Abstract
Teleost fish faced with stressful stimuli launch an endocrine stress response through activation of the hypothalamic-pituitary-interrenal axis to release glucocorticoids, in particular cortisol, into the blood. For the majority of bacterial fish pathogens, stress is considered a key factor in disease outbreaks. Based upon studies in mammals, there is considerable evidence to suggest that, besides impairing the immune system, cortisol can have a direct effect on bacterial cells. Hitherto, this intriguing field of microbial endocrinology has remained largely unexplored in aquatic diseases. The present study investigated in vitro the impact of cortisol on phenotypic traits of the fresh water fish pathogen Flavobacterium columnare. Colonies obtained from the highly virulent (HV) isolates resulted in significantly larger and more spreading colonies compared to those from the low virulent (LV) isolates. High cortisol doses added displayed a direct effect on the bacterial cells and induced a significant decrease in colony size. An additional intriguing finding was the inverse relationship between cortisol concentrations added to the broth and the spreading character of colonies retrieved, with higher cortisol doses resulting in less rhizoid to rough and even smooth colony formation (the latter only in the LV trout isolate), suggesting a dose–response effect. The loss of the rhizoid appearance of the F. columnare colonies upon administration of cortisol, and hence the loss of motility, might indicate a phenotypic change to the biofilm state. These findings form the basis for further research on the impact of glucocorticoids on other virulence factors and biofilm formation of F. columnare.Electronic supplementary materialThe online version of this article (doi:10.1186/s13567-016-0370-9) contains supplementary material, which is available to authorized users.
Highlights
For the majority of bacterial fish pathogens, stress is considered a key factor in disease outbreaks
Based upon studies in mammalian species, there is considerable evidence to suggest that, besides eliciting an impairment of the immune system, stress hormones can have a direct effect on the bacterial cells [1, 2]
The present study aimed to investigate the impact of cortisol on F. columnare in vitro
Summary
For the majority of bacterial fish pathogens, stress is considered a key factor in disease outbreaks. Based upon studies in mammalian species, there is considerable evidence to suggest that, besides eliciting an impairment of the immune system, stress hormones can have a direct effect on the bacterial cells [1, 2]. Hitherto, this intriguing field of microbial endocrinology, whereby micro-organisms, through their long co-existence with animals, have developed sensory systems for detecting host-associated hormones [3], has remained largely unexplored in aquatic diseases. Thereby, fish housed in the same water body are exposed to cortisol levels which may accumulate especially in recirculation aquaculture systems where water renewal is limited and stocking densities are high [16]
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