Evaluating the protective efficacy of antigen combinations against Photobacterium damselae ssp. piscicida infections in cobia, Rachycentron canadum L

Cobia, Rachycentron canadum L., is a very important aquatic fish that faces the risk of infection with the bacterial pathogen Photobacterium damselae ssp. piscicida, and there are few protective approaches available that use multiple antigens. In the present study, potent bivalent antigens from P. damselae ssp. piscicida showed more efficient protection than did single antigens used in isolation. In preparations of three antigens that included recombinant heat shock protein 60 (rHSP60), recombinant α-enolase (rENOLASE) and recombinant glyceraldehyde-3-phosphate dehydrogenase (rGAPDH), we analysed the doses that elicited the best immune responses and found that this occurred at a total of 30 μg of antigen per fish. Subsequently, vaccination of fish with rHSP60, rENOLASE and rGAPDH achieved 46.9, 52 and 25% relative per cent survival (RPS), respectively. In addition, bivalent subunit vaccines--combination I (rHSP60 + rENOLASE), combination II (rENOLASE + rGAPDH) and combination III (rHSP60 + rGAPDH)--were administered and the RPS in these groups (65.6, 64.0 and 48.4%, respectively), was higher than that achieved with single-antigen administration. Finally, in combination IV, the trivalent vaccine rHSP60 + rENOLASE + rGAPDH, the RPS was 1.6%. Taken together, our results suggest that combinations of two antigens may achieve a better efficiency than monovalent or trivalent antigens, and this may provide new insights into pathogen prevention strategies.

An oral delivery system for recombinant subunit vaccine to fish

Oral vaccination is considered as the most desirable method for immunizing fish because it is non-stressful, user-friendly and is capable of easy administration to large numbers of fish. However, many publications have indicated that the current oral vaccines still lack the desired efficacy. Here we reported on an oral vaccine method to deliver recombinant subunit vaccine based on the food chain of fish, in that live Artemia nauplii are encapsulated with recombinant bacteria containing the antigen. The feasibility and efficacy of this method for delivering protein antigen was tested in a zebrafish model, where immunisation took place through feeding Artemia encapsulated with recombinant E. coli. By this oral vaccine method the antigen could be delivered to the hindgut as confirmed by immuno-histochemistry, and its efficacy was demonstrated by the ability to protect vaccinated fish significantly from a direct injection of a native Pseudomonas exotoxin and bacterial pathogen. Combined with the recombinant technology for producing desired protein antigens, this oral vaccine delivery method provides the capacity to deliver a variety of different antigens, including the option for delivery of multivalent vaccines by the oral route.

Simple and rapid detection of Photobacterium damselae ssp. piscicida by a PCR technique and plating method

AIMS

To detect Photobacterium damselae ssp. piscicida using the PCR technique and plating method.

METHODS AND RESULTS

Two strains of P. damselae ssp. piscicida were isolated from cultured cobia (Rachycentron canadum) at two different fish farms in Taiwan. A pair of primers was designed to detect the capsular polysaccharide gene of P. damselae ssp. piscicida by PCR. Reference strains of different genus and different clinical strains were used for this study. The expected product (410 bp) was obtained from both P. damselae ssp. piscicida and P. damselae ssp. damselae, and they were differentiated by culturing on thiosulphate citrate bile salts-sucrose agar (TCBS-1). Photobacterium damselae ssp. damselae grew on TCBS-1 producing green colonies whereas P. damselae ssp. piscicida did not grow.

CONCLUSIONS

The methods used are cost and labour effective when compared with the other methods and commercially available kits.

SIGNIFICANCE AND IMPACT OF THE STUDY

This work provides an integrated set of methods to identify the species P. damselae and to differentiate P. damselae ssp. piscicida from P. damselae ssp. damselae.