Analysis of the potential of environmental Burkholderia sensu lato isolates to cause infection in a zebrafish embryo model

Burkholderia sensu lato (s.l.) regroups several genera of closely related bacterial species with remarkable diversity in metabolic features. They can be free-living or have mutualistic or pathogenic interactions with different host organisms, including insects, plants and animals. Burkholderia s.l. species display numerous characteristics that can be beneficial for agronomical and biotechnological applications, including promotion of plant growth and bioremediation; however, many of the species are known human pathogens. Several recent taxonomical reclassifications of the Burkholderia genus resulted in its separation into 7 distinct genera. Burkholderia sensu stricto (s.s.), which includes the Burkholderia cepacia complex and the pseudomallei group, regroups most of the species that have been shown to cause infection in humans and plants. Little is known about the pathogenic potential of species belonging to the other 6 genera; however there have been occasional reports of human infections.

Here, the vertebrate zebrafish embryo was used to analyse the behaviour of a panel of environmental isolates belonging to different species of the Burkholderia cepacia complex (5), Paraburkholderia (5) and Caballeronia (1). Burkholderia cepacia P14-NS, Burkholderia orbicola P21-NS, and Paraburkholderia sp. ABIP659 (closest to P. tropica , and further named tropica -like) each caused a low percentage (2%) of embryo mortality. While the other strains had not killed the embryos during the 4-day time span of the experiments, overall, Paraburkholderia and Burkholderia s.s. strains persisted equally well in this vertebrate host. Unexpectedly, P. tropica -like showed the highest increase in bacterial burden during infection, and induced the strongest pro-inflammatory response, including abscess formation, and poor prognosis at the endpoint of the experiment. Our study emphasizes that in vivo infection studies are needed to gain more insight into potential pathogenicity of the different Burkholderia s.l. species.