Omics approaches provide new insights into the epidemiology and pathogenicity of the emerging fish-pathogen Francisella noatunensis subsp. orientalis

Abstract

Francisella noatunensis subsp. orientalis (FNO) is an emerging pathogen with large dissemination on Nile tilapia fish farms worldwide. In Brazil, extensive economic losses were associated to francisellosis outbreaks since 2012. Normally the disease cases and the fish mortalities occur when the water temperature is bellow 26ºC. It is an emerging pathogen and thus information about its genetic repertoire and virulence gene expression profiles are still poorly understood. The aims of this work were to compare phylogenetically the Brazilian isolates of each outbreak with sequenced genomes from other countries and to obtain the whole-genome profile expression at the temperatures of 22ºC and 28ºC. The sequenced genome sequences (n = 16) allows the confirmation that this species in under a strong genome decay (high number of pseudogenes and small genome, when compared with free-living species of the same genus), but it does not discard the existence of other strain in other countries. In Brazil, there is only one strain, with slight mutation events and with genetic diversions in the first reported cases, which demonstrates that this bacterium has only one insertion into the country before 2012. Regarding the quantification of the pathogen’s virulence, the observed median lethal doses were smaller (i.e., < 3 logs) when the hosts were maintained at 22ºC in comparison with those maintained at 28ºC. However, in in-vitro assays, the pathogen genes related to virulence did not change the expression when submitted to different temperatures. These results show a divergent evolution of this species as compared with other species of the same genus, which shows a significant regulation of these genes when submitted to similar temperatures as their hosts. This high expression of virulence genes migth be a host niche adaptive strategy. Since fishes are poikilothermic, substantial variations of their body temperature occur as a result of water temperature changes around them. On the other side, metabolism genes of FNO were up- and down-regulated between tested temperatures, which might represent an improved host fitness trait for replication at lower temperatures.