The in-vitro antibacterial effects of organic salts, chemical disinfectants and antibiotics against pathogens of black disease in fairy shrimp of Thailand.

The antibacterial effects of organic salts, chemical disinfectants and antibiotics were evaluated on cultures of Aeromonas hydrophila C03, Aeromonas sobria C26, A. sobria C29, Aeromonas caviae C24 and Acinetobacter sp. SH-94B, the pathogens that cause black disease found in fairy shrimps (Streptocephalus sirindhornae Sanoamuang et al. (2000) and Branchinella thailandensis Sanoamuang, Saengphan & Murugan) of Thailand. The minimal inhibitory concentrations (MICs) of organic salts (sodium chloride and potassium chloride) and antibiotics (oxytetracycline dihydrate, streptomycin sulphate, kanamycin monosulphate, chloramphenicol and ampicillin) were determined using the agar-dilution method. The effect of chemical disinfectants (sodium hypochlorite and chlorine dioxide) was evaluated by exposing bacteria to different concentrations of these chemicals for different periods of time. Interestingly, all strains were intrinsically resistant to 0.25-3% sodium chloride and potassium chloride. The effect of sodium hypochlorite was greater than that of chlorine dioxide, and 5-20 µg mL(-1) of sodium hypochlorite was sufficient to inhibit the growth of these bacteria, but the exposure time varied, depending on the bacterial species. Of the antibiotics tested, chloramphenicol and oxytetracycline dihydrate completely inhibited the selected strains. Chloramphenicol showed the highest antibacterial effect against all pathogenic species--the MIC and minimal bactericidal concentration (MBC) ranged from 0.8 to 3.1 µg mL(-1) from 3.1 to 6.25 µg mL(-1), respectively. To achieve control of black disease during cultivation of fairy shrimp, data derived from this study can be used as a basis for further toxicity tests in vivo.

Clinical observations of black disease in fairy shrimps, Streptocephalus sirindhornae and Branchinella thailandensis, from Thailand and pathogen verification.

In this study, black disease infecting fairy shrimps, Streptocephalus sirindhornae Sanoamuang, Murugan, Weekers & Dumont, and Branchinella thailandensis Sanoamuang, Saengphan & Murugan, in Thailand, was investigated. The typical signs of the disease are the appearance of black spots on the cuticle, located mainly on the dorsal side and thoracopods. A number of rod-shaped bacteria aggregated in the black spots and were visualized by scanning electron microscopy. The histopathological results showed that a haemocytic response to the infection resulted in a dense melanized core of bacteria. In addition, generalized septicaemia by rod-shaped bacteria was also observed in the infected tissue. Of the 31 isolates, Aeromonas spp. were predominantly isolated and six strains were selected for the experimental infections. The most pathogenic strain was identified molecularly as A. hydrophila. When fairy shrimp were infected at bacterial concentrations of 10(4) and 10(6) cfu mL(-1) , the overall infection levels were 73.33 ± 6.67% and 93.33 ± 6.67%, respectively. The experimentally infected fairy shrimp showed abnormal swimming and died within 24-48 h after the appearance of the dark pigment.