Retrospective genome-oriented analysis reveals low transmission rate of multidrug-resistant Pseudomonas aeruginosa from contaminated toilets at a bone marrow transplant unit

Background Prevention of toilet-to-patient transmission of multidrug-resistant Pseudomonas aeruginosa (MDR PA) poses management-related challenges at many bone marrow transplant units (BMTUs). Aim To conduct a longitudinal retrospective analysis of the toilet-to-patient transmission rate for MDR PA under existing infection control (IC) measures at a BMTU with persistent MDR PA toilet colonization. Methods The local IC bundle comprised: (1) patient education regarding IC; (2) routine patient screening; (3) toilet flushing volume of 9 L; (4) bromination of toilet water tanks, and (5) toilet decontamination using hydrogen peroxide. Toilet water was sampled periodically between 2016 and 2021 (minimum every three months: 26 intervals). Upon MDR PA detection, disinfection and re-sampling were repeated until ≤3 cfu/100 mL was reached. Whole-genome sequencing (WGS) was performed retrospectively on all available MDR PA isolates (90 out of 117 positive environmental samples, 10 out of 14 patients, including nine nosocomial). Findings WGS of patient isolates identified six sequence types (STs), with ST235/CT1352/FIM-1 and ST309/CT3049/no-carbapenemase being predominant (three isolates each). Environmental sampling consistently identified MDR PA ST235 (65.5% ST235/CT1352/FIM-1), showing low genetic diversity (difference of ≤29 alleles by core-genome multi-locus sequence typing (cgMLST)). This indicates that direct toilet-to-patient transmission was infrequent although MDR PA was widespread (detection on 79 occasions, detection in every toilet). Only three MDR PA patient isolates can be attributed to the ST235/CT1352/FIM-1 toilet MRD PA population over six years. Conclusion Stringent targeted toilet disinfection can reduce the potential risk for MDR PA acquisition by patients.