Naturally-occurring, strain-specific defects in the retraction of Acinetobacter baumannii type IV pili promote biofilm formation

Type IV pili are helical filaments composed of protein subunits which are produced by numerous taxa of bacteria, including Acinetobacter. Type IV pili are extended out from the cell by extension enzyme complexes, which extract subunits from the membrane and insert them into the base of the filament, but can also be retracted by reverse rotation catalyzed by a retraction enzyme. Type IV pili have diverse functions, and some (twitching motility, DNA-uptake) require retraction while others (host adhesion, bacterial aggregation) do not. Acinetobacter bacteria, including International Clone I (IC-I) and International Clone II (IC-II) strains, show variable phenotypes in assays of type IV pilus-dependent functions. We show this variation is the result of variable efficiency in pilus retraction between pilus subtypes, and from that, a differential balance between retraction-dependent and retraction-independent functions. We define type IV pilus subtypes based on the sequence of the major subunit, PilA. In both naturally-occurring pilA variants from the IC-I and IC-II groups and isogenic strains complemented with IC-I or IC-II pilA, the IC-I pilus subtype promotes greater twitching motility and DNA-uptake while the IC-II pilus subtype promotes biofilm formation while showing reduced capacity for DNA-uptake and twitching motility, similar to a retraction-deficient mutant and consistent with the hypothesis that pilus retraction of the IC-II pilus is naturally deficient. Testing the hypothesis that this defect in retraction was sufficient to increase the level of piliation on the cell surface, we compared the yields of T4P sheared from the cell surface and found that in an isogenic background, complementation with IC-II pilA results in greater levels of surface PilA per cell than equivalent complementation with an IC-I pilA gene.