Abstract
Antimicrobial resistance is a global concern, especially with the backdrop of the development of possible environmental and public health hazards. Flavobacterium columnare has been observed to be multi-drug resistant but highly susceptible to sodium chloride, hence the need for susceptibility profiling. Therapeutic efficacy of commonly used antibiotics and sodium chloride was tested in-vitro against four isolates of F. columnare using the disc diffusion and the pour plate methods, respectively. Comparative in-vivo testing was performed on experimentally infected Clarias gariepinus juveniles in 11 groups of 15 fish each. Nine groups were treated with 1.0, 2.0 and 3.0% NaCl for 5, 10 and 30 min each. A tenth group was exposed to 25 mg L-1 ciprofloxacin for 1 h by immersion, while the eleventh group was not treated (positive control). Growth inhibition was observed to be highest with ciprofloxacin followed by ofloxacin and tetracycline in that order, and also in all NaCl concentrations. There was no mortality in the infected fish groups treated with 25 mg L-1 ciprofloxacin, 1% NaCl, and 2 to 3% NaCl for 5 and 10 min only. However, 46.7 ± 9.4 % mortality observed in 3 % NaCl treatment for 30 min was significantly higher than the 23.3 ± 4.6% observed in 2 % NaCl for 30 min, but not significantly different from the positive control with 36.7±4.7 % mortality. Since short duration sodium chloride bath was found to be effective in the control of columnaris disease, 1% salt disinfection of fry and fingerling stock for 30 min could be incorporated into routine management in catfish hatchery, without the fear of environmental or public health hazards. Key words: Antibiogram, columnaris disease, sodium chloride, susceptibility.  
Highlights
Flavobacterium columnare has been observed by various researchers (Bernardet and Grimont 1989; Hawke and Thune, 1992) to have developed resistance to many antibiotics, the basis for which its isolation is done on medium made selective with the incorporation of antibiotics to which it is resistant such as neomycin
The zone of inhibition to the growth of F. columnare observed for ciprofloxacin ranged between 15 to 26 mm with an average of 21.25 mm
The antibiotic bath and salt disinfection trials reflected the results obtained for the in-vitro susceptibility test as fish treated with 25 mg L-1 of ciprofloxacin and salt concentration of 1% and above were cured of columnaris disease
Summary
Health polymyxin B, and tobramycin (Hawke and Thune, 1992; Starliper, 2003). Blind use of antibiotics is wasteful and constitutes potential environmental and public health hazard since 75% of the antibiotics still finds its way back to the environment and can further exacerbate the already worsening problems of antibiotic-resistant pathogens
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