Phage Henu15-resistant mutant derives fitness trade-offs in Morganella morganii

The rapid emergence of multidrug-resistant strains of Morganella necessitates new therapeutic approaches, such as phage therapy. Phages are considered a promising adjunct to antibiotics. However, the rapid emergence of phage-resistant bacterial mutants poses a significant challenge in clinical practice. Nevertheless, this phenomenon is often accompanied by fitness trade-offs. Therefore, understanding these fitness trade-offs is crucial prior to phage clinical application. In the present study, a new lytic Morganella morganii phage, named Henu15, was isolated and characterized. The phage genome comprises a 52,795 bp double-stranded DNA with 72 open reading frames. Notably, the phage genome lacks genes encoding integrases, known virulence factors, or acquired antibiotic resistance determinants, underscoring its therapeutic potential. A time-kill assay demonstrated that Henu15 exhibits synergistic effects with ciprofloxacin, norfloxacin, and ceftazidime. Importantly, a Henu15-resistant mutant exhibited pleiotropic defects, including impaired adsorption, enhanced sensitivity to polymyxin B and tetracycline, reduced biofilm formation, and diminished in vivo colonization capacity. Through the whole-genome resequencing of the phage-resistant mutant, mutations were found in 7 genes involved in encoding related proteins such as ATP-dependent chaperone protein ClpB, tetrasulfate reductase subunit A, and AlpA family transcriptional regulators, which might be responsible for the observed fitness defects, providing a molecular basis for the attenuated virulence. Together, these findings provide new insights into the evolutionary interactions between phage resistance and bacterial fitness that could potentially offer a novel approach to treating resistant Morganella infections.