Plant Preparations and Compounds with Activities against Biofilms Formed by Candida spp.

Fungi from the genus Candida are very important human and animal pathogens. Many strains can produce biofilms, which inhibit the activity of antifungal drugs and increase the tolerance or resistance to them as well. Clinically, this process leads to persistent infections and increased mortality. Today, many Candida species are resistant to drugs, including C. auris, which is a multiresistant pathogen. Natural compounds may potentially be used to combat multiresistant and biofilm-forming strains. The aim of this review was to present plant-derived preparations and compounds that inhibit Candida biofilm formation by at least 50%. A total of 29 essential oils and 16 plant extracts demonstrate activity against Candida biofilms, with the following families predominating: Lamiaceae, Myrtaceae, Asteraceae, Fabaceae, and Apiacae. Lavandula dentata (0.045–0.07 mg/L), Satureja macrosiphon (0.06–8 mg/L), and Ziziphora tenuior (2.5 mg/L) have the best antifungal activity. High efficacy has also been observed with Artemisia judaica, Lawsonia inermis, and Thymus vulgaris. Moreover, 69 plant compounds demonstrate activity against Candida biofilms. Activity in concentrations below 16 mg/L was observed with phenolic compounds (thymol, pterostilbene, and eugenol), sesquiterpene derivatives (warburganal, polygodial, and ivalin), chalconoid (lichochalcone A), steroidal saponin (dioscin), flavonoid (baicalein), alkaloids (waltheriones), macrocyclic bisbibenzyl (riccardin D), and cannabinoid (cannabidiol). The above compounds act on biofilm formation and/or mature biofilms. In summary, plant preparations and compounds exhibit anti-biofilm activity against Candida. Given this, they may be a promising alternative to antifungal drugs.


Introduction
The genus Candida contains about 150 species; however, most are environmental organisms. The most medically important is Candida albicans, which accounts for about 80% of infections. C. albicans causes more than 400,000 cases of bloodstream life-threatening infections annually, with a mortality rate of about 42% [1]. Candida non-albicans species that

Materials and Methods
In this review, publications available in PubMed and Scopus databases and through the Google search engine were taken into account. The following keywords and their combinations were used: "antifungal," "Candida," "anti-biofilm," "biofilm," "plant," "compound," "extract," and "essential oil." The principal inclusion criterion was the inhibition of biofilm formation by at least 50%. We focused on biofilm inhibition assays, in which the time of culture allowed for Candida biofilm maturation was at least 24 hours. Articles from the year 2000 to the present were taken into account. All articles published in predatory journals were rejected.

Plant Preparations That Display Activity against Candida Biofilms
The present review includes 60 articles in which Candida biofilm formation was inhibited by at least 50%. It has been shown that preparations from 34 plants demonstrate activity against Candida biofilms. Among them were 29 essential oils and 16 extracts. The plants from the following families dominated: Lamiaceae (6 species in 5 genera), Myrtaceae (5 species in 4 genera), Asteraceae (4 species in 4 genera), Fabaceae (4 species in 3 genera), and Apiacae (4 species in 2 genera).
Antibiofilm activity may vary between plants in the same family. For example, in the Lamiaceae family, essential oil from Lavandula dentata acted against C. albicans biofilm at concentrations of 0.045-0.07 µL/mL [21], while essential oil from Satureja hortensis acted against the same biofilm at concentrations of 400-4800 mg/L [51]. There may also be large differences within the same species, due to various reasons. This may be influenced by, for example, different research methodologies, the use of different strains of fungi, and different chemical compositions depending on the plant variety, country, and season of harvest. A notable example of such a difference is observed with Mentha × piperita. In studies by Benzaid et al. [44], essential oil of M. piperita acted against Candida biofilm at a concentration of 10 µL/mL. However, the work of Agarwal et al. [38] showed that the same essential oil was active at 800 µL/mL.
Changes in the content of active substances were described by Gonçalves et al. [56]. They showed that in essential oil from Mentha cervina collected in August, the amount of isomenthone was 8.7% and pulegone was 75.1%. However, in essential oil collected in February, the ratio of the two compounds reversed and amounted to 77.0% for isomenthone and 12.9% for pulegone. The method of obtaining the compounds likewise had an influence on their content in the final essential oil. In a study byĆavar et al. [57], the composition of essential oils of Calamintha glandulosa differed depending on the extraction method. The level of menthone was 3.3% using aqueous reflux extraction, 4.7% using hydrodistillation, and 8.3% using steam distillation, while the concentration of shisofuran was only 0.1% using hydrodistillation and steam distillation, while aqueous reflux yielded 9.7%.
In terms of activity, large differences were found, depending on the authors cited. Eugenol and thymol serve as good examples. Both compounds exhibited excellent activity in some studies (from 12.5 mg/L for eugenol [58] and 1.56 mg/L for thymol [26]), and in other studies, the activity was very poor (up to 80,000 for both [59]). These differences may be related, for example, to a different purity of the compound, a different fungal suspension density, or even to the use of other Candida strains with different sensitivities to chemical substances. A number of other factors, such as the type of culture medium, pH of the medium, incubation time, and temperature may likewise influence the antimicrobial activity [20].

Conclusions
Plant preparations (essential oils and extracts) and pure compounds exhibit antibiofilm activity against Candida species. Some of them are characterized by high activity in concentrations below 16 mg/L. Given this activity at relatively low concentrations, some may prove to be promising alternatives to antifungal drugs, especially in the cases of resistant or multiresistant strains of Candida. Moreover, the simple chemical structures involved and relative ease of extraction from natural sources warrant further research into the development of new, promising, and much-needed plant-based antifungals.