Effect of Flavobacterium columnare inoculation, antibiotic treatments and resident bacteria on rainbow trout Oncorhynchus mykiss eyed egg survival and external membrane structure.

This study was conducted to evaluate the potential pathogenicity of the bacterium Flavobacterium columnare on rainbow trout Oncorhynchus mykiss eyed eggs. Survival to hatching was unaffected by the inclusion in the incubation water of either 300 colony-forming units (CFU) ml(-1) or 3000 CFU ml(-1) of F. columnare at either 10 or 12 degrees C in either McConaughy or Shasta strain eyed eggs. Bacterial numbers, obtained via scanning electron microscopy or culture, and external membrane morphology were also not significantly different among eggs receiving different concentrations of F. columnare. Initial F. columnare burdens were significantly and positively correlated to the presence of biofilm on the egg external membrane, and biofilm was in turn significantly correlated with increased membrane degradation. The use of either streptomycin or tetracyclin antibiotics significantly reduced bacterial numbers on McConaughy strain eggs, and more eggs survived to hatch in those dishes treated with antibiotics.