Comparative plaque- and liquid-based phage susceptibility testing of Pseudomonas aeruginosa isolates in a cystic fibrosis randomized clinical trial

The optimal phage susceptibility testing (PST) method is unclear. Here, two PST methods were compared using Pseudomonas aeruginosa isolates recovered from patients with cystic fibrosis enrolled in a placebo-controlled clinical trial of a single intravenous dose of a four-phage cocktail (WRAIR-PAM-CF1). In total, 508 P. aeruginosa sputum isolates were evaluated, including isolates collected before or after phage or placebo treatment, isolates from the same trial participants over time, and isolates of different colony morphologies from the same participants at the same time point. Susceptibility to phages PaWRA01Φ11, PaWRA01Φ39, PaWRA02Φ83, and PaWRA02Φ87 and a cocktail of all four phages was assessed using a plaque assay (PA) and a Biolog Omnilog-based liquid assay (LA) with the same bacterial inoculum. A total of 170/508 isolates (33%) and 195/508 isolates (38%) were identified as susceptible to the cocktail by PA and LA, respectively. Among the susceptible isolates, 169/170 (99%; PA) and 187/195 (96%; LA) also tested susceptible to at least one of the four phages in the cocktail. Susceptible percent agreement between the PA and LA ranged from 76% to 100% and non-susceptible percent agreement ranged from 89% to 100% for all phages studied and the cocktail. Phage PaWRA02Φ83 demonstrated the greatest individual activity, with 164/508 (32%) and 188/508 (37%) isolates testing susceptible by PA and LA, respectively. The four-phage cocktail provided numerically broader coverage of P. aeruginosa (i.e., had the highest susceptibility rate) than did any single phage. There was high concordance between PST results by PA and LA.