Several biological activities relevant to caries prevention have been identified through various in vitro and in vivo studies. These include induction of the detachment of cariogenic microorganisms from hydroxyapatite, changes in cell surface hydrophobicity, bactericidal activity against cariogenic organisms, and the disruption of signal transduction in Streptococcus mutans.

Glucan is one of the major components of oral biofilm formed by cariogenic bacteria S. mutans and Streptococcus sobrinus. These microorganisms form biofilms on the tooth surface by attaching to the acquired pellicle on the enamel and adhesion to the tooth through cell interactions. The shiitake extracts could reduce preformed biofilms in dextranase, a water-soluble α-(1,6)-linked glucan. This dextran-degrading enzyme, dextranase, has been used to reduce the formation of dental plaque.

An in vitro dental caries biofilm model was developed in combination with cariogenic microorganisms with dental hard tissue substratum (dentin or enamel), allowing modeling of frequent acid challenges by sucrose pulsing with a constant depth film fermentor. The amount of mineral loss quantifies anticariogenicity. This mineral loss is inhibited by low molecular weight (LMW) fractions of shiitake extracts in vitro.

Shiitake mushroom contains erythritol, which is 1,2 and 3,4-butanetetrol, which has 70–80% sweetness compared to sucrose. In the presence of erythritol, S. mutans and S. sobrinus showed no adherence to glass, suggesting that the bacteria do not utilize this sugar to synthesize glucans. Since these bacteria do not use or metabolize it to produce the lactic acid byproduct, it does not cause enamel demineralization.

Both high molar and low molecular mass (LMM) fractions inhibit S. mutans adherence to hydroxyapatite crystals, promoting the detachment of bacteria and inducing biofilm destruction. This is mainly carried out by a compound called adenosine. An in vitro study was conducted to characterize the bacterial receptors for the purified mushroom subfraction-5 to understand better the action of this subfraction on interbacterial and host interactions. S. mutans and Prevotella intermedia were the two bacterial strains used to study the same. Binding subfraction-5 to surface molecules of the bacteria mentioned above may result in the inactivation of their physiological functions.

The results indicated that bacterial surface alterations affected adhesion and biofilm formation at the molecular level. This extract had demonstrated an inhibitory effect on one of the virulence factors of S. mutans when specific pathogen-free rats were infected with S. mutans and fed with a cariogenic diet containing 0.25% shiitake extract and the controls were provided with a cariogenic diet without shiitake extract. A lower caries score was observed in the test group as the extract helped reduce firmly adherent plaque.

This led to an increase in the incidence of non/loosely adherent plaque and a decrease in total plaque formation. A double-blind, three-leg, cross-over, randomized, controlled clinical trial was done to investigate the caries-preventive properties of low molecular weight(LMW) fraction of edible mushroom shiitake extract. Volunteers were asked to rinse twice daily with a solution containing LMW fraction of edible mushroom, placebo (negative control without active ingredients), or meridol (positive control) for 2 weeks with a 2-week washout period between each rinsing period. This trial assessed the shift in plaque pH after a sucrose challenge, the change in the microbial flora, and the intraoral plaque accumulation. Results indicated that frequent rinses with shiitake reduced the metabolic activity of dental plaque.

The LMM fraction of shiitake extracts contains metabolites such as sesquiterpenes, steroids, anthraquinone, benzoic acid derivatives, and quinolones which inhibit the growth of S. mutans. This bacteriostatic action is carried out by inhibiting DNA synthesis and elongating bacteria with interrupted septa. These morphologic changes are similar to those observed in streptococcal thermosensitive temperature or exposed to inhibitory doses of β-lactam antibiotics. Copalic acid is a metabolite of sesquiterpenes which is an active compound displaying promising minimum inhibitory concentration (MIC) values (2.0–6.0 µg/mL) and has both bactericidal and bacteriostatic effects. In a separate in vitro study, the LMM fractions of shiitake mushrooms with a minimum of 2x concentrations inhibited the growth of S. mutans by inhibition of DNA synthesis. This bacteriostatic action was also confirmed by these fractions’ morphological effects, which showed the elongation of bacteria with interrupted septa.

Shiitake mushroom extract has the best efficacy regarding significant biofilm inhibition and disruption. This shiitake mushroom extract at concentrations slightly above the minimum inhibitory concentration (MIC) of Porphyromonas gingivalis showed significant inhibition. Shiitake mushroom extract also contains carvacrol which has potent antimicrobial activity. Carvacrol is a known aromatic monoterpene shown to actively disintegrate the outer membrane of Gram-negative bacteria by releasing lipopolysaccharides from the bacterial cell wall and increasing membrane permeability. An in vitro study was carried out to investigate the biofilm inhibitory effect and disruptive efficacy of shiitake mushroom extract on four individual oral pathogens, namely S. mutans, Fusobacterium nucleatum, Aggregatibacter actinomycetemcomitans, and P. gingivalis and on a mixed culture. This extract showed significant inhibitory and disruptive effects on P. gingivalis. Furthermore, S. mutans and multispecies biofilms also showed inhibition at their MIC.

The results of the in vitro and in vivo animal studies are difficult to transpose to humans because:

  • The human oral environment could influence anti-bacterial activity
  • It also affects the virulence factor of the active compounds on oral pathogens and
  • Food consumption probably allows insufficient contact time between the oral tissues and active compounds in most cases.

Other Oral Health Benefits of Shiitake

A significant oral health benefit of this mushroom is its antigingivitis effect. LMM fractions from shiitake mushroom interfere with the binding of S. mutans cells to hydroxyapatite and P. intermedia cells to gingival cells. LMM fractions bring about the detachment of odonto- and periodonto-pathogenic bacteria from preformed biofilms. The antimicrobial mode of action has been evaluated at MIC, and an antibiotic-like mode of action has been described for the LMM fractions of mushrooms. This activity strongly inhibited DNA and partial RNA synthesis with a 50% reduction in protein synthesis.

As a result of cell division inhibition, a certain degree of cell elongation has been observed in S. mutans, while P. intermedia showed elongation in the form of filaments. The morphogenetic effects are similar to those obtained by treatment with antibiotics such as β-lactams or quinolones. A comparative study was conducted to determine the effectiveness of shiitake mushroom extract to that of the active component in leading gingivitis mouthwash containing chlorhexidine in an artificial mouth model. The total bacterial numbers and eight key taxa in the oral community were investigated over time, and results indicated that the shiitake extract lowered the numbers of some pathogenic taxa.

The disulfide derivative extracted with ethyl acetate has some antifungal activities. Based on its spectroscopic data, the chemical structure is bis-disulfide. Ethanolic mycelial extracts from shiitake also possess antifungal activity against Paramecium caudatum. Lenthionine was also found to be more sensitive to fungi than bacteria.

The antioxidant effects of shiitake are due to L-ergothioneine (2-mercaptohistidine trimethylbetaine). This is due to the ability to trap reactive oxygen/nitrogen species and inflammatory mediators such as hydroxyl radicals, hypochlorous acid, and peroxynitrite, which are formed by the reaction of nitric oxide with superoxide. Reactive oxygen/nitrogen species and nitrous oxide are perceived as important signaling molecules generated during muscle contraction and involved in the regeneration and adaptation of skeletal muscle to physical work. Thus, the extract of shiitake has the potential to modulate reactive oxygen/nitrogen species and nitrous oxide and enhances skeletal muscle regeneration after intense exercise.

Shiitake mushrooms have also been reported to have cancer-preventing properties due to the presence of “myochemicals” in them. These chemicals inhibit the growth of tumor cells which may result from the induction of apoptosis. Lentinan from shiitake mushroom (0.5–1.0 mg/day, intravenously) has been used for adjuvant tumor therapy, which leads to prolonged survival time, restoration of immunological parameters, and improvement of life quality.

Antiviral activities were reported for the mycelial culture medium of shiitake mushroom, and sulfated lentinan completely prevented HIV-induced cytopathic effects. Antilipidemic effects were caused by the compound eritadenin which is a nucleotide derivative.