Loss of the type VII secretion ATPase EssC promotes biofilm formation of Staphylococcus aureus under acidic stress.

Staphylococcus aureus adapts to hostile host-associated niches by dynamically switching between planktonic growth and biofilm lifestyles. Acidic environments, such as the skin surface and intracellular compartments, impose substantial stress on bacterial survival; however, the contribution of the type VII secretion system (T7SS) to biofilm adaptation under acidic conditions remains poorly understood. Here, we investigated the role of the T7SS ATPase EssC in regulating S. aureus biofilm formation under acidic stress. Using an essC deletion mutant in the USA300 background, we demonstrate that loss of EssC markedly enhances biofilm biomass and thickness at pH 5.0, despite reducing bacterial viability within mature biofilms. Mechanistically, essC deletion reprograms multiple stages of biofilm development, including enhanced initial adhesion mediated by upregulated fibronectin-binding proteins (FnBPA and FnBPB), increased intercellular aggregation driven by elevated polysaccharide intercellular adhesin (PIA) production, and biofilm stabilization through augmented autolysis-dependent extracellular DNA release. These phenotypic changes are accompanied by coordinated transcriptional remodeling, characterized by downregulation of the biofilm repressor agr and activation of the arlS-icaA and sigB-icaA regulatory axis. Collectively, our findings uncover an unrecognized link between the T7SS core component EssC and biofilm regulation under acidic stress, highlighting EssC as a potential modulator of S. aureus survival strategies in hostile host microenvironments.