Host adaptations in the fixLJ pathway of the Burkholderia cepacia complex increase virulence

The Burkholderia cepacia complex (BCC) is composed of multiple species, including B. multivorans and B. dolosa, that are significant pathogens for people with cystic fibrosis (CF) and are extensively resistant to many antibiotics. The fixL gene of the fixLJ 2-component system (TCS) in these BCC species shows evidence of positive selection for nonsynonymous mutations during chronic lung infection in CF. Previous work showed that the B. dolosa fixLJ system regulates 11% of the genome and modulates biofilm formation, motility, persistence within macrophages, and virulence in a murine pneumonia model. Here, we assess the impacts of clinically observed FixL evolved variants in fixLJ pathway-mediated phenotypes in B. dolosa and B. multivorans. BCC carrying the ancestral fixL sequence are less pathogenic than constructs carrying evolved variants in both a macrophage infection model and a murine pneumonia model. In vitro phospho-transfer experiments demonstrate that the evolved B. dolosa FixL variants are able to reduce fixLJ pathway activity by either having lower levels of kinase activity or increased phosphatase activity. Notably, the ancestral fixL genotype has increased ability to survive within the soil compared to isogenic constructs with evolved fixL genotypes, demonstrating that increased virulence comes at an expense. Modulation of the FixLJ system has profound effects on many BCC phenotypes including full pathogenicity, and this modulation is critical for BCC adaptation to the host. ### Competing Interest Statement The authors have declared no competing interest.