Draft Genome Sequences of Streptococcus agalactiae Strains Isolated from Nile Tilapia (Oreochromis niloticus) Farms in Thailand

Characterization and virulence of Streptococcus agalactiae deficient in SaeRS of the two-component system

There are a variety of regulatory systems in bacteria, among which the two-component system (TCS) can sense external environmental changes and make a series of physiological and biochemical reactions, which is crucial for the life activities of bacteria. As a member of TCS, SaeRS is considered to be an important virulence factor in Staphylococcus aureus, but its function in tilapia (Oreochromis niloticus)-derived Streptococcus agalactiae remains unknown. To explore the role of SaeRS in regulating virulence in the two-component system (TCS) of S. agalactiae from tilapia, ΔSaeRS mutant strain and CΔSaeRS complementary strain were constructed by homologous recombination. The results showed that the abilities of growth and biofilm formation of ΔSaeRS strain were significantly decreased when cultured in a brain heart infusion (BHI) medium (P < 0.01). Also, the survival rate of the ΔSaeRS strain in blood was decreased when compared with the wild strain S. agalactiae THN0901. Under the higher infection dose, the accumulative mortality of tilapia caused by the ΔSaeRS strain was significantly decreased (23.3%), of which THN0901 and CΔSaeRS strains were 73.3%. The results of competition experiments in tilapia showed that the invasion and colonization abilities of the ΔSaeRS strain were also dramatically lower than those of the wild strain (P < 0.01). Compared with the THN0901, the mRNA expression levels of virulence factors (fbsB, sip, cylE, bca, etc.) in the ΔSaeRS strain were significantly down-regulated (P < 0.01). SaeRS is one of the virulence factors of S. agalactiae. It plays a role in promoting host colonization and achieving immune evasion during the infection of tilapia, which provides a basis for exploring the pathogenic mechanism of S. agalactiae infected with tilapia.

Current status and advances of fish vaccines in Malaysia

Fish diseases have a significant negative influence on the Malaysian aquaculture industry. Since the 1980s, the sector has grown in size, which has resulted in a rise in the prevalence of infectious outbreaks affecting both freshwater and marine cultured fish species. Demand for commercially available fish vaccinations is predicted to increase as infectious disease outbreaks continue to occur. In Malaysia, aquaculture vaccine research and development (R&D) are still in its infancy, with most efforts concentrating on producing vaccines against bacterial infections, most notably streptococcosis, vibriosis, and motile Aeromonas septicemia. Despite several attempts, no homegrown vaccine has been effectively introduced into the manufacturing pipeline to date. At the moment, only three imported aquatic vaccines have received full permission, a far cry from the 314 and 60 vaccines licensed in the poultry and porcine industries, respectively. This review will describe recent findings regarding the development of aquaculture vaccines for certain fish species and diseases in Malaysia. In our opinion, R&D on fish vaccines is critical to the aquaculture industry's viability.

Genetic Relatedness Among Streptococcus agalactiae Isolated from Cattle, Fish, and Humans

Streptococcus agalactiae is well recognized to cause a variety of infections in many animal species and humans. We aimed to investigate genetic relatedness of S. agalactiae strains isolated from humans and animal origins, including cattle and fish, using capsular gene typing, pulsed-field gel electrophoresis (PFGE), and multilocus sequence typing techniques. Our results revealed that S. agalactiae strains with capsular type Ia and ST103 were observed from all bovine isolates (17/17) and one human isolate (1/5). S. agalactiae strains with capsular type III and ST283 were detected among isolates from fish (5/5) and from humans (2/5). Two PFGE clusters containing isolates from mixed origins were demonstrated: one cluster of five fish and one human isolate, and another cluster of one bovine and one human isolate. In conclusion, the close genetic relationship among S. agalactiae strains isolated from humans and animal origins was evident.

Genomic Insights Into the Distribution and Evolution of Group B Streptococcus

Streptococcus agalactiae, also known as Group B Streptococcus (GBS), is a bacteria with truly protean biology. It infects a variety of hosts, among which the most commonly studied are humans, cattle, and fish. GBS holds a singular position in the history of bacterial genomics, as it was the substrate used to describe one of the first major conceptual advances of comparative genomics, the idea of the pan-genome. In this review, I describe a brief history of GBS and the major contributions of genomics to understanding its genome plasticity and evolution as well as its molecular epidemiology, focusing on the three hosts mentioned above. I also discuss one of the major recent paradigm shifts in our understanding of GBS evolution and disease burden: foodborne GBS can cause invasive infections in humans.

Comparative genomics inferred two distinct populations of piscine pathogenic Streptococcus agalactiae, serotype Ia ST7 and serotype III ST283, in Thailand and Vietnam

The genomes of Streptococcus agalactiae (group B streptococcus; GBS) collected from diseased fish in Thailand and Vietnam over a nine-year period (2008-2016) were sequenced and compared (n = 21). Based on capsular serotype and multilocus sequence typing (MLST), GBS isolates are divided into 2 groups comprised of i) serotype Ia; sequence type (ST)7 and ii) serotype III; ST283. Population structure inferred by core genome (cg)MLST and Bayesian clustering analysis also strongly indicated distribution of two GBS populations in both Thailand and Vietnam. Deep phylogenetic analysis implied by CRISPR array's spacer diversity was able to cluster GBS isolates according to their temporal and geographic origins, though ST7 has varying CRISPR1-spacer profiles when compared to ST283 strains. Based on overall genotypic features, Thai ST283 strains were closely related to the Singaporean ST283 strain causing foodborne illness in humans in 2015, thus, signifying zoonotic potential of this GBS population in the country.

MicroRNA regulation of Toll-like receptor signaling pathways in teleost fish

The innate immune system is the first line defense mechanism that recognizes, responds to, controls or eliminates invading pathogens. Toll-like receptors (TLRs) are a critical family of pattern recognition receptors (PRRs) tightly regulated by complex mechanisms involving many molecules to ensure a beneficial outcome in response to foreign invaders. MicroRNAs (miRNAs), a transcriptional and posttranscriptional regulator family in a wide range of biological processes, have been identified as new molecules related to the regulation of TLR-signaling pathways in immune responses. To date, at least 22 TLR types have been identified in more than a dozen different fish species. However, the functions and underlying mechanisms of miRNAs in the regulation of inflammatory responses related to the TLR-signaling pathway in fish is lacking. In this review, we summarize the regulation of miRNA expression profiles in the presence of TLR ligands or pathogen infections in teleost fish. We focus on the effects of miRNAs in regulating TLR-signaling pathways by targeting multiple molecules, including TLRs themselves, TLR-associated signaling proteins, and TLR-induced cytokines. An understanding of the relationship between the TLR-signaling pathways and miRNAs may provide new insights for drug intervention to manipulate immune responses in fish.

Serotype distribution and antimicrobial susceptibilities of Streptococcus agalactiae isolated from infected cultured tilapia (Oreochromis niloticus) in Thailand: Nine-year perspective

Streptococcus agalactiae (group B Streptococcus, GBS) infection remains a major problem associated with high mortality of cultured tilapia worldwide. The present study reports the serotype distribution and antimicrobial susceptibilities of GBS isolated from infected tilapia cultured in Thailand. One hundred and forty-four GBS isolates were identified by biochemical, serological and molecular analyses. Of these 144 GBS isolates, 126 were serotype Ia and 18 were serotype III. Antimicrobial susceptibilities of the 144 GBS isolates were determined by the disc diffusion method. Most GBS isolates were susceptible to lincomycin, norfloxacin, oxytetracycline, ampicillin, erythromycin and chloramphenicol, but resistant to oxolinic acid, gentamicin, sulfamethoxazole and trimethoprim. However, 17 isolates displayed an oxytetracycline-resistant phenotype and harboured the tet(M) gene. The broth microdilution method was used to determine the minimal inhibitory concentrations (MICs) of 17 oxytetracycline-resistant GBS isolates, and then minimal bactericidal concentrations (MBCs) of these isolates were evaluated. Oxytetracyline-resistant isolates were found to be susceptible to ampicillin, lincomycin, norfloxacin, erythromycin and chloramphenicol, with the MIC and MBC ranging from ≤ 0.125 to 0.5 μg ml- 1 and ≤ 0.125 to 2 μg ml- 1, respectively. Moreover, all 17 oxytetracycline-resistant isolates demonstrated resistance to trimethoprim, oxolinic acid, gentamicin, sulfamethoxazole and oxytetracycline, with the MIC and MBC ranging from 16 to ≥ 128 μg ml- 1 and ≥ 128 μg ml- 1, respectively. These findings are useful information for antibiotic usage in fish aquaculture.

Genomic comparison between pathogenic Streptococcus agalactiae isolated from Nile tilapia in Thailand and fish-derived ST7 strains

Streptococcus agalactiae, or Group B streptococcus (GBS), is a highly virulent pathogen in aquatic animals, causing huge mortalities worldwide. In Thailand, the serotype Ia, β-hemolytic GBS, belonging to sequence type (ST) 7 of clonal complex (CC) 7, was found to be the major cause of streptococcosis outbreaks in fish farms. In this study, we performed an in silico genomic comparison, aiming to investigate the phylogenetic relationship between the pathogenic fish strains of Thai ST7 and other ST7 from different hosts and geographical origins. In general, the genomes of Thai ST7 strains are closely related to other fish ST7s, as the core genome is shared by 92-95% of any individual fish ST7 genome. Among the fish ST7 genomes, we observed only small dissimilarities, based on the analysis of clustered regularly interspaced short palindromic repeats (CRISPRs), surface protein markers, insertions sequence (IS) elements and putative virulence genes. The phylogenetic tree based on single nucleotide polymorphisms (SNPs) of the core genome sequences clearly categorized the ST7 strains according to their geographical and host origins, with the human ST7 being genetically distant from other fish ST7 strains. A pan-genome analysis of ST7 strains detected a 48-kb gene island specifically in the Thai ST7 isolates. The orientations and predicted amino acid sequences of the genes in the island closely matched those of Tn5252, a streptococcal conjugative transposon, in GBS 2603V/R serotype V, Streptococcus pneumoniae and Streptococcus suis. Thus, it was presumed that Thai ST7 acquired this Tn5252 homologue from related streptococci. The close phylogenetic relationship between the fish ST7 strains suggests that these strains were derived from a common ancestor and have diverged in different geographical regions and in different hosts.