[Modification of bacterial growth by alloplastic bone substitutes]

To determine the applicability of alloplastic materials as bone substitutes it is now standard procedure to test materials for possible toxic effects and to study their behavior in animal models and cell cultures. This is especially important with respect to middle ear implants that can be put at risk by recurrent infections and require additional testing in a bacterially contaminated environment.

In the present study ionomeric cement (V-O CEM), bioactive glass ceramic and hydroxyapatite were subjected to contamination with S. aureus, E. coli, Pr. mirabilis, Ps. aeruginosa and Enterococci using agar diffusion and microbial suspension tests and examined for their antibacterial activity. A special feature of V-O CEM that had to be considered was that it could be implanted in two physical states (as a viscous substance and a fully hardened material).

The agar diffusion test showed that an antibacterial effect of freshly mixed V-O CEM was demonstrable for up to 60 min. In the microbial suspension test growth of E. coli was found to be promoted after 48-h incubation by V-O CEM set for 1 h. S. aureus exhibited a depressed growth, while Pseudomonas cultures demonstrated cell death after 48 h. V-O CEM set for 24 h and 7 days, respectively, exerted a similar though less pronounced effect. Using the microbial suspension test, a comparison was also made of the antibacterial activities of 24-h V-O CEM, bioactive glass ceramic and hydroxyapatite against cultures of S. aureus, Pseudomonas and E. coli. The inhibitory effect of hydroxyapatite on the growth of S. aureus was found to persist beyond the 48-h incubation period. There was slight growth of E. coli in the presence of bioactive glass ceramic after 48 h, whereas hydroxyapatite produced inhibition of microbial growth. V-O CEM inhibited the growth of Pseudomonas, unlike bioactive glass ceramic and hydroxyapatite, which transiently promoted bacterial growth.

Our findings showed that V-O CEM, bioactive glass ceramic and hydroxyapatite exhibited material-dependent bacterial colonization and thus resembled polymeric bone substitutes (susceptible to invasion by S. epidermidis) and metals (sensitive to S. aureus). In general, users of bone substitutes should conduct preclinical tests in order to obtain advance information on the properties of possible replacement material. Since there can be varying interactions between the materials studied and bacterial growth, material-specific effects on bacterial growth should be investigated. While it is recognized that in vitro studies are an inadequate simulation of the clinical situation, they still provide some insight into the likely behavior of a bone substitutes in human sites.