Genomic epidemiology and transmission dynamics of recurrent Clostridioides difficile infection in Western Australia.

Recurrent cases of Clostridioides difficile infection (rCDI) remain one of the most common and serious challenges faced in the management of CDI. The accurate distinction between a relapse (caused by infection with the same strain) and reinfection (caused by a new strain) has implications for infection control and prevention, and patient therapy. Here, we used whole-genome sequencing to investigate the epidemiology of 94 C. difficile isolates from 38 patients with rCDI in Western Australia. The C. difficile strain population comprised 13 sequence types (STs) led by ST2 (PCR ribotype (RT) 014, 36.2%), ST8 (RT002, 19.1%) and ST34 (RT056, 11.7%). Among 38 patients, core genome SNP (cgSNP) typing found 27 strains (71%) from initial and recurring cases differed by ≤ 2 cgSNPs, suggesting a likely relapse of infection with the initial strain, while eight strains differed by ≥ 3 cgSNPs, suggesting reinfection. Almost half of patients with CDI relapse confirmed by WGS suffered episodes that occurred outside the widely used 8-week cut-off for defining rCDI. Several putative strain transmission events between epidemiologically unrelated patients were identified. Isolates of STs 2 and 34 from rCDI cases and environmental sources shared a recent evolutionary history, suggesting a possible common community reservoir. For some rCDI episodes caused by STs 2 and 231, within-host strain diversity was observed, characterised by loss/gain of moxifloxacin resistance. Genomics improves discrimination of relapse from reinfection and identifies putative strain transmission events among patients with rCDI. Current definitions of relapse and reinfection based on the timing of recurrence need to be reconsidered.