A disease with white spots in internal organs of Nile tilapia occurred in Zhanjiang, southern China. Multiple, white nodules, 0.8-2.2 mm in diameter, were scattered throughout the liver, spleen and kidney of diseased fish. Signs of nodules reproduced after artificial infection with the isolated strain. Isolated bacteria were Gram-negative, facultative anaerobic, motile, short rod-shaped, with a length of 1.2-2.2 μm. Morphological and biochemical tests, as well as phylogenetic analysis, all strongly indicated that the isolate from tilapia is identical to Aeromonas schubertii (A. schubertii) which temporary named LF1708 strain. Antibiotic sensitivity assays showed the LF1708 is sensitive to 24 of 27 tested antibiotics. Pathogenicity test revealed that the isolate at the dose of 3.75 × 10 CFU/g killed 100% of experimental tilapia within 2 days and the dose of 1 × 10 CFU/g killed 100% of experimental zebrafish within 1 day. Histopathology of diseased tilapia infected with A. schubertii showed numerous necrotic lesions widely distributed in spleen, liver and kidney, and infiltration with a large number of bacteria. To our knowledge, this was the first report that associated A. schubertii with mortality in tilapia.

No abstract

Streptococcus agalactiae is a causative agent responsible for massive mortalities of tilapia that has led to catastrophic losses to tilapia culture globally. Bacteriophages represent a new class of antimicrobials against bacteria. In this study, we characterized the bacteriophage HN48, which formed small and round-transparent plaques on a double-layer plate. With a hexagonal head and a long tail, this phage may belong to the Caudovirales according to the International Committee on Taxonomy of Viruses. HN48 was found to have a relatively wide and highly specific host range, to be sensitive to high temperature (60-80°C) and low pH (3-5), and to be relatively stable at alkaline pH (8-10). Intraperitoneal injection with HN48 had no adverse effects on tilapia and effectively inactivated the bacteria in the kidney. Fish that received phage therapy had 60% ± 3.3% survival rates and a delayed mean death time of about 3 days when compared to the control group. To the best of knowledge, this is the first study of tilapia streptococcal phage. Overall, the results indicated that phage HN48 could prevent tilapia from experimental S. agalactiae infection, suggesting it has the potential to control this disease.

A novel permanently growing brain cell line from koi (Cyprinus carpio L.) (KB cell line) was established, and its suitability for detection of koi herpesvirus (KHV) was demonstrated in this study. The KB cell line was optimally maintained at 27°C in Leibovitz's L-15 medium supplemented with 10% foetal bovine serum (FBS). It was subcultured more than 100 times, and chromosome analysis revealed that 51.54% of KB cells at passage 80 maintained the abnormal diploid chromosome number 2n = 96 while the modal chromosome number was 2n = 100. The cell line was cryopreserved in liquid nitrogen at -196°C and was recovered from storage after 1 year with good cell viability and vitality. The results of virus isolation demonstrated that KB cells were susceptible to KHV, which was shown by the presence of an obvious cytopathic effect and abundant virus particles. The viral titres of KHV in KB reached 10 TCID /0.1 ml within 7 days. Immunofluorescence and Western blot assays confirmed that KB replicated KHV. The newly established KB cell line will serve as a useful tool to elucidate KHV disease (KHVD) pathogenesis.

No abstract