusion, the present study brings a partial understanding of the pathophysiological and neuroinflammatory mechanisms in meningitis in Sta infected tilapia, enabling important advances in the therapy of this disease as well as the possibility of using this biological model to understand human meningitis.

Cytotoxicity of Streptococcus agalactiae secretory protein on tilapia cultured cells

Streptococcus agalactiae secrete virulence factors believed to be able of killing host tissues, especially under elevated water temperature. A direct effect of S. agalactiae secretory products on tilapia cells was tested on the tilapia kidney (TK-1) cell culture. The bacteria were cultured under four different temperature levels: 22, 29, 32 and 37°C; the cell-free portion was processed through SDS-PAGE; and distinct bands were identified by LC-MS/MS. At least, three virulence factors were identified, Bsp, PcsB and CAMP factor, with increasing levels as the cultured temperature rose. Expressions of bsp, pcsB and cfb were also up-regulated with the rising of the temperature in S. agalactiae culture. The supernatant from the bacteria cultured under specified temperatures was added into TK-1 cell-cultured wells. Morphological damage and mortality of the cultured cells, as determined by MTT method, were increased progressively from the supernatant treatment according to the rise of temperature in S. agalactiae culture. This study suggests that the production of the three virulence factors of S. agalactiae reported herein is temperature-dependent, and it is likely that CAMP factor directly kills the TK-1 cells since the other two types of protein are involved in S. agalactiae cell division and the bacterial adherence to host tissues.

In silico Prediction of New Drug Candidates Against the Multidrug-Resistant and Potentially Zoonotic Fish Pathogen Serotype III Streptococcus agalactiae

Streptococcus agalactiae is an invasive multi-host pathogen that causes invasive diseases mainly in newborns, elderly, and individuals with underlying health complications. In fish, S. agalactiae causes streptococcosis, which is characterized by septicemia and neurological signs, and leads to great economic losses to the fish farming industry worldwide. These bacteria can be classified into different serotypes based on capsular antigens, and into different sequence types (ST) based on multilocus sequence typing (MLST). In 2015, serotype III ST283 was identified to be associated with a foodborne invasive disease in non-pregnant immunocompetent humans in Singapore, and the infection was related to raw fish consumption. In addition, a serotype III strain isolated from tilapia in Brazil has been reported to be resistant to five antibiotic classes. This specific serotype can serve as a reservoir of resistance genes and pose a serious threat to public health. Thus, new approaches for the control and treatment of S. agalactiae infections are needed. In the present study, 24 S. agalactiae serotype III complete genomes, isolated from human and fish hosts, were compared. The core genome was identified, and, using bioinformatics tools and subtractive criteria, five proteins were identified as potential drug targets. Furthermore, 5,008 drug-like natural compounds were virtually screened against the identified targets. The ligands with the best binding properties are suggested for further in vitro and in vivo analysis.

Quantitative detection of streptococcosis infection in dead samples of Nile Tilapia (Oreochromis niloticus)

AIMS

The aims of the study were to evaluate whether epidemic strains of streptococcosis infected tilapia can be isolated and identified from dead fish for epidemiological investigation.

METHODS AND RESULTS

Firstly, tilapias were inoculated with a lethal dose (1 × 10⁸  CFU per fish) of Streptococcus agalactiae and brain tissues were harvested for bacteriological examination and qPCR assay 3, 12, 24 and 48 h postdeath. Streptococcus agalactiae was the only dominant bacterium cultivated on the brain heart infusion (BHI) plate and the bacterial load was about 10⁷  CFU per mg. Secondly, tilapia were killed via ice water shock and immersed either in an aquarium containing 2·27 × 10⁴  CFU per ml S. agalactiae or in a pond with streptococcosis outbreak. Streptococcus agalactiae failed to grow on the BHI plate but were identified (<6 × 10²  CFU per mg) via qPCR assay. Finally, an epidemiological investigation of streptococcosis was conducted in the main tilapia breeding areas of South China. A total of 387 tilapia samples were collected including 24 suspected healthy, 35 moribund and 328 dead fish. The achieved detection rates were 0, 100 and 94·82% via bacteriological examination, and 0, 100 and 98·78% via qPCR assay respectively. The concentration of S. agalactiae in brain tissues ranged between 10⁵ and 10⁷  CFU per mg.

CONCLUSIONS

Streptococcus agalactiae can survive for 48 h in the brain of dead fish. Dead tilapia can be a useful alternative for epidemiological investigation when the diagnostic analysis of moribund fish is unavailable or impractical.

SIGNIFICANCE AND IMPACT OF THE STUDY

This detection method expands the sampling range, reduces the difficulty of sample collection and improves efficiency. Consequently, this method provides an alternative for epidemiological investigation of tilapia streptococcosis.

Fatal Infection in a Wild Sandbar Shark (Carcharhinus plumbeus), Caused by Streptococcus agalactiae, Type Ia-ST7

Simple Summary

Streptococcus agalactiae (group B Streptococcus, GBS) is a major fish pathogenic bacterium. In this study, we describe a fatal infection of a stranded wild sandbar shark (Carcharhinus plumbeus) by a post-mortem examination, histopathology, classical bacteriology and advanced molecular methods. The bacterial agent was characterized as S. agalactiae, type Ia-ST7.

Abstract

Streptococcus agalactiae is one of the most important fish pathogenic bacteria as it is responsible for epizootic mortalities in both wild and farmed species. S. agalactiae is also known as a zoonotic agent. In July 2018, a stranded wild sandbar shark (Carcharhinus plumbeus), one of the most common shark species in the Mediterranean Sea, was found moribund on the seashore next to Netanya, Israel, and died a few hours later. A post-mortem examination, histopathology, classical bacteriology and advanced molecular techniques revealed a bacterial infection caused by S. agalactiae, type Ia-ST7. Available sequences publicly accessible databases and phylogenetic analysis suggest that the S. agalactiae isolated in this case is closely related to fish and human isolates. To the best of our knowledge, this is the first description of a fatal streptococcosis in sandbar sharks.

Development and evaluation of a quantitative polymerase chain reaction for aquatic Streptococcus agalactiae based on the groEL gene

AIMS

The aim of this study was to develop a TaqMan quantitative polymerase chain reaction (qPCR), based on the Streptococcus agalactiae groEL gene, to specifically quantify levels of bacteria within samples derived from aquatic sources, particularly aquaculture. Enumeration of bacteria by qPCR was compared with culture-based methods.

METHODS AND RESULTS

The qPCR was sensitive to 33 isolates of S. agalactiae, representing 11 clonal complexes from aquatic, bovine and human hosts. The specificity of the assay was 92·5% at a threshold C_q value of 35. No cross-reaction with Streptococcus iniae was noted and of the 22 comparator species screened to test assay specificity, Streptococcus porcinus had a C_q value of 33·7 S, while Streptococcus gallolyticus subsp. macedonicus and Streptococcus ictaluri had one replicate value above the C_q threshold of 35 (34·5 and 34·4 respectively), while only S. agalactiae were detected with a C_q value of 30. The limit of detection of the assay was 1·7 copies per µl at C_q 35. Discrepancies between molecular and culture-based methods of enumeration were noted.

CONCLUSIONS

The qPCR was able to detect a diverse range of S. agalactiae isolates from different clonal complexes (CCs) and could distinguish between S. agalactiae and closely related species, notably S. iniae. The results suggest that a C_q 30 would be a very meaningful cut-off, allowing the detection of infected fish while ruling out all false positives.

SIGNIFICANCE AND IMPACT OF THE STUDY

This rapid and sensitive qPCR assay is useful to quantify DNA copy number in the laboratory and could prove useful for detecting low levels of S. agalactiae in aquaculture systems, including Oreochromis niloticus culture.

Identification and typing of Vagococcus salmoninarum using genomic and proteomic techniques

This study reports on the characterization of Vagococcus salmoninarum using phenotypic, serological, antigenic, genetic and proteomic methods. All strains of V. salmoninarum were resistant to most of the antimicrobials tested, and only 10% of strains were sensitive to florfenicol. Serological analysis demonstrated a high antigenic homogeneity within the species. No cross-reaction was detected with other fish pathogenic species causing streptococcosis (Lactococcus garvieae, Streptococcus parauberis, Streptococcus iniae, Streptococcus agalactiae, Carnobacterium maltaromaticum) using serum against V. salmoninarum CECT 5810. Electrophoretic analysis of cell surface proteins and immunoblot supported the antigenic homogeneity within V. salmoninarum strains. Moreover, limited diversity was detected using genomic (RAPD, ERIC-PCR and REP-PCR) and MALDI-TOF-MS analyses. The phenotypic, genomic and proteomic methods tested allowed the rapid differentiation of V. salmoninarum from the other species causing streptococcosis. However, MALDI-TOF-MS is the most promising method for typing and characterization of V. salmoninarum.

Epizootics of Streptococcus agalactiae infection in captive rays from Queensland, Australia

The aim of this study was to describe two epizootics of high mortalities from infection with Streptococcus agalactiae, occurring in captive rays held in a marine display aquarium in south-east Queensland, Australia, in 2009 and 2010. Five different species of rays were affected, including mangrove whiprays (Himantura granulata), estuary rays (Dasyatis fluviorum), eastern shovelnose rays (Aptychotrema rostrata), white-spotted eagle rays (Aetobatus narinari) and blue-spotted mask rays (Neotrygon kuhlii). This report describes the history of both epizootics including collection, quarantine and husbandry of rays, the disease epizootics, clinico-pathological features of the disease, antimicrobial therapy, autogenous vaccine production, and laboratory studies including clinical and histopathology, bacteriology, PCR, molecular serotyping and sequencing of the bacterium S. agalactiae.

Natural coinfection by Streptococcus agalactiae and Francisella noatunensis subsp. orientalis in farmed Nile tilapia (Oreochromis niloticus L.)

Streptococcus agalactiae and Francisella noatunensis subsp. orientalis (Fno) are important pathogens for farm-raised tilapia worldwide. There are no reports of coinfection caused by S. agalactiae and Fno in fish. This study aimed to determine the aetiology of atypical mortalities in a cage farm of Nile tilapia and to characterize the genetic diversity of the isolates. Fifty-two fish were sampled and subjected to parasitological and bacteriological examination. The S. agalactiae and Fno isolates were genotyped using MLST and REP-PCR, respectively. Whole-genome sequencing was performed to confirm the MLST results. Seven fish were shown coinfected by S. agalactiae and Fno. Chronic hypoxia and a reduction in the water temperature were determined as risk factors for coinfection. Fno isolates were shown clonally related in REP-PCR. The MLST analysis revealed that the S. agalactiae isolates from seven coinfected fish were negative for the glcK gene; however, these were determined to be members of clonal complex CC-552. This is the first description of coinfection by S. agalactiae and Fno in farm-raised Nile tilapia. The coinfection was predisposed by chronic hypoxia and was caused by the main genotypes of S. agalactiae and Fno reported in Brazil. Finally, a new S. agalactiae genotype with glcK gene partially deleted was described.