Antimicrobial resistance is a global health crisis, partly contributed by inappropriate use of antibiotics. The increasing emergence of multidrug resistant infections has led to the resurgent interest in bacteriophages as an alternative treatment. Current procedures assessing susceptibility and breadth of host range to bacteriophage are conducted using large-scale manual processes that are labor-intensive. The aim here was to establish and validate a scaled down methodology for high-throughput screening in order to reduce procedural footprint. Bacteriophages were isolated from wastewater samples and screened for specificity against 29 clinical Pseudomonas aeruginosa isolates and PA01 using a spot test (2 μL/ drop). Host range assessment was performed on four representative P. aeruginosa isolates using both double agar overlay assay on petri dishes and 24-well culture plates. The breadth of host range of bacteriophages that exhibited lytic activity on P. aeruginosa isolates were corroborated between the current standard practice of whole plate phage assay and 24-well phage assay. The high correlation achieved in this study confirms miniaturization as the first step in future automation that could test phage diversity and efficacy as antimicrobials.

Aim: To investigate the potential of phage therapy combined with antibiotics in suppressing bacterial growth of multidrug resistant P. aeruginosa isolated from individuals with cystic fibrosis (CF). Background: P. aeruginosa, a common pathogen colonising the lungs of individuals with CF becomes difficult to treat and contributes to increased morbidity and mortality in this population. With a lack in new antimicrobials, alternative treatment options including bacteriophage (phage) therapy is now critical. Methods: Overnight cultures of nine tobramycin-resistant P. aeruginosa clinical isolates, were adjusted to 10⁵ CFU/mL and 100 µL added to each well of a 96-well culture plate. P. aeruginosa was then treated with 100 µL of tobramycin (4 µg/mL) or phage (KM3, BM12, MM1; MOI 10, 10⁶ PFU/mL) singularly or in combination to assess treatment efficacy. Bacterial suspensions were incubated for 24 h and overall bacterial growth (OD600_nm) measured. Fractional inhibition concentration index (FICi) was calculated to assess for synergism. Results: P. aeruginosa treated with 4 µg/mL of tobramycin (4-16-fold lower than the MIC) and phages resulted in the reduction of overall bacterial growth (KM3: 46.9-88.8%; BM12: 22.8-86.1%; MM1: 17.9-83.5%). The FICi of tobramycin and phages (KM3, BM12 or MM1) were between 0.0-1.9 demonstrating synergistic (≤0.5) or no interaction (0.5-4) activities depending on the clinical isolate. No antagonistic interaction was observed. Conclusions: This study demonstrates the potential of combination treatments, including the delay of resistance development. Furthermore, lower concentrations of antibiotics used may have a positive impact on future stewardship.