Antibiofilm Activity of Zinc-Carbonate Hydroxyapatite Nanocrystals Against Streptococcus mutans and Mitis Group Streptococci

Dental plaque is a complex multispecies biofilm consisting of a dense community of interacting bacteria (*10 cells/ mg) embedded in a self-produced polysaccharide matrix [1, 4]. Plaque control is critical for oral health, since plaque bacteria include pathogens involved in both dental caries and periodontal disease—the most prevalent human microbial diseases—in addition to a variety of opportunistic pathogens that cause endocarditis, bacteraemia and septicaemia [10]. Plaque formation follows a regimented pattern of colonisation steps that begin with specific adhesion of pioneer colonizers to the acquired pellicle on the tooth surface followed by adhesion of secondary colonizers through interbacterial coaggregation [6]. Streptococcus mitis, Streptococcus gordonii, Streptococcus oralis and Streptococcus sanguinis, all mitis group streptococci (MGS), are dominant pioneer species capable of irreversible attachment to the acquired pellicle by stereochemical interactions between bacterial adhesins and pellicle receptors [7]. MGS play a key role in early plaque formation and conversion to a community with the potential to cause caries and periodontal disease via interspecies coaggregation and selective recruitment of species [12]. Indeed, once MGS have adhered to the pellicle, they coaggregate in a specific manner with late colonizers—predominantly gram-negative anaerobic species involved in periodontitis such as Aggregatibacter actinomycetemcomitans, Fusobacterium nucleatum, Porphyromonas gingivalis, Prevotella spp. and Treponema spp.—as well as with cariogenic streptococci of the mutans group such as S. mutans and S. sobrinus [5]. Mutualistic relationships are established between MGS and late colonizers. One of the best characterised coaggregation systems is the one where S. gordonii and S. oralis coaggregate with P. gingivalis, S. gordonii also providing metabolic support for P. gingivalis [9]. A thick biofilm hosting a community of interacting microorganisms eventually forms [2], whose composition becomes stable over time. Inhibition of early biofilm formation by MGS can thus help prevent dental plaque development. This study examines the anti-caries potential of nanocrystals of zinc-carbonate hydroxyapatite (Zn-CHA)—the active component of a recently investigated desensitizing toothpaste (BioRepair Plus; Coswell S.p.A., Funo, Bologna, Italy) [8], by assessing whether they affect biofilm formation by MGS and S. mutans strains in polystyrene microtiter plates at concentrations devoid of antibacterial activity. The streptococcal strains used in this study included: three strains from the American Type Culture Collection (ATCC) [S. oralis ATCC 10557 and S. sanguinis ATCC 10556 (both isolated from cases of endocarditis)], and [S. mutans ATCC 25175 (caries)] and two strains from the collection of the Department of Biomedical Sciences and Public Health of Polytechnic University of Marche [S. gordonii AN-10 and S. mitis AN-12 (dental plaque)]. Streptococci were routinely grown in blood agar base (Oxoid, Basingstoke, UK) supplemented with 5 % defibrinated sheep blood and Brain Heart (BH, Oxoid) broth at 37 C in presence of 5 % CO2. Minimum inhibitory concentrations (MICs) were determined by the agar assay, according to the guide lines of the C. Palmieri G. Magi B. Facinelli (&) Section of Microbiology, Department of Biomedical Sciences and Public Health, Polytechnic University of Marche, Via Tronto 10/A, 60126 Ancona, Italy e-mail: b.facinelli@univpm.it