• Degradable magnesium implant-associated infections by bacterial biofilms induce robust localized and systemic inflammatory reactions in a mouse model.

      Rahim, Muhammad Imran; Babbar, Anshu; Lienenklaus, Stefan; Pils, Marina; Rohde, M; Helmholtz Centre for infection research, Inhoffenstr. 7, 38124 Braunschweig, Germany. (2017-06-01)
      Biomaterial-associated Pseudomonas aeruginosa biofilm infections constitute cascade of host immune reactions ultimately leading towards implant failure. Due to lack of relevant in vivo biofilm models, majority of the studies report host immune responses against free living or planktonic bacteria while bacteria in clinical situations live more frequently as biofilm communities than as single cells. Present study investigated host immune responses against biomaterial-associated P. aeruginosa biofilms in a clinically relevant mouse model. Previously, we reported metallic magnesium, a prospective biodegradable implant, to be permissive for bacterial biofilms in vivo even though it exhibits antibacterial properties in vitro. Therefore, magnesium was employed as biomaterial to investigate in vivo biofilm formation and associated host immune responses by using two P. aeruginosa strains and two mouse strains. P. aeruginosa formed biofilms on subcutaneously implanted magnesium discs. Non-invasive in vivo imaging indicated transient inflammatory responses at control sites whereas robust prolonged interferon-β (IFN-β) expression was observed from biofilms in a transgenic animal reporter. Further, immunohistology and electron microscopic results showed that bacterial biofilms were located in two dimensions immediately on the implant surface and at a short distance in the adjacent tissue. These biofilms were surrounded by inflammatory cells (mainly polymorphonuclear cells) as compared to controls. Interestingly, even though the number of live bacteria in various organs remained below detectable levels, splenomegaly indicated systemic inflammatory processes. Overall, these findings confirmed the resistance of biofilm infections in vivo to potentially antibacterial properties of magnesium degradation products. In vivo imaging and histology indicated the induction of both, local and systemic host inflammatory responses against P. aeruginosa biofilms. Even though the innate host immune defenses could not eliminate the local infection for up to two weeks, there was no apparent systemic bacteremia and all animals investigated survived the infection.
    • Differential magnesium implant corrosion coat formation and contribution to bone bonding.

      Rahim, Muhammad Imran; Weizbauer, Andreas; Evertz, Florian; Hoffmann, Andrea; Rohde, M; Glasmacher, Birgit; Windhagen, Henning; Gross, Gerhard; Seitz, Jan-Marten; Mueller, Peter P; et al. (2017)
      Magnesium alloys are presently under investigation as promising biodegradable implant materials with osteoconductive properties. To study the molecular mechanisms involved, the potential contribution of soluble magnesium corrosion products to the stimulation of osteoblastic cell differentiation was examined. However, no evidence for the stimulation of osteoblast differentiation could be obtained when cultured mesenchymal precursor cells were differentiated in the presence of metallic magnesium or in cell culture medium containing elevated magnesium ion levels. Similarly, in soft tissue no bone induction by metallic magnesium or by the corrosion product magnesium hydroxide could be observed in a mouse model. Motivated by the comparatively rapid accumulation solid corrosion products physicochemical processes were examined as an alternative mechanism to explain the stimulation of bone growth by magnesium-based implants. During exposure to physiological solutions a structured corrosion coat formed on magnesium whereby the elements calcium and phosphate were enriched in the outermost layer which could play a role in the established biocompatible behavior of magnesium implants. When magnesium pins were inserted into avital bones, corrosion lead to increases in the pull out force, suggesting that the expanding corrosion layer was interlocking with the surrounding bone. Since mechanical stress is a well-established inducer of bone growth, volume increases caused by the rapid accumulation of corrosion products and the resulting force development could be a key mechanism and provide an explanation for the observed stimulatory effects of magnesium-based implants in hard tissue. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 105A: 697-709, 2017.