A superoxide reductase contributes to Clostridioides difficile resistance to oxygen

Clostridioides difficile causes a serious diarrheal disease and is a common healthcare-associated bacterial pathogen. Although it has a major impact on human health, mechanistic details of C. difficile intestinal colonization remain undefined. C. difficile is highly sensitive to oxygen and requires anaerobic conditions for in vitro growth. However, the mammalian gut is not entirely devoid of oxygen, and C. difficile tolerates moderate oxidative stress in vivo. The C. difficile genome encodes several antioxidant proteins, including a predicted superoxide reductase (SOR) that is upregulated upon exposure to antimicrobial peptides. The goal of this study was to establish SOR enzymatic activity and assess its role in protecting C. difficile against oxygen exposure. Insertional inactivation of sor rendered C. difficile more sensitive to superoxide indicating that SOR contributes to antioxidant defense. Heterologous C. difficile sor expression in Escherichia coli conferred protection against superoxide-dependent growth inhibition, and the corresponding cell lysates showed superoxide scavenging activity. Finally, a C. difficile SOR mutant exhibited global proteome changes under oxygen stress when compared to its parent strain. Collectively, our data establish the enzymatic activity of C. difficile SOR, confirm its role in protection against oxidative stress, and demonstrate its broader impacts on the vegetative cell proteome. Importance Clotridioides difficile is an important pathogen strongly associated with healthcare settings and capable of causing severe diarrheal disease. While considered a strict anaerobe in vitro, C. difficile has been shown to tolerate low levels of oxygen in its mammalian host. Among other well-characterized antioxidant proteins, the C. difficile genome includes a predicted superoxide reductase (SOR), an understudied component of antioxidant defense in pathogens. The significance of the research reported herein is the characterization of the enzymatic activity of the putative SOR protein, including confirmation of its role in protection of C. difficile against oxidative stress. This furthers our understanding of C. difficile pathogenesis and presents a potential new avenue for targeted therapies.