الفهرس 
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Abstract
The present study was planned for the isolation of F. columnare species from cultured freshwater fish collected from different localities with phenotypic and molecular characterization of isolate species using gene-based PCR amplification. Phylogenetic analysis of 16S rRNA using BLAST (basic local alignment search tool) revealed that the three genetic groupings constituted well-supported and separate clades within the species F. columnare. In addition to determining if each isolate contained the three virulence genes thioredoxin (trx), glycosyltransferase (gtf), and nitric oxide reductase (norB), the study assessed the bacterium’s resistance to antimicrobial agents.
Therefore, 200 cultured African catfish, Clarias gariepinus and Nile tilapia, Oreochromis niloticus, were collected from cultured freshwater fish farms in Alexandria and Behaira governments. The following results were summarised into the following:
- Clinical examination of columnaris infection in C. gariepinus is characterised by the presence of ulceration and redness can be observed on the body surfaces, potentially resulting in a grey-white discoloration in areas such as the head, mouth, gills, fins, and overall body surface. This discoloration may gradually extend to adjacent skin, causing it to undergo a process known as under-running.
However, the clinical manifestations of O. niloticus displayed a lethargic swimming pattern in close proximity to the water surface and collected near the water inlets. Frequent occurrences of fast opercular movements and atypical swimming behaviours, including rotational motions and jumping, were frequently documented. The presence of a dense, yellowish, opaque mucus layer that envelops both the skin and gills was accompanied by the occurrence of skin ulcerations.
- The biochemically identified F. columnare isolates (N = 5) were identified by the VITEK system.
- Representative isolates were confirmed as F. columnare (genetic group 1) strains TIL2015, HNU-06 and AA, F.covae (genetic group 2) strain CC1351 and F.davisii (genetic group 3) strain ARS-15-12 by sequencing of the 16s rRNA gene using BLAST (basic local alignment search tool) and stored in GenBank databases.
- The presence of virulence genes (Thioredoxin, Glycosyltransferase, and Nitric oxide reductase) and hemolytic activity of each isolate were detected.
- Antimicrobial susceptibility testing of five F. columnare isolates revealed that all of them were sensitive to gentamycin (30 μg) and ofloxacin (5 μg). However, F. columnare (genetic group 1) were also sensitive against doxycycline (30 μg) and streptomycin (10 μg). While F. covae (genetic group 2) was sensitive to doxycycline (30 μg) and colistin (10 μg), All isolates were resistant to amoxicillin (30 μg), spiramycin (100μg), and cefalexin (30 μg), This means that the best drug choice for treatment of columnaris was gentamycin (30 μg) and ofloxacin (5 μg).