A genetic screen identifies a role for oprF in Pseudomonas aeruginosa biofilm stimulation by subinhibitory antibiotics

Abstract

Biofilms are surface-associated communities of bacteria that grow in a self-produced matrix of polysaccharides, proteins, and extracellular DNA (eDNA). Sub-minimal inhibitory concentrations (sub-MIC) of antibiotics induce biofilm formation, indicating a potential defensive response to antibiotic stress. However, the mechanisms behind sub-MIC antibiotic-induced biofilm formation are unclear. We show that treatment of Pseudomonas aeruginosa with multiple classes of sub-MIC antibiotics with distinct targets induces biofilm formation. Further, addition of exogenous eDNA or cell lysate failed to increase biofilm formation to the same extent as antibiotics, suggesting that the release of cellular contents by antibiotic-driven bacteriolysis is insufficient. Using a genetic screen to find stimulation-deficient mutants, we identified the outer membrane porin OprF and the extracytoplasmic function sigma factor SigX as important for the phenotype. Similarly, loss of OmpA - the Escherichia coli OprF homologue - prevented sub-MIC antibiotic stimulation of E. coli biofilms. The C-terminal PG-binding domain of OprF was dispensable for biofilm stimulation. Our screen also identified the periplasmic disulfide bond-forming enzyme DsbA and a predicted cyclic-di-GMP phosphodiesterase encoded by PA2200 as essential for biofilm stimulation. The phosphodiesterase activity of PA2200 is likely controlled by a disulfide bond in its regulatory domain, and folding of OprF is influenced by disulfide bond formation, connecting the mutant phenotypes. Addition of the reducing agent dithiothreitol prevented sub-MIC antibiotic biofilm stimulation. Finally, we show that activation of a c-di-GMP-responsive promoter follows treatment with sub-MIC antibiotics in the wild-type but not an oprF mutant. Together, these results show that antibiotic-induced biofilm formation is likely driven by a signalling pathway that translates changes in periplasmic redox state into elevated biofilm formation through increases in c-di-GMP. SignificanceBacterial biofilms cause significant treatment challenges in clinical settings due to their ability to resist antibiotic killing. Paradoxically, sub-inhibitory levels of a number of chemically distinct antibiotics, with different modes of action, can stimulate biofilm formation of multiple bacterial species. In this work we used the biofilm forming opportunistic pathogen Pseudomonas aeruginosa to look for mutants blind to sub-inhibitory antibiotic exposure, failing to demonstrate biofilm stimulation in response to 3 different antibiotics. We identified key players in this response, including the outer membrane porin OprF and the sigma factor SigX. Notably, the hits from our mutant screen connect changes in periplasmic redox state to elevated biofilm formation via c-di-GMP signaling. By understanding these underlying mechanisms, we can better strategize interventions against biofilm-mediated antibiotic resistance in P. aeruginosa and other bacterial species.