Clostridioides difficile strain dependent and independent adaptations to a microaerobic environment

Clostridioides difficile (formerly Clostridium difficile) colonizes the gastrointestinal tract following disruption of the microbiota and can initiate a spectrum of clinical manifestations ranging from asymptomatic to life-threatening colitis. Following antibiotic treatment, luminal oxygen concentrations increase, exposing gut microbes to potentially toxic reactive oxygen species (ROS). Though typically regarded as a strict anaerobe, C. difficile can grow at low oxygen concentrations. How the bacterium adapts to a microaerobic environment and whether those responses to oxygen are conserved amongst strains is not entirely understood. Here, two C. difficile strains (630 and CD196) were cultured in 1.5% oxygen and the transcriptional response was evaluated via RNA-sequencing. During growth in a microaerobic environment, several genes predicted to protect against oxidative stress were upregulated, including ruberythrins and rubredoxins. Genes involved in metal homeostasis were positively correlated with increasing oxygen levels and were also amongst the most differentially transcribed. These included ferrous iron transporters (feo), a zinc transporter (zupT), and predicted siderophore transporters. To directly compare the transcriptional landscape between C. difficile strains, a 'consensus-genome' was generated. On the basis of the identified conserved genes, basal transcriptional differences as well as variations in the response to oxygen were evaluated. While several responses were similar between the strains, there were significant differences in the abundance of transcripts for amino acid and carbohydrate metabolism. Furthermore, homologous metal homeostasis genes were similarly transcribed, but the intracellular metal concentrations significantly varied both in an oxygen-dependent and independent manner. Overall, these results indicate that C. difficile adapts to grow in a low oxygen environment through transcriptional changes, though the specific strategy employed varies between strains. Overall design: Two strains of Clostridioides difficile were grown anaerobically or in microaerobic environment. RNA was isolated from each of the strains and conditions in triplicate and RNA sequencing performed. Transcriptomes and changes in expression patterns were compared across each samples using CLC genomics workbench.