This study compared two methods of enumerating bacteria adhered to the external membrane of eggs of landlocked fall Chinook salmon Oncorhynchus tshawytscha that were subjected to different formalin treatment regimes from egg eye‐up to fry hatch. Bacterial numbers were recorded by either directly counting bacteria via a scanning electron microscope (SEM) or via established bacterial culture methods that provided the number of colony‐forming units. Treatment regimes consisted of a daily 15‐min exposure to formalin at either 500 mg/L or 1,667 mg/ L or no formalin initially followed by exposure to a 1,667‐mg/L dose starting 7 d after eye‐up. A control group did not receive any formalin treatments throughout the experiment. No significant correlation was observed between the number of colony‐forming units (CFUs) per square millimeter of egg membrane determined by plate culture and the number of bacteria per square millimeter counted with the SEM. Bacterial numbers determined by the SEM were 40–120 times greater than those obtained by plate culture. The SEM numbers were correlated to embryo survival, but CFU numbers were not. For both counting methods, bacterial numbers among the untreated controls increased over time. Of the three species of bacteria identified in cultures (Flavobacterium columnare, Sphingobacterium spp., and Pseudomonas spp.), F. columnare was predominant. Embryo survival in the trays that did not receive any formalin was significantly less than that in all of the other treatments, but there was no significant difference in survival among any of the three formalin treatment regimes.
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.
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