Clinically Relevant Acinetobacter spp. Proliferate but Contribute Little to Phosphate Accumulation Under Enhanced Biological Phosphorus Removal-Simulating Conditions

Wastewater treatment plants (WWTPs) receive clinically relevant Acinetobacter species originating from hospitals and municipal sewage, yet their role in enhanced biological phosphorus removal (EBPR) remains unclear. This study evaluated the capacity of Acinetobacter spp. to accumulate orthophosphate (P) under conditions simulating the aerobic phase of EBPR and investigated the physiological and molecular determinants of P accumulation. Clinically relevant A. pittii and A. baumannii exhibited comparable growth to the model P-accumulating species A. junii and successfully proliferated in synthetic wastewater. However, A. junii maintained significantly higher P uptake rates, accumulated more intracellular polyP granules, and continued to remove P after entering the stationary phase, whereas P removal by A. pittii and A. baumannii reached a plateau. While A. junii and A. pittii contained a single ppk1 gene, two distinct ppk1 homologues and the corresponding PPK1 isoforms were identified in A. baumannii. Nevertheless, neither ppk1 copy number, ppk1 expression, nor the abundance of key enzymes involved in canonical polyP metabolism explained the enhanced P accumulation capacity of A. junii. Proteomic analyses suggested that continued P uptake in A. junii was associated with physiological adaptations that support the maintenance of cellular fitness during the stationary phase. Consequently, EBPR systems may favour the persistence and proliferation of clinically relevant Acinetobacter spp. without substantially benefiting from their contribution to P removal.