Study Of DNA Vaccine And Screening Of Effective Chinese Medicine Herbs Against Streptococcus Agalactiae Of Tilapia

In recent years, the outbreak of streptococcosis in China has caused significanteconomic losses in tilapia (Oreochromis niloticus) aquaculture. At present, a commonstrategy in intensive agriculture to control streptococcosis is the use of antibiotics athigh doses. However, such usage has been widely criticized due to negative impactsincluding the accumulation of drug residues in tissues, the development of antibioticresistance, and immunosuppression. As a result, there is an urgent need to decreasethe widespread use of antibiotics and develop an environmentally friendly yeteffective alternative strategy to control streptococcal infections and disease in fish.This paper investigated the epidemiology of Streptococcus agalactiae isolated fromtilapia in south of China, and then studied the DNA vaccine and Chinese herbalmedicine as methods to treat and manage streptococcosis.Firstly, nearly40pathogens were isolated from diseased tilapia cultured inGuangdong, Hainan and Guangxi province. All of the pathogens were identified as S.agalactiae by a specific PCR assay and API20Strep tests. The serotype andmolecular capsular type of these strains were Ⅰa. Both the strain THN, which was isolated from diseased tilapia cultured in Hainan province, and TZH, which wasisolated from diseased tilapia cultured in Guangdong province, were sensitive tothirteen antibiotics (i.e. florfenicol, chloromycetin, nitrofurantoin, doxycycline,tetracycline, minocycline, josamycin, acetylspiramycin, clindamycin, vanguard ADMVI, rifampicin, gatifloxacin and ofloxacin), intermediate sensitive to penicillin, butresistant to six antibiotics (i.e. fluazinam acid, fleroxacin, neomycin, kanamycin,streptomycin, polymyxa amphotericin B). In addition, one S. agalactiae strain(designated strain SDG) was isolated from diseased jade perch (Scortum bacoo)cultured in indoor fish farm in Guangdong province. Our study showed that thecharacteristics (serotype, resistant sperctrum, API20Strep test) of the SDG were assame as THN, this was the first report that jade perch infected by S. agalactiae.DNA vaccination is a technique for protecting an organism against disease byinjecting expression vector with genetically engineered DNA to produce animmunological response. Compared with other vaccines, DNA vaccines has manyadvantages, including easy to operate, stable, easy to store and transport, safe andnon-toxic, etc.. In this paper, we cloned the surface membrane antigen gene, namedsip, from the THN strain, and then integrated into pcDNA3.1eukaryotic expressionvector. Subsequently, the recombinant plasmids pcDNA3.1_sip were injected intotilapia by intramuscular injection. The tilapia was challenged after one monthimmunization. Experimental results showed the recombinant pcDNA3.1_sip providedprotective effect with a relative percent survival (RPS) of46%. This experimentsuggests that the DNA vaccine can be a potential method to treat streptococcosis.Even though vaccine in aquaculture has made some progress in recent years, theuse of vaccine to treat streptococcosis will take some time.There is an urgent need todevelop an environmentally friendly yet effective method to manage streptococcalinfections. Natural approaches such as the use of medicinal herbs and plants do notcause resistant bacteria nor pose as big of an environmental hazard, and should beconsidered for controlling fish diseases in aquaculture. In this study,25TCM herbswith broad antibacterial spectrums were chosen to be tested for their antimicrobialactivity against S. agalactiae in-vitro. Based on the initial screening out of25herbs, Coptis chinensis, Rhus chinensis, Rheum palmatum, Sanguisorba officinalis, Punicagranatum, Scutellaria baicalensis, Eugenia caryophyllata and Phellodendronchinense showed antibacterial activity against S. agalactiae and were selected forfurther experimental study. Further research showed that C. chinensis hold the mosteffective anti-S. agalactiae activity in vitro. In vivo experiments showed that fish feedsupplemented with Coptis chinensis reduced mortality of S. agalactiae infected Niletilapia. The C. chinensis provided protective effect with a RPS of80%after one weekfeed.