Guanosine penta- and tetraphosphate [(p)ppGpp] are two unusual nucleotides implied in the bacterial stringent response. In many pathogenic bacteria, mutants unable to synthesize these molecules lose their virulence. In Gram-positive bacteria such as Enterococcus faecalis, the synthesis and degradation of (p)ppGpp mainly depend on the activity of a bifunctional enzyme, encoded by the relA gene. By analysing ΔrelA and ΔrelQ (which encodes a protein harbouring a ppGpp synthetase activity) deletion mutants, we showed that RelA is by far the main system leading to (p)ppGpp production under our experimental conditions, and during the development of a stringent response induced by mupirocin. We also constructed a mutant (ΔrelAsp) in which a small part of the relA gene (about 0.7 kbp) encoding the carboxy-terminal domain of the RelA protein was deleted. Both relA mutants were more resistant than the wild-type strain to 0.3 % bile salts, 25 % ethanol and acid (pH 2.3) challenges. Interestingly, the ΔrelAsp mutant grew better than the two other strains in the presence of 1 mM H2O2, but did not display increased tolerance when subjected to lethal doses of H2O2 (45 mM). By contrast, the ΔrelA mutant was highly sensitive to 45 mM H2O2 and displayed reduced growth in a medium containing 1 M NaCl. The two mutants also displayed contrasting virulence phenotypes towards larvae of the Greater Wax Moth infection model Galleria mellonella. Indeed, although the ΔrelA mutant did not display any phenotype, the ΔrelAsp mutant was more virulent than the wild-type strain. This virulent phenotype should stem from its increased ability to proliferate under oxidative environments.
Chinese sea bass (Lateolabrax maculatus) is an economically important marine cultured species in China. Interferons (IFNs) play an essential role in innate antiviral immunity. The study on IFN immune system helps prevent and control viral diseases of L. maculatus. We have obtained cloning and characterization of the type I IFNd gene from L. maculatus (LmIFNd) in the present study. The full length of cDNA was 1190 bp, including 5’UTR (untranslated region) of 354 bp, 3’UTR of 278 bp, and an open reading frame (ORF) of 558 bp. It encodes 185 amino acids, and the first 20 amino acids are hypothetical signal peptides. The results of amino acid multiple sequence alignment and phylogenetic tree analysis showed that LmIFNd and mandarin fish (Siniperca chuatsi) IFNd were clustered into one branch, and the gene sequence similarity was as high as 88.9%. The expression of LmIFNd was tissue-specific and highly expressed in the head kidney, spleen, and gill. After infection with Rana grylio virus (RGV), and polyinosinic-polycytidylic acid [Poly(I:C)], the expression of LmIFNd in gill, spleen, and head-kidney was up-regulated significantly. Besides, the expression level of LmIFNd has increased significantly under the stimulation of Vibrio harveyi and Streptococcus iniae. The results show that LmIFNd may play a protective role in both viral and bacterial infections.
Growing evidence suggests a close relationship between gut microbiota and infectious diseases. However, the specific role of gut microbiota in host-pathogen interactions during aquaculture-related infections remains poorly understood. This study investigated the diversity and composition of gut microbiota communities in Aeromonas veronii -infected Lateolabrax maculatus using high-throughput sequencing. The results revealed significant changes in the structure and composition of L. maculatus gut microbiota after A. veronii infection. Over time, Bacteroidetes and Firmicutes decreased significantly, while Proteobacteria increased significantly after A. veronii infection. Most intestinal bacteria showed a decline in abundance over time, with probiotics (such as Lactobacillus ) experiencing a significant decrease and pathogens (such as Aeromonas ) showing a significant increase. Conversely, no differences were observed in the structure and composition of gut microbiota between healthy L. maculatus and those infected with A. veronii after treatment with Lactobacillus plantarum ; no changes in relative abundances of other bacterial phyla or genera except for Aeromonas . Furthermore, intestinal flora’s structural diversity and composition differed significantly from untreated L. maculatus infected with A. veronii . These findings suggest alterations in the structure and composition of gut microbiota following A. veronii infection. L. plantarum can maintain a dynamic balance within the intestinal flora, reducing the potential risk of pathogen infections.
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