Capsule impairs efficient adherence of Streptococcus agalactiae to intestinal epithelium in tilapias Oreochromis sp

Organic acids mitigate Streptococcus agalactiae virulence in Tilapia fish gut primary cells and in a gut infection model

BACKGROUND

Streptococcus agalactiae, a Gram-positive bacterium, has emerged as an important pathogen for the aquaculture industry worldwide, due to its increased induced mortality rates in cultured fish. Developing interventions to cure or prevent infections based on natural alternatives to antibiotics has become a priority, however, given the absence of scientific evidence regarding their mode of action progress has been slow.

METHODS

In this study we aimed to investigate the effect of a mixture of organic acids (natural antimicrobials), AuraAqua (Aq), on the virulence of S. agalactiae using Tilapia gut primary epithelial cells and an in vitro Tilapia gut culture model. Our results show that Aq was able to reduce significantly, in vitro, the S. agalactiae levels of infection in Tilapia gut primary epithelial cells (TGP) when the MIC concentration of 0.125% was tested.

RESULTS AND DISCUSSION

At bacterial level, Aq was able to downregulate bacterial capsule polysaccharide (CPS) gene expression, capC, resulting in a significant decrease in bacterial surface capsule production. The decrease in CPS production was also associated with a reduction in the pro-inflammatory IFNγ, IL1β, TNFα, SOD and CAT gene expression and H2O2 production in the presence of 0.125% Aq (P < 0.0001). The antimicrobial mixture also reduced the levels of S. agalactiae infection in an in vitro gut culture model and significantly reduced the IFNγ, IL1β, TNFα, SOD, CAT gene expression and H2O2 production in infected tissue. Moreover, genes involved in Tilapia resistance to S. agalactiae induced disease, MCP-8 and Duo-1, were also downregulated by Aq, as a consequence of reduced bacterial levels of infection.

CONCLUSION

Conclusively, our study shows that mixtures of organic acids can be considered as potential alternative treatments to antibiotics and prevent S. agalactiae infection and inflammation in the Tilapia fish digestive tract.

A review on the evolution of methods for intestinal in vitro organ culture and its application in veterinary science

Different techniques have been reported in studies of intestinal in vitro organ culture (IVOC). A robust compilation of all available methods is lacking in the literature, making it difficult to choose a method that corresponds to the study's demands. In this review, readers can assess the most available methods, allowing them to evaluate which is more suitable for their purposes and requirements. A simplified view of culturing intestinal explants is presented, highlighting the approachability of IVOC. Relevant findings from diverse veterinarian studies, where explants played a major role, as well as the technique used in each, are described to illustrate its applications. Finally, the strengths and limitations of the innovative intestinal IVOC methods are discussed. This review provides a collection of methods for intestinal explant culture and their possible applications in veterinary research. In this way, it aims to broaden access to IVOC techniques and aid decision-making regarding the best suited for a study's purposes.

CRISPR Contributes to Adhesion, Invasion, and Biofilm Formation in Streptococcus agalactiae by Repressing Capsular Polysaccharide Production

The clustered regularly interspaced palindromic repeat (CRISPR)-associated (Cas) system functions classically as a prokaryotic defense system against invading mobile genetic elements, such as phages, plasmids, and viruses. Our previous study revealed that CRISPR deletion caused increased transcription of capsular polysaccharide (CPS) synthesis-related genes and severely attenuated virulence in the hypervirulent piscine Streptococcus agalactiae strain GD201008-001. Here, we found that CRISPR deficiency resulted in reduced adhesion, invasion, and biofilm formation abilities in this strain by upregulating the production of CPS. However, enhanced CPS production was not responsible for the attenuated phenotype of the ΔCRISPR mutant. RNA degradation assays indicated that inhibited transcription of the cps operon by CRISPR RNA (crRNA) was not due to the base pairing of the crRNA with the cps mRNA but to the repression of the promoter activity of cpsA, which is a putative transcriptional regulator of the capsule locus. IMPORTANCE Beyond protection from invading nucleic acids, CRISPR-Cas systems have been shown to have an important role in regulating bacterial endogenous genes. In this study, we demonstrate that crRNA inhibits the transcription of the cps operon by repressing the activity of promoter PcpsA, leading to increases in the abilities of adhesion, invasion, and biofilm formation in S. agalactiae. This study highlights the regulatory role of crRNA in bacterial physiology and provides a new explanation for the mechanism of crRNA-mediated endogenous gene regulation in S. agalactiae.

Construction of Streptococcus agalactiae sialic acid mutant and evaluation of its potential as a live attenuated vaccine in Nile tilapia (Oreochromis niloticus)

AIMS

This study aimed to develop a live attenuated vaccine as an effective approach to prevent streptococcosis in tilapia (Oreochromis niloticus).

METHODS AND RESULTS

We eliminated the virulence factor, sialic acid (Sia) encoded by the neuA-D gene cluster from the Group B Streptococcus (Streptococcus agalactiae, GBS) strain WC1535, to construct Sia-deficient S. agalactiae (ΔSia) mutant by homologous recombination. Results showed that the ΔSia mutant had higher adherence to HEp-2 cells and lower resistance to RAW264.7 cell phagocytosis than the wild-type S. agalactiae. The virulence of the ΔSia mutant to tilapia dramatically decreased with no virulence recovery. The relative percent survivals (RPSs) were 50.00% and 54.50% at 30 days when challenged at the wild-type WC1535 doses of 1.0 × 10⁷ and 5.0 × 10⁷ CFU fish^-1 , respectively, via intraperitoneal (IP) injection. The tilapia vaccinated via IP injection with the ΔSia mutant induced strong antibody agglutination titers. The expression of IL-1β, TNF-α, MHC-Iα, and MHC-IIβ could be enhanced in the intestine, spleen, and head kidney for tilapia administered with the ΔSia mutant.

CONCLUSIONS

GBS Sia plays a critical role in adherence to HEp-2 cells and resistance to the immune clearance of RAW264.7 cells. Moreover, the ΔSia mutant is a safe, stable, and immunogenic live attenuated vaccine candidate to protect tilapia against GBS infection.

SIGNIFICANCE AND IMPACT OF STUDY

The results offer more evidence of the importance of Sia in GBS and may be instructive in the control of tilapia streptococcosis.

Laboratory-controlled challenges of streptococcosis in Nile tilapia using the oral route (infected-feed) for infection

The study aimed to evaluate the oral infected-feed, intragastric-gavage, and intraperitoneal routes of the Streptococcus agalactiae infection in Nile tilapia (Oreochromis niloticus). For this purpose, 270 juveniles of Nile tilapia, with an average weight of 2 g, were distributed in 18 experimental units of 90 L, acclimatized, and raised for 55 days, until reaching 50 g (median weight). The experimental design was entirely randomized in six treatments, three of which were composed by bacterial infection routes: intraperitoneal 100 μL fish^-1 [10⁸ CFU], intragastric 100 μL fish^-1 [10⁸ CFU], feed inoculum 100 μL g feed^-1 [10⁹ CFU], and three respective control groups. Clinical signs were observed, and mortalities monitored until reaching 50% in the infected groups. Then, tissue samples from the spleen, liver, intestine, brain, and blood were collected from 20 fish per treatment for histopathological and hemato-immunological analyses. In addition, a related mortality curve was established at the end of the experimental challenge. The intraperitoneal and intragastric routes were more aggressive than the oral inoculum, causing greater brain damage, acute hemato-immunological response, and early mortality. While the orally fed inoculum, fish presented brain lesions with less intensity, and a chronic haemato-immunological response, the mortalities occurred twice as long as the other routes. The present research demonstrated that the S. agalactiae oral (feed inoculum) administration can be an innovative methodology to future experimental challenges in aquaculture research.

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.

The Last Half Century of Fish Explant and Organ Culture

Explants are three-dimensional tissue fragments maintained outside the organism. The goals of this article are to review the history of fish explant culture and discuss applications of this technique that may assist the modern zebrafish laboratory. Because most zebrafish workers do not have a background in tissue culture, the key variables of this method are deliberately explained in a general way. This is followed by a review of fish-specific explantation approaches, including presurgical husbandry, aseptic dissection technique, choice of media and additives, incubation conditions, viability assays, and imaging studies. Relevant articles since 1970 are organized in a table grouped by organ system. From these, I highlight several recent studies using explant culture to study physiological and embryological processes in teleosts, including circadian rhythms, hormonal regulation, and cardiac development.

The Response of Microbiota Community to Streptococcus agalactiae Infection in Zebrafish Intestine

Recently, Streptococcus agalactiae has become a major pathogen leading to Streptococcosis. To understand the physiological responses of zebrafish (Danio rerio) to S. agalactiae, the intestinal microbiota composition of the intestine (12 and 24 h post-infection, hpi, respectively) in zebrafish infected with S. agalactiae were investigated. The intestinal bacterial composition was analyzed using PacBio high-throughput full-length 16S rRNA gene sequencing. The most predominant bacteria in the zebrafish intestine were the Fusobacteria phylum and Sphingomonas genus. S. agalactiae infection affected the composition of partially intestinal microbiota. At the species level, the relative abundance of the pathogenic intestinal bacteria Aeromonas veronii, S. agalactiae, and Clostridium tarantellae significantly increased after S. agalactiae infection (p < 0.05), while that of the beneficial intestinal bacteria Bacillus licheniformis, Comamonas koreensis, and Romboutsia ilealis significantly decreased (p < 0.05), showing that S. agalactiae infection aggravates the zebrafish disease through promoting abundance of other intestinal pathogenic bacteria. This study is the first PacBio analyses of the zebrafish intestinal microbiota community under pathogenic infection. Results suggest that the S. agalactiae infection alters the intestinal flora structure in zebrafish.

Morphological characterization of the adherence and invasion of Streptococcus agalactiae to the intestinal mucosa of tilapia Oreochromis sp.: An in vitro model

Streptococcosis in tilapia Oreochromis sp. is possibly the most important bacterial disease for fish production worldwide. In Colombia, streptococcosis is caused by Streptococcus agalactiae (GBS), but in other countries, Streptococcus iniae is also involved. Prevention of streptococcosis is required and must be addressed for economic, social, international trade and public health reasons. This research used an in vitro culture of tilapia intestine to detail the intestinal mucosal response once the pathogen contacts the epithelium. We show that S. agalactiae sheds off its capsule to adhere to the epithelium. The bacterium adheres as a single individuum, in groups or in chains and is able to divide on the apical border of enterocytes. GBS adheres at and invades exclusively through the apical portion of the intestinal folds, using the transepithelial route. Once within the cytoplasm of enterocytes, the bacteria continue to divide. On the basolateral side of the epithelium, the microorganisms leave the cells to reach the propria and travel through the microcirculation. No evidence of an immuno-inflammatory reaction or goblet cell response in the epithelium or the lamina propria was seen during the process of adherence and invasion of the pathogen.

Capsular polysaccharide of Streptococcus agalactiae is an essential virulence factor for infection in Nile tilapia (Oreochromis niloticus Linn.)

Streptococcus agalactiae (Group B Streptococcus, GBS) is associated with diverse diseases in aquatic animals. The capsule polysaccharide (CPS) encoded by the cps gene cluster is the major virulence factor of S. agalactiae; however, limited information is available regarding the pathogenic role of the CPS of serotype Ia piscine GBS strains in fish. Here, a non-encapsulated mutant (Δcps) was constructed by insertional mutagenesis of the cps gene cluster. Mutant pathogenicity was evaluated in vitro based on the killing of whole blood from tilapia, in vivo infections, measuring mutant survival in tilapia spleen tissues and pathological analysis. Compared to wild-type (WT) GBS strain, the Δcps mutant had lower resistance to fresh tilapia whole blood in vitro (p < 0.01), and more easily cleared in tilapia spleen tissue, and was highly attenuated in tilapia and zebrafish. Additionally, compared to the Δcps mutant, numerous GBS strains and severe tissue necrosis were observed in the tilapia spleen tissue infected with WT strains. These results indicated that the CPS is essential for GBS pathogenicity and may serve as a target for attenuation in vaccine development. Gaining a better understanding of the role, the GBS pathogenicity in fish will provide insight into related pathogenesis and host-pathogen interactions.

CD44 facilitates adherence of Streptococcus equi subsp. zooepidemicus to LA-4 cells

Streptococcus equi subsp. zooepidemicus (S. zooepidemicus) causes a wide variety of infections in many species. CD44 is a transmembrane adhesion molecule, expressed by various cell types, which has been implicated in several infection processes. The aim of this study was to examine the role of CD44 in S. zooepidemicus adherence to LA-4 cells (mouse lung adenoma). Dose-dependent adhesion with LA-4 may be effectively studied by flow cytometry. Adherence of S. zooepidemicus is reduced after treatment of cells with anti-CD44 antibody. Treatment of S. zooepidemicus with recombinant CD44 significantly reduced bacteria adherence. In addition, CD44 can directly bind to wild-type S. zooepidemicus, while the binding was decreased in the capsule deletion isogenic mutant. These data suggest that CD44 facilitates adherence of S. zooepidemicus to LA-4 cells.

Capsular Switching and ICE Transformation Occurred in Human Streptococcus agalactiae ST19 With High Pathogenicity to Fish

Although Streptococcus agalactiae (GBS) cross-infection between human and fish has been confirmed in experimental and clinical studies, the mechanisms underlying GBS cross-species infection remain largely unclear. We have found different human GBS ST19 strains exhibiting strong or weak pathogenic to fish (sGBS and wGBS). In this study, our objective was to identify the genetic elements responsible for GBS cross species infection based on genome sequence data and comparative genomics. The genomes of 11 sGBS strains and 11 wGBS strains were sequenced, and the genomic analysis was performed base on pan-genome, CRISPRs, phylogenetic reconstruction and genome comparison. The results from the pan-genome, CRISPRs analysis and phylogenetic reconstruction indicated that genomes between sGBS were more conservative than that of wGBS. The genomic differences between sGBS and wGBS were primarily in the Cps region (about 111 kb) and its adjacent ICE region (about 106 kb). The Cps region included the entire cps operon, and all sGBS were capsular polysaccharide (CPS) type V, while all wGBS were CPS type III. The ICE region of sGBS contained integrative and conjugative elements (ICE) with IQ element and erm(TR), and was very conserved, whereas the ICE region of wGBS contained ICE with mega elements and the variation was large. The capsular switching (III–V) and transformation of ICE adjacent to the Cps region occurred in human GBS ST19 with different pathogenicity to fish, which may be related to the capability of GBS cross-infection.

Transcriptome and Proteome of Fish-Pathogenic Streptococcus agalactiae Are Modulated by Temperature

Streptococcus agalactiae is one of the most important pathogens associated with streptococcosis outbreaks in Nile tilapia farms worldwide. High water temperature (above 27°C) has been described as a predisposing factor for the disease in fish. At low temperatures (below 25°C), fish mortalities are not usually observed in farms. Temperature variation can modulate the expression of genes and proteins involved in metabolism, adaptation, and bacterial pathogenicity, thus increasing or decreasing the ability to infect the host. This study aimed to evaluate the transcriptome and proteome of a fish-pathogenic S. agalactiae strain SA53 subjected to in vitro growth at different temperatures using a microarray and label-free shotgun LC-HDMS^E approach. Biological triplicates of isolates were cultured in BHIT broth at 22 or 32°C for RNA and protein isolation and submitted for transcriptomic and proteomic analyses. In total, 1,730 transcripts were identified in SA53, with 107 genes being differentially expressed between the temperatures evaluated. A higher number of genes related to metabolism, mainly from the phosphotransferase system (PTS) and ATP-binding cassette (ABC) transport system, were upregulated at 32°C. In the proteome analysis, 1,046 proteins were identified in SA53, of which 81 were differentially regulated between 22 and 32°C. Proteins involved in defense mechanisms, lipid transport and metabolism, and nucleotide transport and metabolism were upregulated at 32°C. A higher number of interactions were observed in proteins involved in nucleotide transport and metabolism. We observed a low correlation between the transcriptome and proteome datasets. Our study indicates that the transcriptome and proteome of a fish-adapted S. agalactiae strain are modulated by temperature, particularly showing differential expression of genes/proteins involved in metabolism, virulence factors, and adaptation.

A metabolomic investigation into the effects of temperature on Streptococcus agalactiae from Nile tilapia (Oreochromis niloticus) based on UPLC-MS/MS

Streptococcosis caused by Streptococcus agalactiae is one of the most serious diseases in farmed tilapia, and temperature is one of the most important environmental factors related to its outbreak. To elucidate the influence of temperature variation on the pathogen from a metabolic perspective, the global metabolomics of 2 pathogenic strains of S. agalactiae from sick tilapia were analyzed at 35°C and 25°C using ultra performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS) combined with pattern recognition approaches and pathway analysis. The result showed that the metabolic status of S. agalactiae was extensively affected by its culture temperature. Based on the results of metabolites contributing to these differences, a large number of nucleotides and their ramifications were markedly elevated at 35°C. Various energy substances, components of the cell wall and substances associated with stress regulation such as glyceraldehyde 3-phosphate, pyroglutamic acid, glutamate, d-Alanyl-d-alanine, glycerophosphocholine, dephospho-CoA, and oxidized glutathione increased when the strains were cultured at 35°C. Additionally, a general decrease in various precursors of capsule, antigen, and virulence protein formation were detected including mannose, maltotriose, N-acetyl-d-glucosamine 6-phosphate, uracil, proline, and citrulline. These metabolic changes indicated that metabolic activity decreased, while adaptive ability to environment and pathogenicity to host increased at high temperature. This study is the first to determine the metabolomic responses of S. agalactiae to temperature, and the results are useful to reveal its pathogenic mechanism and find effective disease control.