Scientists studied a protein called BfpR in a bacterium and found that when this protein is missing, the bacteria become more resistant to the human antimicrobial peptide LL‑37. The same change also makes the bacteria form more biofilm and grow better inside human immune cells.

Response regulators are a critical part of the two-component system of gene expression regulation in bacteria, transferring a signal from a sensor kinase into DNA binding activity resulting in alteration of gene expression. In this study, we investigated a previously uncharacterized response regulator in <i>Francisella novicida</i>, FTN_1452 that we have named BfpR (Biofilm-regulating <i>Francisella</i> protein Regulator, <i>FTN_1452</i>). In contrast to another Francisella response regulator, QseB/PmrA, BfpR appears to be a negative regulator of biofilm production, and also a positive regulator of antimicrobial peptide resistance in this bacterium. The protein was crystallized and X-ray crystallography studies produced a 1.8 &#xc5; structure of the BfpR N-terminal receiver domain revealing interesting insight into its potential interaction with the sensor kinase. Structural analysis of BfpR places it in the OmpR/PhoP family of bacterial response regulators along with WalR and ResD. Proteomic and transcriptomic analyses suggest that BfpR overexpression affects expression of the critical <i>Francisella</i> virulence factor iglC, as well as other proteins in the bacterium. We demonstrate that mutation of <i>bfpR</i> is associated with an antimicrobial peptide resistance phenotype, a phenotype also associated with other response regulators, for the human cathelicidin peptide LL-37 and a sheep antimicrobial peptide SMAP-29. <i>F. novicida</i> with mutated <i>bfpR</i> replicated better than WT in intracellular infection assays in human-derived macrophages suggesting that the down-regulation of iglC expression in <i>bfpR</i> mutant may enable this intracellular replication to occur. Response regulators have been shown to play important roles in the regulation of bacterial biofilm production. We demonstrate that <i>F. novicida</i> biofilm formation was highly increased in the <i>bfpR</i> mutant, corresponding to altered glycogen synthesis. Waxworm infection experiments suggest a role of BfpR as a negative modulator of iglC expression with de-repression by Mg²⁺. In this study, we find that the response regulator BfpR may be a negative regulator of biofilm formation, and a positive regulator of antimicrobial peptide resistance in <i>F. novicida</i>.