New genetic tools in Finegoldia magna identify a conserved adhesin required for the formation of stress-tolerant aggregates

The Gram-positive obligate anaerobe Finegoldia magna is a member of the healthy human microbiota, but also acts as an opportunistic pathogen to cause persistent, biofilm-associated infections. Despite its prevalence on the human host, little is known about F. magna biology or the mechanisms underlying its clinically relevant phenotypes, partly due to the lack of genetic tools. We address this gap by establishing genetic approaches for investigating gene function in F. magna, which we apply to identify genetic determinants of aggregate biofilm formation. We found that F. magna isolates are naturally competent, allowing for targeted chromosomal integration of linear DNA constructs via homologous recombination. Transformation frequency varied substantially among strains and was also affected by factors such as homologous flank length, DNA concentration, and incubation method. To identify genes that mediate aggregate biofilm formation, we used experimental evolution to select for F. magna mutants that had lost the ability to aggregate. This approach identified a conserved locus encoding a putative adhesin that we named FafA (Finegoldia adhesion factor A). Next, we applied targeted mutagenesis tools to show that deletion of fafA markedly reduced autoaggregation but does not impair other modes of biofilm formation, including surface attachment or aggregation during agitation. Finally, we demonstrate that FafA-mediated aggregation protects F. magna from antibiotic and oxidative stress. Together, these findings establish a genetic framework for mechanistic studies in F. magna and identify FafA as a conserved adhesin that promotes aggregation and stress tolerance in this anaerobic pathobiont.