Resistome and phylogenomic analysis of trimethoprim–sulfamethoxazole-resistant Stenotrophomonas maltophilia complex isolates obtained from Bulgarian hematopoietic stem cell transplant recipients

The present study aimed to investigate the resistome of four trimethoprim-sulfamethoxazole (SXT)-resistant Stenotrophomonas maltophilia complex (Smc) isolates from Bulgarian hematopoietic stem cell transplantation (HSCT) recipients and to subject them to phylogenomic analysis involving all sul1-positive strains of the identified species with available genomes worldwide. Preliminary identification by MALDI-TOF mass spectrometry determined all four isolates as S. maltophilia. The sources of isolation were stools (SM175, SM176, and SM179) and urine (SM178). SM176 and SM178 also showed high-level levofloxacin resistance. All isolates demonstrated in vitro susceptibility to minocycline and cefiderocol. Whole-genome sequencing (WGS) assigned SM175, SM176, and SM178 as Stenotrophomonas forensis. Two types of class 1 integrons were detected in the four isolates, namely SM175 and SM179 carried empty integrons, whereas SM176 and SM178 carried a gene cassette (3,748 bp in length) consisting of aac6'-Ib-cmlB-blaOXA-9. Alignment against public databases revealed that this cassette has not been found in Stenotrophomonas species so far, but it was present in Pseudomonas aeruginosa and Enterobacterales. Phylogenomic analysis of our assembled sequences, together with all 26 sul1-positive S. maltophilia and S. forensis genomes, indicated that S. maltophilia SM179 was not part of any S. maltophilia cluster. SM175, SM176, and SM178 were closely related (differences of 35-101 SNPs). To the best of our knowledge, this is the first report of SXT-resistant Smc isolates from post-HSCT patients with hematological malignancies in Bulgaria, which presents WGS-based resistome and phylogenomic analyses. We also report on the first sul1-containing S. forensis clinical isolates. Our findings reveal high global heterogeneity of sul1-positive S. maltophilia.