Temperature change modulates pathogenicity of Francisella noatunensis subsp. orientalis to Nile tilapia but not the expression of its virulence genes

Francisellosis outbreaks have been reported in the literature as occurring seasonally when water temperature is below 24°C. The aim of this study was to quantify the median lethal doses (LD50) of FNO in experimental challenges at 28°C and 22°C, and to investigate the impact of temperature changes in whole genome expression using microarray technology. The LD50 for Nile tilapia at 28°C was ~105.7, whereas at 22°C, the LD50 was ~102.2, showing that the decrease in temperature enhanced pathogen virulence. Out of 1,917 genes screened, a total of 31 and 19 genes were down- and up-regulated at 22°C, respectively. These genes were grouped by orthology into functional categories of: amino acid, inorganic ion, and carbohydrate transport and metabolism; transcription; and posttranslational modification, protein turnover, and chaperones. Expression of genes related to metabolism, oxidative stress, and thermal shock were regulated by temperature changes, reflecting an ability of FNO to adapt to the environment. Expression of virulence genes usually required for the Francisella genus was not changed between tested temperatures, including that of genes located on the Francisella Pathogenicity Island. The F. noatunensis subsp. orientalis FNO12 isolated from diseased fish was used in this study. Quadruplicate culture of this strain were incubated at 22°C or 28°C and harvested at the mid-exponential phase of bacterial growth (OD600 = 0.2) for RNA extraction. The RNA level expression are analyzed on Agilent Microarray platform.