The rapid increase in the number of worldwide human infections caused by the extremely antibiotic resistant bacterial pathogen Stenotrophomonas maltophilia is cause for concern. An alternative treatment solution in the post-antibiotic era is phage therapy, the use of bacteriophages to selectively kill bacterial pathogens. In this study, the novel bacteriophage AXL3 (vB_SmaS-AXL_3) was isolated from soil and characterized. Host range analysis using a panel of 29 clinical S. maltophilia isolates shows successful infection of five isolates and electron microscopy indicates that AXL3 is a member of the Siphoviridae family. Complete genome sequencing and analysis reveals a 47.5 kb genome predicted to encode 65 proteins. Functionality testing suggests AXL3 is a virulent phage and results show that AXL3 uses the type IV pilus, a virulence factor on the cell surface, as its receptor across its host range. This research identifies a novel virulent phage and characterization suggests that AXL3 is a promising phage therapy candidate, with future research examining modification through genetic engineering to broaden its host range.

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Stenotrophomonas maltophilia is a ubiquitous environmental bacterium capable of causing disease in humans. Antibiotics are largely ineffective against this pathogen due to numerous chromosomally encoded antibiotic resistance mechanisms. An alternative treatment option is phage therapy, the use of bacteriophages to selectively kill target bacteria that are causing infection. To this aim, we isolated the Siphoviridae bacteriophage AXL1 (vB_SmaS-AXL_1) from soil and herein describe its characterization. Host range analysis on a panel of 30 clinical S. maltophilia strains reveals a moderate tropism that includes cross-species infection of Xanthomonas, with AXL1 using the type IV pilus as its host surface receptor for infection. Complete genome sequencing and analysis revealed a 63,962 bp genome encoding 83 putative proteins. Comparative genomics place AXL1 in the genus Pamexvirus, along with seven other phages that infect one of Stenotrophomonas, Pseudomonas or Xanthomonas species. Functional genomic analyses identified an AXL1-encoded dihydrofolate reductase enzyme that provides additional resistance to the antibiotic combination trimethoprim–sulfamethoxazole, the current recommended treatment option for S. maltophilia infections. This research characterizes the sixth type IV pilus-binding phage of S. maltophilia and is an example of phage-encoded antibiotic resistance.