Antimicrobial potential of strontium against bacteria associated with peri-implantitis
Authors:   , Alshammari Hatem 
Published: March 2023

Dental implants are nowadays a standard treatment to replace missing teeth and restore function and aesthetics. However, biological complications associated with implants, e.g., peri-implant infections, are common and jeopardize the success of treatment. The main aim of this thesis was to explore strontium (Sr) as a possible prevention strategy against peri-implant infections, since Sr has been shown to have antibacterial action and also to promote titanium (Ti) implant osseointegration. In this thesis, a systematic appraisal of the literature about the antimicrobial potential of Sr-functionalized Ti surfaces for oral applications was performed, and was followed by a series of in vitro studies assessing the antimicrobial potential of Sr against micro-organisms associated with peri-implantitis. In Study I, the systematic appraisal of the literature resulted in an initial list of 1081 potentially relevant publications, where from nine publications from in vitro studies met the inclusion criteria. Most of the included studies showed that Sr-functionalized Ti exerted a limited immediate (i.e., 24 h) antimicrobial effect, likely due to a low Sr ion release; a relevant antimicrobial effect and biofilm inhibition potential against Streptococcus aureus was observed at both early and late timepoints, with an adequate Sr ion release. Study II assessed, in vitro, five different concentrations of soluble Sr(OH)2 (100, 10, 1, 0.1, and 0.01 mM) against 6 different mono-species bacteria (Streptococcusmitis, Staphylococcus epidermidis, Aggregatibacter actinomycetemcomitans, Porphyromonas gingivalis, Escherichia coli, and Fusobacterium nucleatum) in terms of cell growth, minimal inhibitory concentration (MIC), and biofilm viability. In the agar diffusion test, zones of inhibition were only observed for 0.01, 0.1, and 1 mM of Sr(OH)2 against P. gingivalis. Growth inhibition in planktonic cultures was achieved at 10 mM for all species tested. In the biofilm viability assay, 10 and 100 mM Sr(OH)2 showed potent bactericidal effect against S. mitis, S. epidermidis, A. actinomycetemcomitans, E. coli, and P. gingivalis. It was thus concluded that Sr(OH)2 has antimicrobial properties against bacteria associated with peri-implantitis. Study III assessed the in vitro early- and late bacteriostatic and bactericidal effect of Sr-functionalized wafers on bacteria associated with peri-implantitis (E. coli,S. aureus, Streptococcus oralis, Actinomyces naeslundii, Parvimonas micra, P.gingivalis and F. nucleatum) as mono-species after 2 and 24 hours, and as multispecies at day 1, 3, and 6. Sr-functionalized wafers, compared to Ti controls, were associated with statistically significant less viable cells in both mono- and multispecies tests. Number of colony forming units (CFUs) within the biofilm were significantly higher in Ti wafers, compared to Sr-functionalized wafers, for S. aureus at all time-points of evaluation and for E. coli at day 1. Gingipain activity was higher in Ti wafers compared to Sr-functionalized ones, and the qPCR showed that P. gingivalis comprised 15% of the total biofilm on Ti wafers at day 6, while it remained below detection levels at Sr-coated wafers. In Study IV, the impact of Ti surface roughness (turned vs moderately rough) on the antimicrobial effect of Sr on bacterial associated with peri-implantitis (S. oralis, P. micra, A. naeslundii, F. nucleatum, P. gingivalis, S. aureus, and E. coli), grown in different multispecies consortia, was assessed in vitro. Bacterial viability and biofilm formation, and well as, proteolytic activity of P. gingivalis were assessed at day 1, 3, and 6. Sr-functionalized surfaces were associated with statistically significant reduction in number of viable cells compared to nonfunctionalized surface at all times of investigation for all multispecies tested. Higher proteolytic activity of P. gingivalis was found at non-functionalized Ti disks compared to Sr-functionalized ones. Sr-functionalised surfaces were associated with notable growth inhibition of both E. coli and S. aureus, while P. gingivalis remined undetected at all time points of evaluation on all disks. The turned surface had a slightly higher release of Sr ion compared with the moderately rough surface in the first 24 hours, while both surfaces showed a sustained release for up to 15 days. Overall, the data generated with this series of projects indicate Sr surfaces exerts an antimicrobial potential on bacteria associated with peri-implantitis and it is worthwhile to further explore the potential of Sr-functionalized Ti in the prevention of peri-implant infections.