Abstract
Aim
Toxin-antitoxin (TA) systems are small genetics elements found in the majority of bacteria which encode a toxin causing bacterial growth arrest and an antitoxin counteracting the toxic effect. In Salmonella and E. coli, TA systems were shown to be involved in the formation of persisters. Persisters are a bacterial subpopulation with low growth rate and high tolerance to antibiotics. They could be responsible for antibiotic treatment failure in chronic infections and relapses, notably in bone and joint infections (BJI) caused by Staphylococcus aureus. Currently, two type II TA system families were described in S. aureus, mazEF and axe/txe, but their physiological roles are not well described. In this work, we studied the importance of mazEF in the intracellular survival of S. aureus inside osteoblasts, one of the mechanisms considered in the chronicity of S. aureus BJI.
Methods
Using an ex vivo model of intracellular infection of human osteoblast-like cells (MG-63), two strains of S. aureus HG003 wild type and its isogenic mutant HG003 ΔmazEF were compared in terms of : i) internalization and intracellular survival by lysostaphin protective assay and ii) cytotoxicity by quantifying LDH in the culture supernatant, 24h and 48h after infection.
Results
The comparison of the two strains revealed that HG003 ΔmazEF had a lower capacity to be internalized by osteoblasts compared to the wild type (p=0.02). However, intracellular survival was greater for HG003 ΔmazEF compared to the wild type 24h and 48h post-infection (p=0.02 and 0.001 respectively). Concerning the bacteria-induced cell death, HG003 ΔmazEF appeared to be less cytotoxic than the wild type strain at 24h post infection (p=0.007) whereas no more differences could be observed after 48h. This delayed cytotoxicity with HG003 ΔmazEF was also observed after incubation of culture supernatants with osteoblasts during 8 hours, suggesting that the differences observed could be caused by a secreted molecule.
Conclusions
Our results suggest that the mazEF system could be involved in S. aureus BJI physiopathology regulating cytotoxicity and persistence in osteoblasts. Our prospect is to identify the target of the mazF toxin which could be a therapeutic target.
