Rapid identification of Streptococcus iniae by specific PCR assay utilizing genetic markers in ITS rDNA

Streptococcus iniae in aquaculture: a review of pathogenesis, virulence, and antibiotic resistance

One of the main challenges in aquaculture is pathogenic bacterial control. Streptococcus iniae stands out for its ability to cause high mortality rates in populations of commercially important fish populations and its recent recognition as an emerging zoonotic pathogen. The rise in identifying over 80 strains some displaying antibiotic resistance coupled with the emerging occurrence of infections in marine mammal species and wild fish underscores the urgent need of understanding pathogenesis, virulence and drug resistance mechanisms of this bacterium. This understanding is crucial to ensure effective control strategies. In this context, the present review conducts a bibliometric analysis to examine research trends related to S. iniae, extending into the mechanisms of infection, virulence, drug resistance and control strategies, whose relevance is highlighted on vaccines and probiotics to strengthen the host immune system. Despite the advances in this field, the need for developing more efficient identification methods is evident, since they constitute the basis for accurate diagnosis and treatment.

Bacteriophage endolysin treatment for systemic infection of Streptococcus iniae in hybrid striped bass

Streptococcus iniae, a zoonotic Gram-positive pathogen, poses a threat to finfish aquaculture, causing streptococcosis with an annual economic impact exceeding $150 million globally. As aquaculture trends shift towards recirculating systems, the potential for horizontal transmission of S. iniae among fish intensifies. Current vaccine development provides only short-term protection, driving the widespread use of antibiotics like florfenicol. However, this practice raises environmental concerns and potentially contributes to antibiotic resistance. Thus, alternative strategies are urgently needed. Endolysin therapy, derived from bacteriophages, employs hydrolytic endolysin enzymes that target bacterial peptidoglycan cell walls. This study assesses three synthetic endolysins (PlyGBS 90-1, PlyGBS 90-8, and ClyX-2) alongside the antibiotic carbenicillin in treating S. iniae-infected hybrid striped bass (HSB). Results demonstrate that ClyX-2 exhibits remarkable bacteriolytic potency, with lytic activity detected at concentrations as low as ∼15 μg/mL, approximately 8-fold more potent than the PlyGBS derivatives. In therapeutic effectiveness assessments, both carbenicillin and ClyX-2 treatments achieved significantly higher survival rates (85 % and 95 %, respectively) compared to placebo and PlyGBS-based endolysin treatments. Importantly, no statistical differences were observed between ClyX-2 and carbenicillin treatments. This highlights ClyX-2 as a promising alternative for combating S. iniae infections in aquaculture, offering potent bacteriolytic activity and high survival rates.

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.

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.

Efficacy of Feed-Based Formalin-Killed Vaccine of Streptococcus iniae Stimulates the Gut-Associated Lymphoid Tissues and Immune Response of Red Hybrid Tilapia

Red hybrid tilapia were fed a formalin-killed oral Streptococcus iniae vaccine (FKV) in the present study was assessed. Three hundred Red hybrid tilapia 80 ± 10 g were divided into five groups (1A, 1B, 2A, 2B, and Cx), each consisting of 60 fish. Fish from Groups 1A, 1B, 2A, and 2B were fed with FKV over different periods of administration, while Group 2B was the only group of fish to receive an oral booster vaccination on day 14- and 21-days post-vaccination (dpv). Group Cx was fed with normal pellets containing no vaccine as a control group. At four weeks post-vaccination (wpv), all fish were experimentally infected with S. iniae. Groups 2A and 2B had the lowest level of mortalities following vaccination (45% and 30%, respectively) compared to Groups 1A and 1B (80% and 55%, respectively), while the level of mortalities in Group Cx was 100%. All vaccinated groups showed a significant increase in anti-S. iniae IgM levels (p < 0.05) in serum, mucus, and gut-lavage, while Group Cx did not (p > 0.05) and all fish in this group died by five weeks post-infection. In conclusion, fish fed with the S. iniae FKV had a greater level of protection against S. iniae, with increased specific antibody response to the vaccine and there was also evidence of GALT stimulation by the vaccine.

Development of a real-time PCR assay for detection and quantification of Streptococcus iniae using the lactate permease gene

The aim of this study is the development and evaluation of a rapid and accurate quantitative PCR (qPCR)-based protocol for detection of zoonotic pathogen Streptococcus iniae in bacterial cultures and tissues of diseased fish. For this purpose, the lactate permease-encoding (lldY) gene was selected as a target for the design of S. iniae-specific primers based on comparative genomic analysis using 45 sequences retrieved from NCBI genome database. Specificity and applicability of these primers were tested using 115 bacterial strains and fish tissues infected with S. iniae. Sensitivity, reproducibility and efficiency of qPCR assay were also determined. The developed qPCR assay showed 100% specificity with pure bacterial cultures or DNA extracted from S. iniae or tissues of fish infected with the bacterium. The method has high sensitivity with a detection limit of 1.12 × 10¹ amplicon copies per assay (equivalent to 2 × 10^-9  ng/µl) using bacterial DNA and of 1.44 × 10¹ gene copies in tissues of fish infected with S. iniae. In conclusion, this qPCR protocol provides an accurate and sensitive alternative for the identification of S. iniae and its detection on fish tissues that can be implemented as a routine tool in microbiological laboratories.

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.

Multilocus sequence analysis of diverse Streptococcus iniae isolates indicates an underlying genetic basis for phenotypic heterogeneity

Streptococcus iniae is a Gram-positive, opportunistically zoonotic bacterium infective to a wide variety of farmed and wild fish species worldwide. Outbreaks in wild fish can have detrimental environmental and cultural impacts, and mortality events in aquaculture can result in significant economic losses. As an emerging or re-emerging pathogen of global significance, understanding the coalescing factors contributing to piscine streptococcosis is crucial for developing strategies to control infections. Intraspecific antigenic and genetic variability of S. iniae has made development of autogenous vaccines a challenge, particularly where the diversity of locally endemic S. iniae strains is unknown. This study genetically and phenotypically characterized 11 S. iniae isolates from diseased wild and farmed fish from North America, Central America, and the Caribbean. A multilocus sequence analysis (MLSA) scheme was developed to phylogenetically compare these isolates to 84 other strains of Streptococcus spp. relevant to aquaculture. MLSA generated phylogenies comparable to established genotyping methods, and isolates formed distinct clades related to phenotype and host species. The endothelial Oreochromis mossambicus bulbus arteriosus cell line and whole blood from rainbow trout Oncorhynchus mykiss, Nile tilapia Oreochromis niloticus, and white sturgeon Acipenser transmontanus were used to investigate the persistence and virulence of the 11 isolates using in vitro assays. In vivo challenges using an O. niloticus model were used to evaluate virulence by the intragastric route of infection. Isolates showed significant differences (p < 0.05) in virulence and persistence, with some correlation to genogroup, establishing a basis for further work uncovering genetic factors leading to increased pathogenicity.

First identification and characterization of Streptococcus iniae obtained from tilapia (Oreochromis aureus) farmed in Mexico

This is the first study to isolate, identify and characterize Streptococcus iniae as the causative disease agent in two tilapia (Oreochromis aureus) populations. The populations were geographically isolated, of distinct origins, and did not share water sources. Affected fish showed various external (e.g., exophthalmia and cachexia, among others) and internal (e.g., granulomatous septicaemia and interstitial nephritis, among others) signs. All internal organ samples produced pure cultures, two of which (one from each farm, termed S-1 and S-2) were subjected to biochemical, PCR and 16S rRNA sequencing (99.5% similarity) analyses, confirming S. iniae identification. The two isolates presented genetic homogeneity regardless of technique (i.e., RAPD, REP-PCR and ERIC-PCR analyses). Pathogenic potentials were assessed through intraperitoneal injection challenges in rainbow trout (Oncorhynchus mykiss) and zebrafish (Danio rerio). Rainbow trout mortalities were respectively 40% and 70% at 10⁴ and 10⁶  CFU per fish with the S-1 isolate, while 100% mortality rates were recorded in zebrafish at 10² and 10⁴  CFU per fish with the S-2 isolate. The obtained data clearly indicate a relationship between intensified aquaculture activities in Mexico and new disease appearances. Future studies should establish clinical significances for the tilapia industry.

Staphylococcus epidermidis ΔSortase A strain elicits protective immunity against Staphylococcus aureus infection

Staphylococcus aureus and Staphylococcus epidermidis are two of the most significant opportunistic human pathogens, causing medical implant and nosocomial infections worldwide. These bacteria contain surface proteins that play crucial roles in multiple biological processes. It has become apparent that they have evolved a number of unique mechanisms by which they can immobilise proteins on their surface. Notably, a conserved cell membrane-anchored enzyme, sortase A (SrtA), can catalyse the covalent attachment of precursor bacterial cell wall-attached proteins to peptidoglycan. Considering its indispensable role in anchoring substrates to the cell wall and its effects on virulence, SrtA has attracted great attention. In this study, a 549-bp gene was cloned from a pathogenic S. epidermidis strain, YC-1, which shared high identity with srtA from other Staphylococcus spp. A mutant strain, YC-1ΔsrtA, was then constructed by allelic exchange mutagenesis. The direct survival rate assay suggested that YC-1ΔsrtA had a lower survival capacity in healthy mice blood compare with the wild-type strain, indicating that the deletion of srtA affects the virulence and infectious capacity of S. epidermidis YC-1. YC-1ΔsrtA was then administered via intraperitoneal injection and it provided a relative percent survival value of 72.7 % in mice against S. aureus TC-1 challenge. These findings demonstrate the possbility that YC-1ΔsrtA might be used as a live attenuated vaccine to produce cross-protection against S. aureus.