ABSTRACT
Flavobacterium columnre is one of the most important bacterial pathogens of wild, farmed and ornamental fish in warm, and also in fresh and sea water causing columnaris disease. In the present study one hundred and forty Nile tilapia were collected from different localities of farms from El-Behera governorate and Kafr El-sheikh provinces. The clinical signs of the disease were detected and the pathological lesions were recorded. Bacteriological examination of samples of naturally infected fish with Flavobacterium columnare revealed that the characteristics colonies on cytophaga agar media. Sixty seven suspected isolates were isolated and biochemically identified by API-20 E system. The distribution of the gained isolates among organs denoted 33 isolates from gills [49.25%], 20 isolates from kidneys [29-85%], 12 isolates from deep layer of dorsal musculature [17.91%], one isolate from both spleen and intestine [1.49%], but it was not isolated from the liver. The pathogencity experiment indicated wide variation among Flavobacterium columnare isolates. Isolate number 10 was the most virulent isolate as it caused 100% mortality two weeks post injection followed by isolate number 7, 3 caused 45, 25% mortality. The clinical signs and the post mortem examination revealed the same signs and lesions of natural infection with Flavobacterium columnare beside severe oedematous swelling at site of injection. The median lethal dose [LD50] of the highly virulent isolate was 10[5] viable bacterial cells /ml. Stress factors, temperature, PH, unionized ammonia and dissolved oxygen affects on the pathogencity of the organism. In vitro antibiotic sensitivity test of 5 isolates revealed that the most effective drug Cefotaxime [60%]. While the less effective one, were Neomycin, Norfloxacin and Rifampcin [100%] then Doxycyclin, Cephalexin, Florfenicel, Gentamycin and Marbocyl [80%] then Cefalor, Tetracycline and Ciprofloxacin [60%]. On the other hand, Erthromycin, Ampicillin [80%] had no effect