Abstract. Screening and selecting of indigenous gastrointestinal bacteria and nipah worm fecal pellets are very important before being applied as probiotics. Previous studies have succesfully isolated 10 bacterial isolates that having  cellulolytic and proteolytic abilities from intestinal and fecal pellets of nipah worm. The purpose of this study was to determine the pathogenicity of all isolates against nipah worms in vitro and in vivo. Testing of pathogenicity in vitro was carried out on blood agar and DNAse agar, while in vivo testing was carried out by injecting 0.1 ml of bacterial suspension into the nipah worm body which was then cultured for 14 days. The results showed that only 10% (3 from 30 isolates) of all isolates were suspected having pathogenic activity. Isolates NrBF6, NrBF 9, and NrBC4 had been indicated from haemolysis activity in blood agar and lysed DNA on DNAse agar medium. In vivo pathogenicity tests through injection into gastrointestinal caivity showed that isolates NrBF6, NrBF9 and NrBC4 had LD50 at the suspension dose of 10³ bacterial cells. LD50 reached for 5, 8 and 20 days, respectively. Symptoms of infection that appeared most dominantly in nipah worms were wounds on the surface of the body, broken body segments, and pale.

Keywords: pathogenicity, gastrointestinal bacteria, nipah worm, Namalycastis