Proteomic analysis reveals Renibacterium salmoninarum grown under iron-limited conditions induces iron uptake mechanisms and overproduction of the 57-kDa protein

Genomic and proteomic aspects of p57 protein from Renibacterium salmoninarum: Characteristics in virulence patterns

Renibacterium salmoninarum is one of the oldest known fish bacterial pathogens. This Gram-positive bacterium is the causative agent of Bacterial Kidney Disease (BKD), a chronic infection that primarily infects salmonids at low temperatures. Externally, infected fish may show exophthalmos, skin blisters, ulcerations, and hemorrhages at the base of the fins and along the lateral line. Internally, the kidney, heart, spleen, and liver may show signs of inflammation. The best characterized virulence factor of R. salmoninarum is p57, a 57 kDa protein located on the bacterial cell surface and secreted into surrounding fish tissue. The p57 protein in fish is the main mediator in suppressing the immune system, reducing antibody production, and intervening in cytokine activity. In this review, we will discuss aspects such as single nucleotide polymorphisms (SNPs) that modify the DNA sequence, variants in the number of copies of MSA genes, physical-chemical properties of the signal peptides, and the limited iron conditions that can modify p57 expression and increase the virulence of R. salmoninarum.

Host–Pathogen Interactions of Marine Gram-Positive Bacteria

Simple Summary

Complex interactions between marine Gram-positive pathogens and fish hosts in the marine environment can result in diseases of economically important finfish, which cause economic losses in the aquaculture industry. Understanding how these pathogens interact with the fish host and generate disease will contribute to efficient prophylactic measures and treatments. To our knowledge, there are no systematic reviews on marine Gram-positive pathogens. Therefore, here we reviewed the host–pathogen interactions of marine Gram-positive pathogens from the pathogen-centric and host-centric points of view.

Abstract

Marine Gram-positive bacterial pathogens, including Renibacterium salmoninarum, Mycobacterium marinum, Nocardia seriolae, Lactococcus garvieae, and Streptococcus spp. cause economic losses in marine fish aquaculture worldwide. Comprehensive information on these pathogens and their dynamic interactions with their respective fish–host systems are critical to developing effective prophylactic measures and treatments. While much is known about bacterial virulence and fish immune response, it is necessary to synthesize the knowledge in terms of host–pathogen interactions as a centerpiece to establish a crucial connection between the intricate details of marine Gram-positive pathogens and their fish hosts. Therefore, this review provides a holistic view and discusses the different stages of the host–pathogen interactions of marine Gram-positive pathogens. Gram-positive pathogens can invade fish tissues, evade the fish defenses, proliferate in the host system, and modulate the fish immune response. Marine Gram-positive pathogens have a unique set of virulence factors that facilitate adhesion (e.g., adhesins, hemagglutination activity, sortase, and capsules), invasion (e.g., toxins, hemolysins/cytolysins, the type VII secretion system, and immune-suppressive proteins), evasion (e.g., free radical quenching, actin-based motility, and the inhibition of phagolysosomal fusion), and proliferation and survival (e.g., heme utilization and siderophore-mediated iron acquisition systems) in the fish host. After infection, the fish host initiates specific innate and adaptive immune responses according to the extracellular or intracellular mechanism of infection. Although efforts have continued to be made in understanding the complex interplay at the host–pathogen interface, integrated omics-based investigations targeting host–pathogen–marine environment interactions hold promise for future research.

Comparative pan-genomic analysis of 51 Renibacterium salmoninarum indicates heterogeneity in the principal virulence factor, the 57 kDa protein

Renibacterium salmoninarum, a Gram-positive intracellular pathogen, is the causative agent of bacterial kidney disease (BKD), the impacts of which are high mortalities and economic losses for the salmon industry. This study provides novel analyses for the whole-genome sequences of 50 R. salmoninarum isolates and the reference strain ATCC 33209 using a pan-genomic approach to elucidate phylogenomic relationships and identify unique and shared genes associated with pathogenicity and infection mechanisms. Genome size varied from 3,061,638 to 3,155,332 bp; gene count from 3452 to 3580; and predicted coding sequences from 3402 to 3527. Comparative analyses revealed an open, but approaching closed, pan-genome. The pan-genome analysis recovered 4064 genes, with a core genome containing 3306 genes. Phylogenetic analysis of R. salmoninarum showed high genomic homogeneity, apart from one isolate obtained from Salmo trutta in Norway. All genomes presented the 57-kDa protein (p57). Strain ATCC 33209 and the Chilean isolates H-2 and DJ2R presented two copies of the msa gene, while the remaining isolates had one copy. The pan-genome analysis further identified differences in the number of copies and length of the signalling peptide for p57, the principal virulence factor reported for this bacterium. This heterogeneity could be associated with the secretion levels of p57, potentially influencing virulence. Additionally identified were numerous common genes related to iron uptake, the stress response and regulation, and cell signalling-all of which constitute the pathogenic repertoire of R. salmoninarum. This investigation provides information that is applicable in future studies for identifying therapeutic targets and/or for designing new strategies (e.g., vaccines) to prevent BKD infections in salmon farming.