Natural occurrence of a slow lytic pseudomonas phage in a Pediatric case of multidrug-resistant P. aeruginosa severe pneumonia.

Pseudomonas aeruginosa (P. aeruginosa) is widely distributed in the environment. As an opportunistic pathogen, it commonly causes infections in immunocompromised individuals, including respiratory tract infections and burn wound infections. P. aeruginosa possesses multiple antibiotic resistance mechanisms, including efflux pumps, resistance genes, and population dynamics. Phage therapy is a potential approach for addressing drug-resistant P. aeruginosa infections; however, clinical experience and standardized guidelines for its application in severe pneumonia remain limited. A 14-month-old infant was hospitalized for pneumonia. Four days later, he developed acute pneumonia and was sent to the ICU for 38 days of antibiotic therapy; nonetheless, P. aeruginosa remained detectable in the patient's respiratory secretions. During the clinical course, phage zjk6 was detected from a longitudinal P. aeruginosa isolate in the absence of phage therapy. This finding documents the coexistence of a naturally detected phage and MDR P. aeruginosa during prolonged pneumonia, but does not establish that the phage mediated bacterial clearance or clinical recovery. We performed whole-genome sequencing on P. aeruginosa isolates from patients to ascertain if they were infected by the same infection and assessed their antibiotic resistance using drug sensitivity testing. We isolated phages using the drip technique and double-layer plate method, examined their appearance by transmission electron microscopy, and assessed their biological properties through one-step growth curve analysis and lysis spectrum detection. Genome sequencing and comparative genomic analyses were performed to characterize phage zjk6 and representative bacterial isolates and to evaluate phage-host genomic relatedness. P. aeruginosa was isolated repeatedly during 49 days of treatment. Comparative genomic analysis of representative longitudinal isolates revealed multiple strain backgrounds, including distinct ST508 and ST266 lineages and a closely related ST836 lineage. Phage zjk6 was isolated from the fifth clinical isolate, which served as the propagation/reference host. This phage possesses an elongated tail and a limited lysis spectrum, which is capable of gradually lysing the fifth isolated P. aeruginosa strain. Genomic analysis showed that zjk6 formed plaques and displayed slow lytic behavior under the tested conditions, while also carrying lysogeny-associated regulatory modules, indicating temperate potential rather than a strictly lytic lifestyle. A naturally detected slow lytic Pseudomonas phage may coexist with MDR P. aeruginosa during prolonged infection. These findings support further study of phage-bacterium interactions in clinical infections, while the therapeutic significance of zjk6 requires additional validation.