9781888569551-ch37

Proceedings of the Third World Fisheries Congress: Feeding the World with Fish in the Next Millenium—The Balance between Production and Environment

A Fragment Amplified from Aeromonas sobria Has Protective Immunity against Diseases Caused by A. hydrophila in Soft-Shelled Turtle Trionyx sinensis

Qiao-zhen Ye, Siu-ming Chan, Jian-guo He, Wei-jun Dai, Jing-yi Zhu, Jun-teng Xie, Jie Chen

doi: https://doi.org/10.47886/9781888569551.ch37

Aeromonas hydrophila is a gram-negative, facultatively anaerobic, short straight-rod freshwater bacterium that causes disease in humans and other vertebrates, including mammals, amphibians, fish, and reptiles. It is a mafjor pathogen in the culture of aquatic animals such as freshwater fish and soft-shelled turtle Trionyx sinensis that causes fatal hemorrhagic septicemia in China (Lu 1992; Ye 1999). It also has been implicated as a causative agent in human wound infections and diarrhea (Altwegg and Geiss 1989). The pathogenicity of this bacterium may involve several extracellular products, including hemolysin, enterotoxin, aerolysin, protease, and cell surface layers such as outer membrane proteins (OMPs), S-layer protein, and lipopolysaccharide (LPS; Austin et al. 1996).

In the past, diseases caused by A. hydrophila were controlled by using antimicrobial compounds and vaccines. Antibiotic treatment is not presently recommended because it can cause ecological problems. The widespread use of antimicrobial compounds over the past 50 years has increased resistance in several different bacteria species, destroyed the balance of normal bacteria in the environment and within host animals, and allowed some antibiotic-resistant bacteria to become dominant and emerge as pathogens. Inactivated whole cells, administered to fish orally by immersion or injection, have provided protection in several studies (Schachte 1978; Lamers and de Haas 1983; Ruangpan et al. 1986). The use of these vaccines is limited by the antigenic diversity of A. hydrophila strains in different geographical areas. Until now, no vaccines to protect farmed fish against A. hydrophila infections have been commercially available. This problem may be resolved by using vaccines containing common antigens that induce protection.

Aeromonas
cytotoxic enterotoxin (Act), an aerolysin-like toxin, possesses hemolytic, cytotoxic, and enterotoxic activities simultaneously (Rose et al. 1989). Qiu (1997) purified a toxin from a soft-shelled turtle isolate, A. hydrophila, that had the same biological activity and molecular mass (52 kDa) as Act. Further study revealed that this toxin is a potential protective antigen. Because the toxin is secreted at a low and unstable level in A. hydrophila and the procedure of formalin inactivation induces significant change in the antigen, recombinant technology may be the best way to overcome these problems.

For this study, primers in polymerase chain reaction (PCR) were designed based on the published Act gene sequence (Chopra et al. 1993). The amplification fragment from A. sobria was expressed as a recombinant protein, and the ability of this protein to provide protective immunity against A. hydrophila was tested.