Abstract
Cryptocaryon irritans, an important marine ciliate protozoan that causes a disease known as marine white spot disease, has already become one of the most harmful parasites in Chinese marine aquaculture industry. Previous studies indicated that the rabbitfish (Siganus oramin) showed low susceptibility to C. irritans infections, and the serum of rabbitfish had killing effect against C. irritans. Practically, the skin mucus of fish has been considered to be the first defense against infections caused by various pathogens. Whether the skin mucus of rabbitfish has the same parasiticidal activity is the main research purpose of this paper. In this study, we compared the antiparasitic and antibacterial activities of skin mucus from S. oramin, Epinephelus coioides, Sciaemops ocellatus, Plectorhinchus cinctus and Trachinotus ovatus. The parasite-killing effects of skin mucus were tested by immobilization assay for three species of parasites, C. irritans, I. multifiliis, and T. brucei brucei. The antimicrobial properties of skin mucus were determined using the agar diffusion method and the minimum inhibitory concentration for six species of bacteria (Staphylococcus aureus, Bacillus subtilis, Streptococcus iniae, Escherichia coli, Vibrio cholera, Aeromonas sobria, Edwardsiella tarda, Photobacterium damselae subsp. and Vibrio alginolyticus) and two species of fungi (Aspergillus niger and Gibberella saubinetii) were also examined. We found that compared with the skin mucus isolated from E. coioides, S. ocellatus, P. cinctus and T. ovatus, the rabbitfish skin mucus was lethal to C. irritans, I. multifiliis, and T. brucei brucei. The minimum parasiticidal concentration (MPC) of rabittfish skin mucus for C. irritans, I. multifiliis and T. brucei brucei were 4.0, 5.0, and 3.0 mg/mL, respectively. After the theronts of C. irritans and I. multifiliis were co-incubated with the skin mucus of rabbitfish, we observed the cilia falling off, body swelling, outer membrane rupture and contents release under a electron microscope. In addition, the results of bactericidal activity showed that the skin mucus of rabbitfish could inhibit the growth of S. aureus, V. cholera, S. iniae, A. sobria, V. alginolyticus, V. Parahaemolyticus, N. asteroids and P. damselae subsp. The minimum inhibitory concentration (MIC) of rabbitfish skin mucus for S. aureus, V. cholera, S. iniae, V. alginolyticus, A. sobria and V. Parahaemolyticus were 0.16, 0.16, 0.08, 0.63, 0.63 and 0.63 mg/mL, respectively. However, the skin mucus of rabbitfish had no effect on the growth of two tested fungi species - G. saubinetii and A. niger. This result suggested that the skin mucus of rabbitfish played an important role in the innate host defensive mechanism to combat against C. irritan and other pathogenic organisms. Base on our observation, further studies on purification and identification of these skin-mucus antiparasitic factors need to be carried out. This type of investigation will provide a new method for controlling the C. irritans. Therefore, we expect that our ongoing work will have great significance to the studies on the innate immunity of fish and the biocontrol of the parasites.
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