Fracture-related infections (FRIs) caused by multidrug-resistant (MDR) bacteria, such as *Klebsiella pneumoniae*, pose significant challenges due to limited treatment options and the need for frequent surgical interventions. *K. pneumoniae*'s ability to form biofilms, evade the immune system, and rapidly acquire resistance genes (like extended-spectrum β-lactamases (ESBLs) and carbapenemases) exacerbates the problem. Biofilms, complex bacterial communities attached to implants or tissue, harbor persister cells—dormant bacteria highly resistant to antibiotics. Treating FRIs with MDR bacteria often requires long-term, high-dose broad-spectrum antibiotics, leading to adverse effects and low success rates. Phage therapy, using bacteriophages (bacteria-infecting viruses), offers a potential solution. However, phages typically target only specific bacterial strains, necessitating personalized approaches. This often involves selecting phages from a pre-prepared bank and potentially pre-adapting them *in vitro* to increase infectivity and reduce resistance development. This study explores the efficacy of a combined pre-adapted phage and antibiotic therapy for a severe FRI caused by a pandrug-resistant *K. pneumoniae* strain.

The increasing prevalence of MDR bacterial infections, particularly those caused by *Klebsiella pneumoniae*, necessitates the exploration of alternative therapeutic strategies. The literature highlights the challenges associated with treating *K. pneumoniae* infections due to biofilm formation and rapid resistance development. Existing studies underscore the potential of phage therapy as an adjunct to conventional antibiotic treatment, but emphasize the need for personalized approaches, including phage pre-adaptation to enhance efficacy and minimize resistance emergence. Previous research demonstrates the effectiveness of phage therapy in treating various bacterial infections, including those involving orthopedic implants. The use of pre-adapted phages, specifically tailored to target the patient's bacterial strain, is increasingly recognized as a crucial step in improving treatment outcomes. The existing literature supports the potential synergistic effects of combining phage therapy with antibiotic treatment to combat MDR bacterial infections, but further investigation is necessary to optimize treatment protocols.

This case study involved a 30-year-old female victim of a suicide bombing who developed a severe FRI caused by a pandrug-resistant *K. pneumoniae* strain after extensive antibiotic treatment (over 700 days). The patient’s *K. pneumoniae* isolates were sent to the Eliava Institute in Tbilisi, Georgia, for phage selection and pre-adaptation. Phage vB_KpnM_M1 (M1) was selected based on its lytic activity against the patient's isolates. The phage was characterized extensively: its morphology was determined via transmission electron microscopy; its host range was assessed using a broad panel of *Klebsiella* spp. isolates; its adsorption rate, latent period, burst size, pH stability, and thermostability were evaluated using standard techniques. A one-step growth experiment revealed that phage M1 has a productive reproduction cycle with a short adsorption period (91% of phage particles adsorbed in 2 min, 99% in 6 min) (Fig. 2c), causing bacterial cell lysis within 8–10 min after a latent period of 35 min, and an average burst size of 43 phage particles per infected cell (Fig. 2d). Maximal phage activity was observed over the pH range of 5.0–10.0 (Fig. 2e) and the phage was thermostable until 50 °C (Fig. 2f). The phage was then pre-adapted using Appelmans' method for 15 rounds to reduce resistance development. Whole-genome sequencing of both the original and pre-adapted phage M1 was performed to identify mutations responsible for improved lytic activity. Antibiotic susceptibility testing was performed using the VITEK 2 system and disk diffusion methods (Table 1). After ethical approval and informed consent, local phage therapy, combined with antibiotic treatment (meropenem and colistin initially, then ceftazidime/avibactam), was initiated. The patient's clinical, microbiological, and radiological status were monitored before and after treatment. The efficacy of the combined phage and antibiotic therapy was also investigated *in vitro* using mature biofilms and bacterial suspensions to assess potential synergistic effects. Phage neutralization by patient sera was assessed over time to investigate potential immune response.

The patient's *K. pneumoniae* isolates exhibited a pandrug-resistant (PDR) phenotype, resistant to almost all available antibiotics. The pre-adapted phage M1 displayed high lytic activity against the patient's *K. pneumoniae* strains both *in vitro* and in mature biofilms. Genomic analysis revealed that phage M1 belonged to the *Slopekvirus* genus and lacked genes associated with toxicity, lysogeny, or antibiotic resistance. A single missense mutation (Thr281Arg) in the distal tail fiber protein was identified in the pre-adapted phage, potentially contributing to its enhanced lytic activity. Combination therapy with pre-adapted phage M1 and antibiotics (meropenem, colistin, and ceftazidime/avibactam) resulted in significant clinical, microbiological, and radiological improvement. *In vitro* studies demonstrated a synergistic effect of the phage-antibiotic combination in reducing bacterial counts in mature biofilms compared to either treatment alone (Fig. 8). Similarly, *in vitro* studies in suspensions showed enhanced efficacy of the phage-antibiotic combination (phage M1 with ceftazidime/avibactam and meropenem, but not colistin) against the PDR *K. pneumoniae* isolate (Fig. 9). The patient's serum showed evidence of phage-neutralizing antibodies after phage application, but these antibodies disappeared after 3 years. Three months after the combined phage-antibiotic therapy, the FRI was controlled, and the patient's clinical status had dramatically improved. Long-term follow-up (3 years) showed no recurrence of the infection.

This case study demonstrates the potential of pre-adapted phage therapy as a valuable addition to the treatment arsenal for infections caused by PDR bacteria. The successful outcome highlights the synergistic effect of combining phage therapy with appropriately selected antibiotics. The observed *in vitro* synergy suggests that pre-adapted phages may enhance antibiotic efficacy by targeting persister cells within biofilms and making them more susceptible to antibiotics. The development of neutralizing antibodies is a common occurrence in phage therapy, but in this case, their presence did not seem to negatively impact the therapeutic success. The long-term clinical success and absence of recurrence indicate a potential long-lasting therapeutic effect. The results support the further exploration of individualized, combined phage-antibiotic therapies for MDR bacterial infections, particularly in cases where conventional antibiotic treatment fails.

This case study provides strong evidence for the efficacy of a combined pre-adapted phage and antibiotic therapy in resolving a challenging FRI caused by a PDR *K. pneumoniae* strain. The successful outcome underscores the potential of personalized phage therapy as a valuable tool in combating MDR bacterial infections. Further research should focus on optimizing phage-antibiotic combinations, understanding the underlying mechanisms of synergy, and conducting larger clinical trials to validate these findings.

This study is a case report involving a single patient, limiting the generalizability of the findings. The local administration of phages prevents drawing firm conclusions regarding systemic effects. The precise mechanisms underlying the observed synergy between the phage and antibiotics remain to be fully elucidated. While the study provides compelling evidence of therapeutic success, larger, well-controlled clinical trials are needed to confirm the efficacy and safety of this approach in a wider patient population.