Nitric oxide inhibits biofilm formation by Vibrio fischeri via the nitric oxide sensor HnoX

Abstract

Nitric oxide (NO) is an important defense molecule secreted by the squid Euprymna scolopes and sensed by the bacterial symbiont, Vibrio fischeri, via the NO-sensor HnoX. HnoX inhibits colonization through an unknown mechanism. The genomic location of hnoX adjacent to hahK, a recently identified positive regulator of biofilm formation, suggested that HnoX may inhibit colonization by controlling biofilm formation, a key early step in colonization. Indeed, deletion of hnoX resulted in early biofilm formation in vitro, an effect that was dependent on HahK and its putative phosphotransfer residues. An allele of hnoX that encodes a protein with increased activity severely delayed wrinkled colony formation. Control occurred at the level of transcription of the syp genes, which produce the polysaccharide matrix component. The addition of NO abrogated biofilm formation and diminished syp transcription, effects that required HnoX. Finally, an hnoX mutant formed larger symbiotic biofilms. This work has thus uncovered a host-relevant signal controlling biofilms and a mechanism for inhibition of biofilm formation by V. fischeri. The study of V. fischeri HnoX permits us to understand not only host-associated biofilm mechanisms, but also the function of HnoX domain proteins as regulators of important bacterial processes.

Candida albicans biofilm inactivated by cold plasma treatment in vitro and in vivo

“…Indeed, many biofilm inactivation methods are developed based on hydroxyl radical formation, and gaseous ozone therapy is also considered as an efficient approach for oral biofilm inactivation . Furthermore, biofilm formation can be inhibited by nitric oxide …”

Should they stay or should they go? Nitric oxide and the clash of regulators governing Vibrio fischeri biofilm formation

“…The current work by Thompson, Tischler et al . () not only generates new questions in the V. fischeri‐E. scolopes symbiosis, it also contributes to a larger interest in NO and biofilm regulation.…”

“…SypG is phosphorylated and activated by the sensor kinase SypF, which itself can be modulated by another sensor kinase RscS (Darnell et al ., ; Hussa et al ., ; Norsworthy and Visick, ). Conversely, SypF can reduce SypG activity, although the conditions determining whether SypF has a net positive or negative effect on SypG activation are not yet understood (Thompson et al ., b). The versatile SypF also modulates the activity of SypE, which represses Syp‐biofilm formation post‐transcriptionally (Morris et al ., ; Morris and Visick, ).…”

“…In this issue of Molecular Microbiology , Thompson, Tischler et al . () now connect a negative regulator of Syp biofilm formation to a regulatory cue with its own rich history of study in this and other symbioses: nitric oxide (NO). As noted above, HahK is an indirect activator of syp .…”

“…Although there is a good deal of evidence supporting the idea that V. fischeri responds to NO produced by the host, Thompson, Tischler et al () rightly point out that endogenous NO production by V. fischeri cannot be ruled out as a contributing signal. In Azospirillum brasilense , endogenous NO production as a byproduct of denitrification can stimulate biofilm formation, and this stimulation can be blocked by a disruption of the Nap periplasmic nitrate reductase system (Arruebarrena Di Palma et al ., ).…”