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The human pathogen  Clostridioides difficile  has evolved into the leading cause of nosocomial diarrhea. The bacterium is capable of spore formation, which even allows survival of antibiotic treatment. Although  C. difficile  features an anaerobic lifestyle, we determined a remarkably high oxygen tolerance of the laboratory reference strain 630Δ erm . A mutation of a single nucleotide (single nucleotide polymorphism [SNP]) in the DNA sequence (A to G) of the gene encoding the regulatory protein PerR results in an amino acid substitution (Thr to Ala) in one of the helices of the helix-turn-helix DNA binding domain of this transcriptional repressor in  C. difficile  630Δ erm . PerR is a sensor protein for hydrogen peroxide and controls the expression of genes involved in the oxidative stress response. We show that PerR of  C. difficile  630Δ erm  has lost its ability to bind the promoter region of PerR-controlled genes. This results in a constitutive derepression of genes encoding oxidative stress proteins such as a rubrerythrin ( rbr1 ) whose mRNA abundance under anaerobic conditions was increased by a factor of about 7 compared to its parental strain  C. difficile  630. Rubrerythrin repression in strain 630Δ erm  could be restored by the introduction of PerR from strain 630. The permanent oxidative stress response of  C. difficile  630Δ erm  observed here should be considered in physiological and pathophysiological investigations based on this widely used model strain.   IMPORTANCE  The intestinal pathogen  Clostridioides difficile  is one of the major challenges in medical facilities nowadays. In order to better combat the bacterium, detailed knowledge of its physiology is mandatory.  C. difficile  strain 630Δ erm  was generated in a laboratory from the patient-isolated strain  C. difficile  630 and represents a reference strain for many researchers in the field, serving as the basis for the construction of insertional gene knockout mutants. In our work, we demonstrate that this strain is characterized by an uncontrolled oxidative stress response as a result of a single-base-pair substitution in the sequence of a transcriptional regulator.  C. difficile  researchers working with model strain 630Δ erm  should be aware of this permanent stress response.

A Point Mutation in the Transcriptional Repressor PerR Results in a Constitutive Oxidative Stress Response in Clostridioides difficile 630Δ erm