With the increase in carbapenem-resistant A. baumannii (CRAB) infections, there has been a resurgence in the use of polymyxins, specifically colistin (COL). Since the reintroduction of COL-based regimens in treating CRAB infections, several COL-resistant A. baumannii isolates have been identified, with the mechanism of resistance heavily linked with the loss of the lipopolysaccharide (LPS) layer of the bacterial outer membrane through mutations in lpxACD genes or the pmrCAB operon. SPR206, a novel polymyxin derivative, has exhibited robust activity against multidrug-resistant (MDR) A. baumannii. However, there is a dearth of knowledge regarding its efficacy in comparison with other A. baumannii-active therapeutics and whether traditional polymyxin (COL) mediators of A. baumannii resistance also translate to reduced SPR206 activity. Here, we conducted susceptibility testing using broth microdilution on 30 A. baumannii isolates (17 COL-resistant and 27 CRAB), selected 14 COL-resistant isolates for genomic sequencing analysis, and performed time-kill analyses on four COL-resistant isolates. In susceptibility testing, SPR206 demonstrated a lower range of minimum inhibitory concentrations (MICs) compared with COL, with a four-fold difference observed in MIC₅₀ values. Mutations in lpxACD and/or pmrA and pmrB genes were detected in each of the 14 COL-resistant isolates; however, SPR206 maintained MICs ≤ 2 mg/L for 9/14 (64%) of the isolates. Finally, SPR206-based combination regimens exhibited increased synergistic and bactericidal activity compared with COL-based combination regimens irrespective of the multiple resistance genes detected. The results of this study highlight the potential utility of SPR206 in the treatment of COL-resistant A. baumannii infections.

Gram-positive bacteria covalently anchor specific proteins to the peptidoglycan cell wall via sortase, a cysteine transpeptidase that targets proteins with a cell wall sorting signal. Sortase enzymes are critical for bacterial pathogenesis, and their inhibitors have become promising therapeutic targets for infection management. Filifactor alocis, a Gram-positive anaerobic bacterium, is now proposed as a diagnostic indicator of periodontal disease. Unlike other bacteria, F. alocis encodes a single putative sortase, FA1364. In this study, we functionally characterized the putative sortase FA1364 and found that it belongs to the class A family (SrtA). The SrtA-anchored surface proteins (FA1006, FA1336, FA1424, and FA1750) were identified, and MS/MS analysis confirmed that SrtA is required for their cell-surface localization. The recombinant SrtA protein could recognize and cleave the LPKTG sorting motif with cysteine 191 and arginine 200 as essential catalytic residues. F. alocis FLL101 (ΔFA1364::ermF) showed reduced ability to coaggregate and form biofilm, along with decreased collagen binding and survival in epithelial cells. Additionally, the FA1364-defective mutant exhibited increased sensitivity to air exposure. Collectively, these results suggest that the F. alocis SrtA protein is an important virulence factor and may represent a novel therapeutic target for the control of periodontal diseases.