A global to local genomics analysis of Clostridioides difficile ST1/RT027 identifies cryptic transmission events in a northern Arizona healthcare network

Clostridioidesdifficile is a ubiquitous, diarrheagenic pathogen often associated with healthcare-acquired infections that can cause a range of symptoms from mild, self-limiting disease to toxic megacolonand death. Since the early 2000s, a large proportion of C. difficile cases have been attributed to the ribotype027 (RT027) lineage, which is associated with sequence type 1 (ST1) in the C. difficile multilocus sequence typing(MLST) scheme. The spread of ST1 has been attributed, in part, to resistance to fluoroquinolones used to treat un-related infections, which creates conditions ideal for C. difficile colonization and proliferation. In this study, we analyzed 27 isolates from a healthcare network in northern Arizona, USA, and 1,352 public ST1 genomes to place locally-sampled isolates into a global context. Core genome, single nucleotide polymorphism (SNP) analysis demonstrated that at least 6 separate introductions of ST1 were observed in healthcare facilities in northern Arizona over an 18-month sampling period. A reconstruction of transmission networks identified potential nosocomial transmission of isolates following two of these introductions, which were only identified via whole genome sequenceanalysis. Antibiotic resistance heterogeneity was observed among ST1 genomes, including variability in resistance profiles among locally sampled ST1 isolates. To investigate why ST1 genomes are so common globally, we compared all high-quality C. difficile genomes and identified that ST1 genomes have gained and lost a number of genomic regions compared to all other C. difficile genomes; analyses of other toxigenic C. difficile sequence types demonstrates that this loss may be anomalous and could be related to niche specialization. These results suggest that a combination of antimicrobial resistance and gain and loss of specific genes may explain the prominent association of this sequence type with C. difficile infection cases worldwide. The degree of genetic variability in ST1 suggests that classifying all ST1 genomes into a quinolone-resistant hypervirulent clone category may not be appropriate. Whole genome sequencingof clinical C. difficile isolates provides a high-resolution surveillance strategy for monitoring persistence and transmission of C. difficile and for assessing the performance of infection prevention and control strategies.