Phage-antibiotic combination is a superior treatment against Acinetobacter baumannii in a preclinical study

BACKGROUND: Clinical phage therapy is often delivered alongside antibiotics. However, the phenomenon of phage-antibiotic synergy has been mostly studied in vitro. Here, we assessed the in vivo bactericidal effect of a phage-antibiotic combination on Acinetobacter baumannii AB900 using phage øFG02, which binds to capsular polysaccharides and leads to antimicrobial resensitisation in vitro. METHODS: We performed a two-stage preclinical study using a murine model of severe A. baumannii AB900 bacteraemia. In the first stage, with an endpoint of 11 h, mice (n = 4 per group) were treated with either PBS, ceftazidime, phage øFG02, or the combination of phage and ceftazidime. The second stage involved only the latter two groups (n = 5 per group), with a prolonged endpoint of 16 h. The primary outcome was the average bacterial burden from four body sites (blood, liver, kidney, and spleen). Bacterial colonies from phage-treated mice were retrieved and screened for phage-resistance. FINDINGS: In the first stage, the bacterial burden (CFU/g of tissue) of the combination group (median: 4.55 × 10(5); interquartile range [IQR]: 2.79 × 10(5)–2.81 × 10(6)) was significantly lower than the PBS (median: 2.42 × 10(9); IQR: 1.97 × 10(9)–3.48 × 10(9)) and ceftazidime groups (median: 3.86 × 10(8); IQR: 2.15 × 10(8)–6.35 × 10(8)), but not the phage-only group (median: 1.28 × 10(7); IQR: 4.71 × 10(6)–7.13 × 10(7)). In the second stage, the combination treatment (median: 1.72 × 10(6); IQR: 5.11 × 10(5)–4.00 × 10(6)) outperformed the phage-only treatment (median: 7.46 × 10(7); IQR: 1.43 × 10(7)–1.57 × 10(8)). Phage-resistance emerged in 96% of animals receiving phages, and all the tested isolates (n = 11) had loss-of-function mutations in genes involved in capsule biosynthesis and increased sensitivity to ceftazidime. INTERPRETATION: øFG02 reliably drives the in vivo evolution of A. baumannii AB900 towards a capsule-deficient, phage-resistant phenotype that is resensitised to ceftazidime. This mechanism highlights the clinical potential of using phage therapy to target A. baumannii and restore antibiotic activity. FUNDING: National Health and Medical Research Council (Australia).