Abstract
Dental caries and periodontitis are the most common oral disease of all age groups, affecting billions of people worldwide. These oral diseases are mostly associated with microbial biofilms in the oral cavity. Streptococcus gordonii, an early tooth colonizing bacterium and Candida albicans, an opportunistic pathogenic fungus, are the two abundant oral microbes that form mixed biofilms with augmented virulence, affecting oral health negatively. Understanding the molecular mechanisms of the pathogen interactions and identifying non-toxic compounds that block the growth of biofilms are important steps in the development of effective therapeutic approaches. In this in vitro study we report the inhibition of mono-species or dual-species biofilms of S. gordonii and C. albicans, and decreased levels of biofilm extracellular DNA (eDNA), when biofilms were grown in the presence of gymnemic acids (GAs), a non-toxic small molecule inhibitor of fungal hyphae. Scanning electron microscopic images of biofilms on saliva-coated hydroxyapatite (sHA) surfaces revealed attachment of S. gordonii cells to C. albicans hyphae and to sHA surfaces via nanofibrils only in the untreated control, but not in the GAs-treated biofilms. Interestingly, C. albicans produced fibrillar adhesive structures from hyphae when grown with S. gordonii as a mixed biofilm; addition of GAs abrogated the nanofibrils and reduced the growth of both hyphae and the biofilm. To our knowledge, this is the first report that C. albicans produces adhesive fibrils from hyphae in response to S. gordonii mixed biofilm growth. Semi-quantitative PCR of selected genes related to biofilms from both microbes showed differential expression in control vs. treated biofilms. Further, GAs inhibited the activity of recombinant S. gordonii glyceraldehyde-3-phosphate dehydrogenase (GAPDH). Taken together, our results suggest that S. gordonii stimulates the expression of adhesive materials in C. albicans by direct interaction and/or signaling, and the adhesive material expression can be inhibited by GAs.
Highlights
Dental caries is a polymicrobial biofilm-induced disease affecting 3.5 billion people globally (Kassebaum et al, 2017)
To determine the minimum amount of gymnemic acids (GAs) needed to inhibit maximum biofilm growth of S. gordonii and C. albicans, an Minimum Biofilm Inhibition Concentrations (MBICs) assay was performed with increasing concentrations of GAs (0–600 μg/mL)
Biofilms were quantified by CV staining, and the results showed a concentration-dependent antibiofilm activity of GAs against S. gordonii and C. albicans (Figures 1A,C, respectively)
Summary
Dental caries is a polymicrobial biofilm-induced disease affecting 3.5 billion people globally (Kassebaum et al, 2017). Candida albicans is a fungus that is the etiologic agent of oral thrush and denture stomatitis, two mucosal oral biofilm infections that affect immunocompromised patients and elderly people, respectively (Odds, 1987). C. albicans and Streptococcus bacterial species are abundant in the oral cavity and readily form mixed biofilms which are resistant to antimicrobials and serve as a source for systemic infections (Dongari-Bagtzoglou et al, 2009; Silverman et al, 2010; Diaz et al, 2012; Ricker et al, 2014; O’Donnell et al, 2015). Some of the streptococci (e.g., Streptococcus mutans) are the causative agents of dental caries and gum disease. Recent studies have shown that a complex interaction and aggregation occurs between streptococci and C. albicans, and the molecular mechanisms are poorly understood (Dutton et al, 2014; Hwang et al, 2017)
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