Pathogenic bacterium Vibrio harveyi: an endosymbiont in the marine parasitic ciliate protozoan Cryptocaryon irritans

Vibrio harveyi, known as a pathogenic bacterium caused severe secondary bacterial infections of the large yellow croaker Larimichthys crocea, was identified as an endosymbiont in the marine parasitic ciliate protozoan Cryptocaryon irritans. Meta 16 S sequencing method was used to identify the bacterial flora in C. irritans, and V.harveyi was isolated via culture-dependent method. Vibrio harveyi was observed in cytoplasm of C. irritans at the stage of tomont both by transmission electron microscopy and by Fluorescence in situ hybridization; no signal,however, was detected in nucleus area. The relationship between V. harveyi and C. irritans and the role of endosymbiotic V. harveyi in C. irritans merit further investigation.

Antibacterial and antiparasitic activities analysis of a hepcidinlike antimicrobial peptide from Larimichthys crocea

As an economically important marine fish,the large yellow croaker Larimichthys crocea suffered from marine white spot disease caused by the ectoparasite Cryptocaryon irritans in recent years.This disease not only could result in physiological damage,but also lead to secondary bacterial invasion.Reports indicated some AMPs(antimicrobial peptides)were of antiparasitic activity to C.irritans.Hepcidin-like(Lc-HepL)was one of the significant differential expression genes excavated from the transcriptome following a challenge with C.irritans.In this study,we characterized this AMP’s bioactivity based on the levels of mRNA and protein.After challenged by C.irritans,qRT-PCR showed Lc-HepL was significantly upregulated in six tissues,including gill,muscle,liver,head kidney and spleen during theront infection,trophont falling off,and secondary bacterial invasion stages,which implicated a role Lc-HepL played in the immune defense against C.irritans and secondary bacterial infection.Recombinant Lc-HepL(rLc-HepL)was induced and purified successfully.rLc-HepL exhibited antibacterial activity to certain bacteria in a dose-and time-dependent manners.Anti-C.irritans activity was explored for the first time and found it could cause the theronts membrane rupture and contents leakage.These results provided the first evidence that Lc-HepL had strong antiparasitic activity against marine fish ectoparasites C.irritans theronts.Together,data indicated that Lc-HepL might be an important component in the innate immune system against C.irritans and has the potential to be employed in future drug development.