Chinese gallnut (Galla chinensis) against Vibrio parahaemolyticus: In vitro activity and the use of medicated bath method to treat infected mud crab Scylla paramamosain

Abstract

Natural herbs, especially traditional Chinese herbs (TCHs), are gaining attention as alternatives to treat bacterial diseases in aquaculture. In order to assess the possibility of using TCHs to cure bacterial diseases of mud crab Scylla paramamosain, firstly, the antibacterial effects of twelve TCHs were determined using the in vitro antimicrobial and minimal inhibitory concentration (MIC) methods on Vibrio parahaemolyticus, the main pathogen of S. paramamosain. The results showed that Galla chinensis displayed the highest antibacterial effect on the pathogen, with an inhibitory plaque diameter > 35 mm, and MIC <0.2 mg∙mL−1. Subsequently, V. parahaemolyticus-infected S. paramamosain was subjected to medicated bath (20 mg∙L−1 G. chinensis) for three days and monitored for seven days (therapeutic group). At the end of the experiment, the crabs in positive control group (V. parahaemolyticus-infected crabs, non-medicated bath) showed the highest mortality (all died 6 days later), with noticeable changes in total bacteria counts (TBC), as well as total haemocyte counts (THC), and phenoloxidase (PO), lysozyme (LSZ), acid phosphatase (ACP) and alkaline phosphatase (ALP) activities compared to the negative control group (saline-injected crabs, non-medicated bath). In the therapeutic group, low cumulative mortality (CM) (25%) was observed. The indices of TBC, THC, and PO improved in the therapeutic group compared to positive control group two days after treatment but returned to normal, with no difference to the negative control group at the end of the experiment. Compared with safety group (saline-injected crabs, medicated bath) and negative control group, 20 mg∙L−1 G. chinensis extract showed no adverse effect on crabs other than causing some decrease in THC, 24 h after immersion. It was therefore concluded that certain TCH are able to inhibit the V. parahaemolyticus, and G. chinensis may be useful in the treatment of crab diseases caused by V. parahaemolyticus.