Candida is a heterogeneous fungal genus composed by more than 150 species. Although some species of Candida coexist as human commensals, they can cause superficial and systemic infections under certain circumstances [28]. Most candidiasis are caused by C. albicans, but in the recent years other non-C. albicans species have manifested a pathogenic capacity. Among the most frequently isolated from clinical specimens are Candida glabrata, C. parapsilosis and C. tropicalis [29].
The pathogenicity of Candida responds to a set of virulence factors, including dimorphism, secretion of hydrolytic enzymes (proteases, lipases and haemolysins) and adhesion and biofilm formation on the mucous epithelium and on medical devices [30, 31]. Formation of biofilms is a complex sequential process that depends on the invasive agent and the structure on which it is hosted [32]. Yeast colonization of biotic and abiotic surfaces is the first step in the development of biofilms, followed by cell division and microcolonies generation that contribute to the maturation of a biofilm characterized by the presence of hyphae and yeasts (sessile cells) embedded in an extracellular matrix and, finally, the detachment of some of these cells [33]. The release of planktonic cells into the environment allows them to colonize new surfaces and to develop new foci of candidiasis.
Because the diagnostic criteria of the peri-implant diseases have been in constant change, the diagnosis of peri-implantitis in the included studies have differed from one to another, due to being published over a long period of time, from 1991 to 2020. Yet most of the reviewed studies collected data about bleeding and/or suppuration on probing, probing depth and radiographic bone loss. Moreover, implant mobility and presence of keratinized mucosa was evaluated in three studies [15, 16, 20, 21]. For all these reasons, although it cannot be guaranteed that all the implants studied in this work have been correctly categorized as healthy or diseased, the margin of error could not be very wide [5, 34, 35]. In regards to the risk factor of PI, none of the studies excluded patients with history of periodontitis and two discarded smokers [10, 22]. Still, only one of the nine selected articles did not state whether they found Candida or not [17].
According to the included studies of this review, implants with peri-implantitis (range, 3–76.7%) had a higher presence of Candida than those without peri-implantitis (range, 9–50%). However, we do not know why prevalence of Candida was significantly bigger in the studies with individuals form Saudi Arabia [10, 22]. Since fungal assessment and sample size were similar to other studies [16, 18], we believe these particular results may be related to special geographical and sociocultural factors, such as diet, and the fact that these authors excluded female patients and smokers [10, 22].
Moreover, the fact that 10–71% of implants, regardless of peri-implantitis, showed the presence of this fungus demonstrates that colonization of Candida in the peri-implant environment is independent of the disease. Also, because a slightly higher number of patients with bar-retained overdentures had Candida, in contrast to locator-retained overdentures, time and hygiene of implants might also be important factors of fungal colonization.
Schwarz et al. [20] were the sole authors to state a direct relationship between Candida and other microorganisms. Thereby, from three implants with PI, C. boidinii was isolated alongside Mycoplasma salivarum, Veillonella parvula, Porphyromonas gingivalis, Parvimonas micra and Tannerella forsythia in one of them, and in the other two, C. albicans was also found with V. parvula, T. forsythia, M. salivarum, P. gingivalis and P. micra. On the other hand, C. dubliniensis was accompanied by M. salivarum, V. parvula, Staphylococcus aureus, P. micra and T. forsythia in one healthy implant. The latter species of Candida, C. dubliniensis has been isolated from patients suffering from different oral pathologies, resembling C. albicans in many virulence factors including hypha formation and hydrolytic enzyme production [36, 37]. In this context, Candida colonization could be linked to the presence of other periodontopathogens, like T. forsythia, P. micra or P. gingivalis. Although the exact role of Candida in the beginning of the peri-implant disease is unknown, we believe that this fungus could play an important function in the latter stages of PI, when the bacterial microenvironment is already established, as demonstrated in experimental studies [38].
Knowledge of the involvement of Candida infection in disorders of bone remodelling is limited, but it has been described in Candida arthritis and osteomyelitis [39, 40], nosological entities described very rarely in the jaws [41, 42]. Candida arthritis and osteomyelitis develop by haematogenous invasion, mainly in patients with immunodeficiency, being C. albicans is usually the most frequently isolated, although C. tropicalis, C. glabrata and C. parapsilosis has also been isolated [40]. As in the latter processes, Candida would act as a modifying agent in chronic inflammation around dental implants that activates the bone resorption response.
Anaerobiosis, as occurs in peri-implant pockets, can promote the virulence of C. albicans, increasing the activity of secreted aspartyl proteinases (Sap) [43]. These Sap proteins are associated to Candida adherence, tissue damage and modulation of immune response, maintaining inflammatory stimuli that attract other periodontopathogens [44, 45]. This role of Saps is important because the ability to form thick biofilms is easier for C. albicans, both under aerobic and anaerobic conditions, whereas for the rest of the species of Candida, growth is much greater only under aerobic conditions [46]. C. albicans hyphae secrete candidalysin, a 31-amino-acid peptide toxin that damages the epithelial cells and has an immunomodulatory capacity by binding to epidermal growth factor receptor (ErbB1 or Her1) [47]. Yeast and hyphal morphologies are present during asymptomatic C. albicans colonization of human mucosal surfaces. However, hypha formation can lead to candidalysin secretion, tissue damage and immune modulation (Fig. 2). Furthermore, C. albicans 95-kDa metallopeptidase, localized in cell wall, owns the capacity to destroy different elements of the peri-implant soft and hard tissues like type I collagen (connective tissue, alveolar bone and cement), type IV collagen (basement membrane of the mucosa), fibronectin (periodontal ligament) and laminin (basement membrane of the mucosa and cement) [48,49,50].
Interestingly, one study [22] pointed that patients with dental implants in which Candida was isolated, also had a higher presence of these fungi at the buccal, lingual and palatal mucosa. Although this study did not differentiate between implants with and without PI, it evidences the existence of an oral reservoir for Candida that facilitates the entry of this fungus into the peri-implant sulcus.
This systematic review has limitations. First, only nine studies analysed the presence of Candida in patients with peri-implantitis including very heterogeneous samples (range, 20–126 studied specimens). Second, few studies reported the number of Candida colony-forming units (CFU), which is a fundamental data for the mycological analysis [9, 51]. Alrabiah et al. [10] showed significant differences in the quantity of Candida between specimens from patients suffering PI (3147.54 CFU/mL) and from patients without PI (496.68 ± 100.2 CFU/mL). These findings are similar to those found by Alsahhaf et al. [22] (2316.26 vs 177.6 CFU/ml). Third, it was impossible to analyse the differences of Candida in patients with PI, regarding the type, composition, design and surface of the implants, because only Leonhardt et al. [16], said that their implants were Nobel Biocare AB (Gothenburg, Sweden). We are convinced of the importance of studying the implant characteristics, since they can be strongly related to colonization and infection, according to how they allow Candida adherence. There is a strong association between implant properties and microbial adhesion, titanium being one of the most resistant to Candida colonization and biofilm development [8]. Moreover, there is a considerable heterogeneity of the selected studies regarding the microbial methods and study design then results obtained from different microbiological methods did not allow for a direct comparison.