Pathogenicity of Aeromonas hydrophila causing mass mortalities of Procambarus clarkia and its induced host immune response

Probable Role of Type IV Pili of Aeromonas hydrophila in Human Pathogenicity

BACKGROUND

Aeromonas hydrophila is a widely recognized broad-spectrum pathogen that primarily targets the gastrointestinal tract. Type IV pili (T4P) are proteinaceous nano-machines located on the bacterial cell surface, playing a crucial role in host colonization and infection. Regrettably, the T4P systems of A. hydrophila remain largely underexplored.

METHODS

A. hydrophila genomes with complete genome assembly and annotation reports up to 31 March 2023, were obtained from the NCBI Genome database or KEGG genome database, followed by a global search for T4P secretion system genes. Protein sequences of these manually curetted genes were used as secondary quarry for Synteny analysis. Protein-protein interaction analysis was performed by string analysis and in silico study of genomic islands.

RESULTS

We identified 27 orthologs of type IV pili (T4P) nano-machine components in A. hydrophila. These orthologs are primarily distributed across three operons: pilABCD, pilMNOPQ, and pilVWXY. While the first two operons are commonly found in all experimental genomes, the presence of the pilVWXY operon, coding for 11 orthologs, is reported here for the first time in A. hydrophila. Notably, the complete pilVWXY operon is absent in nonvirulent strains. A genomic islands study between a nonvirulent and hypervirulent strain also confirms absence of most of the genes coded by pilVWXY in nonvirulent strain. Interestingly, among the 51 experimental genomes analyzed, the pilVWXY operon was completely absent in 10 strains, most of which are categorized as nonvirulent; Conclusions: The distribution of two major type IV pili (T4P) nano-machines, PilABCDMNOPQ and PilVWXY, is reported here for the first time in A. hydrophila. Additionally, this study suggests a potential role for the PilVWXY nano-machine in establishing human disease.

Identification and functional analysis of cytosolic phospholipase A2 (cPLA2) from the red swamp crayfish Procambarus clarkii: The first evidence of cPLA2 involved in immunity in invertebrates

Cytosolic phospholipase A2 (cPLA2) specifically liberates the arachidonic acids from the phospholipid substrates. In mammals, cPLA2 serves as a key control point in inflammatory responses due to its diverse downstream products. However, the role of cPLA2 in animals lower than mammals largely remains unknown. In the current research, a homolog of cPLA2 was first identified and characterized in the red swamp crayfish Procambarus clarkii. The full-length cDNA of PccPLA2 was 4432 bp in length with a 3036 bp-long open reading frame, encoding a putative protein of 1011 amino acids that contained a protein kinase C conserved region 2 and a catalytic subunit of cPLA2. PccPLA2 was ubiquitously expressed in all examined tissues with the highest expression in the hepatopancreas, and the expression in hemocytes as well as hepatopancreas was induced upon the immune challenges of WSSV and Aeromonas hydrophila. After the co-treatment of RNA interference and bacterial infection, the decline of bacteria clearance capability was observed in the hemolymph, and the expression of some antimicrobial peptides (AMPs) was significantly suppressed. Additionally, the phagocytosis of A. hydrophila by primary hemocytes decreased when treated with the specific inhibitor CAY10650 of cPLA2. These results indicated the participation of PccPLA2 in both cellular and humoral immune responses in the crayfish, which provided an insight into the role that cPLA2 played in the innate immunity of crustaceans, and even in invertebrates.

Complete Mitochondrial Genome Sequence and Phylogenetic Analysis of Procambarus clarkii and Cambaroides dauricus from China

To enhance the management and protection of crayfish genetic diversity and germplasm resources in Cambaroides dauricus (C. dauricus), a common species of Procambarus clarkii (P. clarkii) was used as a control group to compare the whole mitochondrial genome sequence using Illumina sequencing technology. This study found that the mitochondrial genome of C. dauricus is 15580 bp in length, with a base composition of A (31.84%), G (17.66%), C (9.42%), and T (41.08%) and a C + G content of 27.08%. The C + G in the D-loop is rich in 17.06%, indicating a significant preference. The mitochondrial genome of C. dauricus contains 13 protein-coding genes, 22 tRNA genes, and 2 rRNA genes, with most of the genes labeled in the negative direction, except for a few genes that are labeled in the positive direction. The start codons of the ten coding sequences are ATG, and the quintessential TAA and TAG are the stop codons. This study also found that the Ka/Ks ratios of most protein-coding genes in the mitochondria of both shrimps are lower than 1, indicating weak natural selection, except for nad 2, nad 5, and cox 1. The Ka/Ks ratio of cox 3 is the lowest (less than 0.1), indicating that this protein-coding gene bears strong natural selection pressure and functional constraint in the process of mitochondrial genetic evolution of both shrimps. Furthermore, we constructed phylogenetic analyses based on the entire sequence, which effectively distinguishes the high body from other shrimp species of the genus based on the mitochondrial genome. This study provides molecular genetic data for the diversity investigation and protection of fishery resources with Chinese characteristics and a scientific reference for the evolutionary study of Procambarus.

Cloning, identification, and functional characterization of novel prophenoloxidases (ShproPO) from the freshwater crab Sinopotamon henanense in response to cadmium exposure and Aeromonas hydrophila infection

Prophenoloxidase (proPO) is essential in the prophenoloxidase-activating system (proPO-AS) which is important for defense against foreign infection in crustaceans. However, most studies have focused on expression in the presence of a single pathogenic bacterium, and very few have addressed the presence of environmental contaminants simultaneously, such as cadmium (Cd) and Aeromonas hydrophila. Our study aimed to investigate the function of proPO in the freshwater crab Sinopotamon henanense and the changes in its expression by Cd and infection of A. hydrophila. A novel proPO from the hemocytes of S. henanense (ShproPO) was found in this research, the full-length cDNA of ShproPO was 2620 bp of encoding a protein of 678 amino acids containing three typical hemocyanin domains. The ShproPO protein could be found in both the granular (GHc) and the semi-granular hemocytes (SGHc). The ShproPO mRNA was found to be abundantly expressed in hemocytes and could be influenced by A. hydrophila infection. These results indicate that ShproPO could be involved in the antibacterial process. Further research found that low concentrations of Cd could promote its expression after infection with A. hydrophila. Therefore, it was hypothesized that Cd disrupted the response of crabs to A. hydrophila infection. Subsequently, PO enzyme activity was found to be significantly reduced through in vivo RNA interference with ShproPO, and the results suggested that ShproPO is likely to be a key enzyme in the melanization response. Finally, ShproPO was found to significantly enhance the phagocytosis of A. hydrophila-infected hemocytes by in vitro recombination, confirming that ShproPO is involved in hemocyte-mediated melanization and phagocytosis. Our findings reveal completely new insight into the immunotoxicity of Cd and the immune function of ShproPO in S. henanense.

Effects of Copper Exposure on Oxidative Stress, Apoptosis, Endoplasmic Reticulum Stress, Autophagy and Immune Response in Different Tissues of Chinese Mitten Crab (Eriocheir sinensis)

High concentrations of copper (Cu²⁺) pose a great threat to aquatic animals. However, the mechanisms underlying the response of crustaceans to Cu²⁺ exposure have not been well studied. Therefore, we investigated the alterations of physiological and molecular parameters in Chinese mitten crab (Eriocheir sinensis) after Cu²⁺ exposure. The crabs were exposed to 0 (control), 0.04, 0.18, and 0.70 mg/L of Cu²⁺ for 5 days, and the hemolymph, hepatopancreas, gills, and muscle were sampled. The results showed that Cu²⁺ exposure decreased the antioxidative capacity and promoted lipid peroxidation in different tissues. Apoptosis was induced by Cu²⁺ exposure, and this activation was associated with the mitochondrial and ERK pathways in the hepatopancreas. ER stress-related genes were upregulated in the hepatopancreas but downregulated in the gills at higher doses of Cu²⁺. Autophagy was considerably influenced by Cu²⁺ exposure, as evidenced by the upregulation of autophagy-related genes in the hepatopancreas and gills. Cu²⁺ exposure also caused an immune response in different tissues, especially the hepatopancreas, where the TLR2-MyD88-NF-κB pathway was initiated to mediate the inflammatory response. Overall, our results suggest that Cu²⁺ exposure induces oxidative stress, ER stress, apoptosis, autophagy, and immune response in E. sinensis, and the toxicity may be implicated following the activation of the ERK, AMPK, and TLR2-MyD88-NF-κB pathways.

The immune response of fairy shrimp Streptocephalus sirindhornae against bacterial black disease by de novo transcriptome analysis

To enhance genomic resources and to understand the molecular immune mechanisms underlying the response of fairy shrimp (Streptocephalus sirindhornae) to pathogens, we first performed a comparative gene transcription analysis from Aeromonas hydrophila-immunized shrimp and from a control group through RNA sequencing. Meanwhile, the differentially expressed genes (DEGs) were investigated, and a total of 46,958,894 clean reads were obtained and then assembled into 73,297 unigenes with an average length of 993 bp and an N50 of 1,458 bp. Unigenes were annotated by comparison with the NR/NT/KO/SwissProt/PFAM/GO and KOG databases, and 28,198 unigenes (38.47%) were annotated in at least one database. After a bacterial challenge, 143 and 287 genes were identified as markedly up- or downregulated, respectively, and 345 were associated with 142 pathways, including the classic immune-related apoptosis, toll-like receptor and MAPK signaling pathways. Moreover, ten differently expressed immune-related genes were confirmed by using quantitative real-time PCR. This study characterized a gene expression pattern for normal and Aeromonas hydrophila-immunized S. sirindhornae for the first time and shed new light on its molecular mechanisms, thus enabling the future efforts of disease control programs for this valuable aquaculture species.