gms | German Medical Science

Objectives: Gram-positive Staphylococcus aureus are the most common cause for septic arthritis in humans. The mature thermonuclease Nuc1 (known as NucA) is secreted by S. aureus and plays an important role in its virulence. NucA degrades extracellular DNA and RNA, and thus contributes to bacterial escape from neutrophil extracellular traps (NET) and killing. This in turn boosts staphylococcal persistence in host cells. This study aimed at investigating the role of NucA in the development of S. aureus-induced septic arthritis.

Methods: Mouse studies were reviewed and approved by the Ethics Committee of Animal Research, Gothenburg (Ethical number 2021/3405). 200 µL of S. aureus Newman wild-type strain (NWT) or its Dnuc1 mutant were intravenously injected (2.8x106 CFU/mouse) into NMRI female mice (N 5/group), aged 8 weeks, to induce a septic arthritis. Mice were monitored for weight loss and clinical signs of arthritis for up to 7 days. On day 7, mice were sacrificed and blood (cytokines by ELISA), kidneys (bacterial load and abscess score), and joints (µCT) were analyzed. Additionally, the effect of the different bacterial strains on cytokine secretion of RAW264.7 macrophages, MM6 monocytes, and neutrophils was investigated. Non-parametric group comparison was performed to assess statistical significance.

Results and conclusion: Mice injected with S. aureus NWT strain developed severe arthritis within 7 days after injection, including bone erosion in the joint area and bacterial load in the kidneys with a high kidney abscess score. In contrast, mice injected with S. aureus Dnuc1 mutant showed hardly any signs of septic arthritis. These mice showed almost no weight loss, hardly any one erosion at the joints and decreased kidney bacterial load and kidney abscess score. In addition, serum levels of IL-6, CXCL1, and S100A8/A9 were significantly decreased in mice injected with the S. aureus Dnuc1 mutant when compared to in mice injected with the S. aureus NWT strain. In vitro, deletion of Nuc1 in the S. aureus JE2 strain stabilized DNA integrity but had no significant influence on IL-6 secretion by the immune cells investigated. However, JE2Dnuc1 showed decreased host cell invasion and was killed much better by human primary neutrophils than the parent strain.

In summary, deletion of Nuc1 S. aureus NWT strain significantly reduced the clinical signs of septic arthritis in the here presented mouse model, suggesting, that Nuc1 is an important virulence factor during S. aureus infection.

Figure 1 [Fig. 1]