Salivary Metals Predict Commensal Candida albicans Levels in Healthy Adults

Objectives: The impact of nutrient metal availability on oral health, including carriage and pathogenicity of Candida species, is an emerging focus in oral microbiology. We detected five metals in human whole saliva (WS): iron, zinc, copper, manganese, and nickel; and examined the relationship between these metals and commensal Candida albicans carriage.
Methods: Unstimulated WS (10 ml) was collected on ice from healthy donors; then plated (0.5 ml) onto YPD containing streptomycin/penicillin to determine number of Candida albicans cells. WS was centrifuged (900 X g) to remove cells, and cell-free undiluted WS was sent for inductively coupled plasma mass spectrometry (ICP-MS) (RTI International) to determine metal concentrations. Metal concentrations below the limit of detection were imputed using the Richardson and Ciampi method. Bivariate dependences of five metals were examined using Spearman correlation. Ability of salivary metal concentrations to predict carriage was tested in five separate zero-inflated negative-binomial models or using only non-zero carriage counts. Principle component analysis (PCA) was performed on individuals with detected carriage; input included five metal concentrations, age, and sex.
Results: Correlated metals differed between individuals with and without detectable carriage, suggesting the oral environment differs when Candida is present or absent. Manganese, iron, and copper significantly predicted fungal carriage: a unit increase in log(ng/mL) metal resulted in an 89, 134 and 220% increase in expected carriage count, respectively; suggesting that these metals are important for fitness of commensal Candida. PCA revealed that input variables cluster individuals separately by low, middle and high carriage, indicating that salivary metals have a bimodal interaction with carriage.
Conclusions: Salivary metals and fungal carriage are interdependent, one possibility being that high metal levels promote fungal proliferation, but increased oral Candida numbers may also impact metal availability in saliva. Thus, salivary metal levels may be a valuable predictive metric for the risk of C. albicans overgrowth.