Resistance to anti-fungal drugs increased seen in severe childhood dental caries
PHILADELPHIA, U.S.: Some young children experience severe dental caries that resists normal therapies. According to a recent study carried out by researchers at the University of Pennsylvania School of Dental Medicine, such tooth decay is associated with dental biofilms containing fungi and bacteria. It demonstrated how the sticky matrix produced by plaque bacteria acts as a shield, protecting yeasts from antifungals. However, the research made new findings that point to a way around this defense.
The study demonstrated that, in many cases, early childhood caries results from dental plaque that contains both bacteria and fungi working together to make the biofilm on the teeth more pathogenic and difficult to remove. Now, the researchers have shown that these two types of microorganisms synergize to enhance drug resistance, enabling the fungal cells to avoid being killed by antifungal therapies. However, simultaneously targeting the matrix produced by the bacteria along with the fungus offers a way to address this.
The scientists observed that dental plaque in children with early childhood caries often contained Candida albicans, a fungal species that normally colonizes mucosal surfaces, in addition to Streptococcus mutans, the bacterium generally associated with caries. The research demonstrated that an enzyme produced by the bacteria, termed GtfB, can bind to C. albicans and a sticky polymeric matrix forms on its cell surface when sugar is present, enabling the fungus to bind to teeth and associate with bacterial counterparts. Once together, these organisms work in concert to increase severity of caries, as established in a rodent model.
It was investigated whether a two-pronged approach might disrupt the synergistic association to promote effective treatment of the biofilm. Antifungal medicine Fluconazole and povidone-iodine, an antiseptic agent with antibacterial properties, were selected for this purpose. Used alone to treat biofilms grown on a toothlike material in the laboratory, the drugs had only moderate effects, confirming that monotherapy does not work very well against polymicrobial biofilms. In combination, the results were more impressive.
To understand why the combination approach was so effective against C. albicans, even without killing many more bacteria, the researchers looked closely at high-resolution microscopic images of the biofilms treated with the various combinations. They observed that, in untreated biofilms and those treated with solely fluconazole, the fungus was coated with an abundant sticky matrix, which seemed to serve as a protective shield against the antifungal compound. However, in biofilms treated also with povidone iodine, the matrix was substantially reduced, leaving the fungus exposed to the fluconazole.
In the literature, the researchers found that iodine-containing drugs can inhibit the activity of GtfB. Based on this, they performed a series of experiments, in which they observed that povidone iodine acted as a powerful inhibitor of the production of the sticky matrix. This led to their hypothesis that the matrix was serving as a “drug-trapping shield,” preventing the fluconazole from accessing and killing the fungal cells.
Using three different assays to disrupt the matrix, either by directly degrading the matrix or using bacteria defective in GtfB, the researchers found that the antifungal ability of fluconazole could be completely restored, confirming the role of the bacteria-produced matrix in promoting antifungal drug resistance.
“Available evidence shows that biofilm-associated diseases are polymicrobial in nature, including a mix of bacterial and fungal species; therefore, a treatment aimed at just one type of microorganism may not be effective. I think this work gives us a glimpse into alternative ways to disrupt cross-kingdom biofilm, a combinatorial approach that considers the fungal and bacterial components,” said senior author Prof. Hyun Koo from the Department of Orthodontics at the dental school.
The study, titled “Bacterial-derived exopolysaccharides enhance antifungal drug tolerance in a cross-kingdom oral biofilm,” was published online in the ISME Journal on April 18, 2018, ahead of inclusion in an issue. The research was conducted with Tel Aviv University, Israel, and the University of Wisconsin–Madison, U.S.