The stress hormone cortisol: a (co)regulator of biofilm formation in Flavobacterum columnare?

Authors: DECLERCQ, ANNELIES - Ghent University; CAI, WENLONG - Auburn University; NARANJO, EBER - Auburn University; THONGDA, WILLAWAN - Auburn University; ARIAS, COVADONGA - Auburn University; DE LA FUENTE, LEONARDO - Auburn University; Beck, Benjamin; Lange, Miles; PEATMAN, ERIC - Auburn University; AERTS, J - Ghent University; DECOSTERE, ANNEMIE - Ghent University

Submitted to: International Conference on Diseases of Fish and Shellfish
Publication Type: Abstract Only
Publication Acceptance Date: 4/26/2017
Publication Date: 9/5/2017
Citation: Declercq, A.M., Cai, W., Naranjo, E., Thongda, W., Arias, C., De La Fuente, L., Beck, B.H., Lange, M.D., Peatman, E., Aerts, J., Decostere, A.M. 2017. The stress hormone cortisol: a (co)regulator of biofilm formation in Flavobacterum columnare? [abstract]. International Conference on Diseases of Fish and Shellfish. p. 18.

Interpretive Summary: Flavobacterium columnare is becoming the principal bacterial pathogen of different farmed fish species globally, causing immense economic losses. Previous work on the effect of cortisol on F. columnare growth gave new perspective into host-pathogen interactions. Stress induced in fish increases cortisol levels in the skin and gill mucus that can trigger F. columnare to form biofilms on the host tissue. Overall susceptibility to columnaris disease varies between isolates which could be due how fish react under stressful conditions. Biofilm formation is an important part of the disease process, therefore in this study multiple isolates of F. columnare were examined under laboratory conditions that facilitate biofilms. It was observed that supplementing the bacterial growth medium with cortisol resulted in increased biofilm growth. A series of bacterial genes believed to be involved in biofilm formation were assayed and showed that their gene expression was increased in free-living bacterial cells exposed to the cortisol as compared to the fixed biofilm cells. These results allow for insight into the mechanisms of pathogenesis which may be co-regulated by cortisol released by the host. Additional work to better understand these mechanisms will lead to better treatments and production methods to reduce disease.

Technical Abstract: Previously, we demonstrated a direct effect of cortisol on Flavobacterium columnare, a notorious fish pathogenic bacterium, engendering a new perspective to bacteria-host communication in aquaculture. As stressed fish harbour increased cortisol levels in the skin and gill mucus, highly virulent F. columnare isolates could be more triggered to form biofilm on the host tissue. This could explain individual variation in terms of disease susceptibility in stress situations. Indeed, biofilm formation seems to be crucial in the pathogenesis of columnaris disease. Gliding motility in particular is a well-known characteristic involved in the colonization of surfaces. During the latter, planktonic F. columnare cells undergo important morphological changes in the transition to the biofilm state. Methodology and results In this study, we cultivated F. columnare isolates of different virulence in the presence or absence of cortisol to investigate its impact on bacterial biofilm formation. To mimic flow conditions of water over the gill and skin as realistically as possible, biofilm formation was studied by using microfluidic chambers. Intriguingly, supplementing cortisol to the bacterial broth resulted in a thicker biofilm, suggesting it has a (co-)regulatory effect on biofilm formation by F. columnare. To refine this hypothesis, the expression of a select series of gliding genes (gldB, gldC, gldH, sprA, and sprB) was investigated using qPCR following culturing the bacteria in the absence or presence of cortisol. The addition of cortisol resulted in a significantly higher expression of gliding genes in the planktonic cells and a significantly lower expression of gliding genes in the sessile cells compared to control planktonic and sessile cells to which no cortisol was added, respectively.