Oral fungal-bacterial biofilm models in vitro: a review

Inclusion of fungi as commensals in oral biofilm is an important innovation in oral biology, and this work aimed to review the literature on the available biofilm and related disease in vitro models. Actually, thousands of bacterial and around one hundred of fungal phylotypes can colonize the oral cavity. Taxonomic profiling combined with functional expression analysis has revealed that Candida albicans, Streptococcus mutans and prominent periodontopathogens are not always present or numerically important in candidiasis, caries, or periodontitis lesions. However, C. albicans combined with Streptococcus spp. co-increase their virulence in invasive candidiasis, early childhood caries or peri-implantitis. As Candida species and many other fungi are also members of oral microcosms in healthy individuals, mixed fungal-bacterial biofilm models are increasingly valuable investigative tools, and new fungal-bacterial species combinations need to be investigated. Here we review the key points and current methods for culturing in vitro mixed fungal-bacterial models of oral biofilms. According to ecosystem under study (health, candidiasis, caries, periodontitis), protocol design will select microbial strains, biofilm support (polystyrene plate, cell culture, denture, tooth, implant), pre-treatment support (human or artificial saliva) and culture conditions. Growing mixed fungal-bacterial biofilm models in vitro is a difficult challenge. But reproducible models are needed, because oral hygiene products, food and beverage, medication, licit and illicit drugs can influence oral ecosystems. So, even though most oral fungi and bacteria are not cultivable, in vitro microbiological models should still be instrumental in adapting oral care products, dietary products and care protocols to patients at higher risk of oral diseases. Microbial biofilm models combined with oral epithelial cell cultures could also aid in understanding the inflammatory reaction.