Abstract
Background: Cysteine can affect the expression level of Clostridioides difficile toxins. CysE and cysK are the key genes involved in de novo cysteine synthesis in C. difficile. This study used the cysE/cysK gene as the key point to investigate the responsiveness of different genotypes of C. difficile to cysteine addition and the causes of this reactivity. Results: The different clinical isolates presented various changes in the copy numbers of cysE/cysK and tcdA/tcdB in response to cysteine addition. The results of the RNA-seq and metabolomics analyses of CD12038 C. difficile revealed that the differentially expressed genes (DEGs) between the strains with and without cysteine addition were enriched mainly in the carbon fixation pathways of prokaryotes, ribosomes, butanoate metabolism, the phosphotransferase system (PTS) and sulfur metabolism pathways. Moreover, the RT-qPCR results suggested that bacterial iron transport, uptake, and chelation related genes were upregulated in the presence of added cysteine. Compared with that in normal C. difficile cultures, the iron concentration in cysteine-supplemented BHI broth decreased sharply. Caco-2 cell coculture results indicated that the effect of cysteine addition on the virulence or pathogenic abilities of strains of C. difficile was inconsistent across genotypes and clinical isolates or reference strains. Conclusions: Different reference strains of C. difficile and clinical isolates had different doses and durations of responsiveness to cysteine addition, and the critical link involved the availability of iron in the environment of the bacteria. This is possibly the response of bacteria to the regulation of gene expression through the maintenance of iron homeostasis.