The involvement of Aeromonas salmonicida virulence factors in bacterial translocation across the rainbow trout, Oncorhynchus mykiss (Walbaum), intestine

Abstract

The pathogenic bacterium Aeromonas salmonicida is the causative agent of furunculosis, a lethal disease in salmonids. The mode of lateral transmission has not been conclusively defined, but A. salmonicida is able to translocate across the intestinal epithelium of salmonids, making the intestinal route a probable candidate. This study investigated some of the virulence mechanisms used by the bacteria to promote translocation. Intestinal segments were placed in modified Ussing chambers to investigate epithelial functions during exposure to bacterial factors. The factors were: extracellular products (ECP), lipopolysaccharide (LPS) or live or heat-inactivated A. salmonicida. Fluorescein isothiocynate (FITC)-labelling enabled detection of translocated bacteria by fluorometry. Live A. salmonicida translocated to a greater degree than heat-inactivated bacteria, suggesting that the bacteria utilize a heat sensitive surface-bound virulence factor which promotes translocation. The epithelium was negatively affected by ECP, manifested as decreased net ion transport, indicating a disturbance in ion channels or cell metabolism. LPS did not affect the epithelium in vitro when administered on the luminal side of the intestinal segment, but significantly increased epithelial translocation of fluorescent bacterial-sized microspheres when administered on the serosal side. This is suggested to be caused by increased transcellular transport, as the paracellular permeability was unaffected indicating maintained epithelial integrity.

Aeromonas salmonicida Type I pilus system contributes to host colonization but not invasion

“…However, the data presented here only show that the invasive A. salmonicida were protected from the antibiotic and do not allow us to determine if the invasive bacteria were internalized within the GI epithelial cells or were resident in the intracellular space. The results of Jutfelt et al (2008), however, suggest that once internalized within the epithelial cell, A. salmonicida can translocate across the epithelium to the serosal side of the intestinal wall, from where it can spread systemically.…”

“…It was not possible to use skin as, in contrast, many bacterial species present in the resident tank water are chloramphenicol-resistant, making the discrimination of otherwise unmarked Aeromonas salmonicida strains from the environmental background difficult, and it is impossible to adequately sterilize fish skin without destroying its structural integrity. Ex vivo A. salmonicida adherence and invasion in the salmonid GI has been reported (Ringø et al 2004, Jutfelt et al 2008) and most aeromonad species (e.g. A. hydrophilia, A. caviae, A. bestiarum) and the related taxon Pleisomonas are GI pathogens.…”

“…However, the data presented here only show that the invasive A. salmonicida were protected from the antibiotic and do not allow us to determine if the invasive bacteria were internalized within the GI epithelial cells or were resident in the intracellular space. The results of Jutfelt et al (2008), however, suggest that once internalized within the epithelial cell, A. salmonicida can translocate across the epithelium to the serosal side of the intestinal wall, from where it can spread systemically.In summary, the fim pili are major adhesive factors that allow Aeromonas salmonicida to adhere to salmon intestinal epithelial cells. Lack of fim causes only slight reduction in overall virulence however, reinforcing the notion that A. salmonicida virulence is complex and multifactorial.…”

“…Recent work (Jutfelt et al 2008) indicates that live Aeromonas salmonicida can cross the salmon intestinal epithelium by a trans-cellular route and that this translocation requires bacteria with intact and functional surface structures. Our continued interest in A. salmonicida adhesins leads us to ask if we could identify factors important for adherence to salmon intestinal tissue.…”

Evaluation of Prebiotic and Probiotic Effects on the Intestinal Gut Microbiota and Histology of Atlantic salmon (Salmo salar L.)

“…Furthermore, as the EU has recommend reductions of in vivo experiments and the numbers of animals used in experiments (Revision of the EU directive for the protection of animals used for scientific purposes [Directive 86/609/EEC]; 8 th of September 2010), attempts have been made to use alternative ex vivo methods (e.g. the Ussing chamber, everted sack and intestinal sack methods) [8][9][10][11][12][13][14][15].…”