The Stress Hormone Cortisol Directly Impactsflavobacterium Columnarein Vitrogrowth

THE STRESS HORMONE CORTISOL DIRECTLY IMPACTSFLAVOBACTERIUM COLUMNAREIN VITROGROWTH CHARACTERISTICS

A.M. Declercq*, J. Aerts, B. Ampe, F. Haesebrouck, S. De Saeger, A. Decostere

Ghent University, Ghent, Belgium.

Institute for Agriculture and Fisheries Research, Melle, Belgium.

Abstract

For the majority of bacterial fish pathogens, the role of stressors is considered a key factor in disease outbreaks.Based upon studies in mammals, there is considerable evidence to suggest that, besides eliciting an impairment of the immune system, stress hormones can have a direct effect on bacterial cells. 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,has remained largely unexplored in aquatic diseases.In this respect, the purpose of the present study was to investigate the in vitro impact of the stress hormone cortisol on Flavobacterium columnare, the causative agent of columnaris disease, a much-feared and predominant bacteriosis of freshwater fish.To do so, F. columnare isolates of different virulence were cultivated in the presence of cortisol and the impact on bacterial titres and colony morphology assessed.

Exposure of the highly virulent isolates to cortisol resulted in decreased bacterial titres. In addition, the retrieved colonies were smaller and displayed a significantly less rhizoid appearance and spreading as exhibited by shorter radiating tendrils compared to colonies procured from unsupplemented broth. For the low virulent isolates, no significant differences were noted whether or not the growth medium was supplied with cortisol.

These results are particularly interesting as rhizoid colonies with spreading edges reflect the gliding motility of the harbouredF. columnare cells. This gliding trait is lost as bacteria become immobile and start forming microcolonies on the fish’s skin and gills, as a first step in biofilm formation, which is a well-known feature in fish succumbing to columnaris disease.This urges us to speculate that fish harbouring high cortisol levels in the skin and gill mucus, are more prone to microcolony and consequent biofilm formation by highly virulent F. columnare isolates. This may explain the individual variation in terms of disease susceptibility as the magnitude of mucus cortisol levels may differ markedly in between fish following stressstimuli.

This study is the first to demonstrate a direct effect of cortisol on a fish pathogenic bacterium and hence engenders a new perspective to bacteria-host communications in aquaculture.