Citrobacter spp. are facultative anaerobic, Gram-negative bacteria that have been known to cause infections in the urinary tract and bloodstream. These infections are predominantly nosocomial in nature. Hospitals are considered to be reservoirs for multiresistant bacteria; as such, their wastewater microbiome is characterized by these bacteria. In the present study, a carbapenem-resistant Citrobacter braakii strain was isolated from hospital sewage in Greifswald, Germany. This particular strain, designated C. braakii GW-Imi-1b1, was the focus of a comprehensive investigation including analyses of antibiotic resistances and proteome profile. Antibiotic resistance tests revealed a broad resistance spectrum, including chloramphenicol, tetracycline, and ertapenem. In addition, an investigation into the defense mechanisms revealed a constitutive presence of a β-lactamase at an unusually high concentration. The growth curve with different meropenem concentrations revealed almost no effect on growth at 20 µg/ml meropenem. Consequently, the high prevalence of β-lactamase may be a contributing factor to the observed carbapenem resistance. Furthermore, due to its localization on the plasmid, it may be capable of being transferred within the wastewater microbiome. Antibiotic treatment of bacterial infections can be substituted or accompanied by phage therapy. The lytic phage vB_CbrP_HGW_001 was isolated from sewage and has been shown to be capable of lysing C. braakii GW-Imi-1b1. Sequencing and microscopic analysis revealed a Kayfunavirus within the Caurovicetes. Subsequent properties of vB_CbrP_HGW_001 indicated the potential for a promising alternative treatment to antibiotics. Phage therapy, particularly when employed in combination with antibiotics, demonstrates a valuable tool in the combat against infections caused by multidrug-resistant bacteria.

The authors have declared no competing interest.

This study was funded by the Helmholtz foundation.

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All data produced in the present study are available upon reasonable request to the authors.