Modulation of Oral Biofilms by Arginine and Zinc Salts

Objectives: Good oral hygiene habits help to control oral biofilms, thus facilitating host-microbe homeostasis. Agents such as zinc salts in oral care products can inhibit a range of physiological bacterial functions. Additional ingredients such as arginine can disrupt biofilms. This study investigated these combined effects on planktonic and biofilm-grown oral streptococci and pathogens.
Methods: Planktonic bacteria were grown in a matrix of arginine and zinc salts to test their influence on bacterial growth. Five-species biofilms, grown anaerobically for 14d on hydroxyapatite-coated pegs of the Calgary biofilm device, comprised early colonisers (Streptococcus salivarius and Actinomyces naeslundii), the bridge organism Fusobacterium nucleatum, and periodontal pathogens (Porphyromonas gingivalis and Prevotella intermedia). Mature biofilms were treated with preparations (w/v) of 1.5% arginine, 0.5% zinc citrate and 1% zinc oxide.
Results: Growth of most streptococci was unaffected by ≤2% (w/v) arginine, while F. nucleatum, T. denticola, P. gingivalis and P. intermedia were inhibited. All bacteria were strongly inhibited by zinc citrate and zinc oxide; however, arginine increased tolerance of most streptococci to low concentrations of zinc citrate. Exposure of biofilms to zinc citrate with arginine significantly reduced viable counts of all five strains (P<0.001), as compared to PBS or arginine alone. The synergistic effects were shown when ZnO was added to the solution of zinc citrate with arginine. Consistent to this observation, zinc compounds with arginine caused bacterial death in biofilms assessed by confocal laser scanning microscopy and disrupted biofilm structures observed by scanning electron microscopy.
Conclusions: Zinc exerted antimicrobial effects on the bacterial species studied while the influence of arginine was not antimicrobial but more subtle. Additionally, the combination of arginine, zinc citrate and zinc oxide increased bacterial death and biofilm disruption compared to arginine and zinc citrate alone. This study lays the foundation for future studies of more complex biofilm models and their interactions with host cells.