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In the oral cavity, Actinomyces form a fundamental component of the indigenous microflora, being among initial colonizers in polymicrobial biofilms. However, some differences may exist between different species in terms of their attachment not only to teeth but also to biomaterials. In this study we investigated the distribution of Actinomyces in 33 dental implant fixtures explanted from 17 patients. The identification was based on comprehensive biochemical testing and gas-liquid chromatography and when needed, 16S rRNA sequencing. Actinomyces was the most prevalent bacterial genus in these failed implants, colonizing 31/33 (94%) of the fixtures. Proportions of Actinomyces growth of the total bacterial growth in the Actinomyces-positive fixtures varied from 0.01% up to 75%. A. odontolyticus was the most common Actinomyces finding, present in 26/31 (84%) Actinomyces-positive fixtures. Actinomyces naeslundii and A. viscosus were both detected in 10/31 (32%) and A. israelii in 7/31 (23%) fixtures. Other Actinomyces species, including A. georgiae, A. gerencseriae and A. graevenitzii, were detected less frequently. Our results suggest that Actinomyces species are frequent colonizers on failed implant surfaces, where A. odontolyticus was the far most prominent Actinomyces species.

In clinical microbiology laboratories, the identification of Actinomyces-like bacteria can be very laborious and problematic. In the present study, we focused on reactivity patterns of 4 commercial test kits, RapID ANA II, RapID 32A, RapID CB Plus, and BBL Crystal ANR ID, that could be used for rapid preliminary identification of Actinomyces isolates belonging to newly described Actinomyces and closely related species. Out of the 54 strains tested, 25 strains (46%) were correctly identified to the genus/group level by BBL Crystal ANR ID system, 16 strains (30%) by RapID 32 A, 11 strains (20%) by RapID CB Plus, and 7 strains (13%) by RapID ANA II. The main problems with these kits were due to occasional weak enzymatic and sugar fermentation reactions. In conclusion, chromogenic substrate sensitivity and specificity need to be enhanced in order to improve the reliability of the test results of these kits, and the present database updated in order to more precisely identify newly described Actinomyces and closely related species.

We evaluated four commercially available kits for rapid identification of Actinomyces and related species. The kits identified correctly 26 to 65% of "classical" Actinomyces strains to the species level and 13 to 49% of newly described Actinomyces strains to the genus level, thus indicating relatively poor applicability and a need to update these kits.

Recent advancements in chemotaxonomic and molecular biology-based identification methods have clarified the taxonomy of the genus Actinomyces and have led to the recognition of several new Actinomyces and related species. Actinomyces-like gram-positive rods have increasingly been isolated from various clinical specimens. Thus, an easily accessible scheme for reliable differentiation at the species level is needed in clinical and oral microbiology laboratories, where bacterial identification is mainly based on conventional biochemical methods. In the present study we designed a two-step protocol that consists of a flowchart that describes rapid, cost-efficient tests for preliminary identification of Actinomyces and closely related species and an updated more comprehensive scheme that also uses fermentation reactions for accurate differentiation of Actinomyces and closely related species.