*3.5. Other Candidiasis*

Diabetic patients are highly predisposed to cutaneous fungal infections due to the higher blood sugar levels. These infections are frequently characterized by thick biofilms, and sometimes the use of medical devices to drain these lesions is mandatory. Foot infection (tinea pedis and toenail onychomycosis) is particularly important to diabetic individuals due to the high incidence of diabetic foot in these patients [329,330]. The most significant *Candida* sp. causing onychomycosis are *C. albicans* and *C. parapsilosis* and it is known that DM patients have a high rate of tinea pedis and onychomycosis. Thus, this infection is now considered to be a predictor of diabetic foot syndrome [331]. The predisposing factors for tinea pedis et unguium are presented in Table 4.

Diabetic foot ulcers are a serious cause of diagnostic and therapeutic concern, and Non-*albicans Candida* spp. with potential biofilm forming abilities are emerging as a predominant cause of this problematic condition. Indeed, in a recent study, the prevalence of different *Candida* sp. was identified as *C. tropicalis* (34.6%), *C. albicans* (29.3%), *C. krusei* (16.0%), *C. parapsilosis* (10.6%), and *C. glabrata* (9.33%) [332]. In order to find the frequency of fungal infections among cutaneous lesions of diabetic patients and to investigate azole antifungal agen<sup>t</sup> susceptibility of the isolates, Raiesi et al. [333] studied type 1 and type 2 DM patients with foot ulcers (38.5%) and with skin and nail lesions (61.5%). Results showed that 24.5% had fungal infections and were at a higher frequency in patients with skin and nail lesions (28%) than in foot ulcers (19.1%). *C. albicans* and *Aspergillus flavus* were the most common species isolated, and a high prevalence of fluconazole-resistant *Candida* sp., particularly in diabetic foot ulcers, was determined [333].


**Table 4.** Physiopathology and etiology related to the occurrence of nail fungal diseases linked to *Candida* sp. in diabetics.

Diabetics with onychomycosis have a greater risk of having a diabetic foot ulcer [336,337,339,340], as confirmed by numerous studies. In Germany, Eckhard and colleagues [337] found that out of 95 patients with type 1 DM, 84.6% had a fungal infection. The most frequent *Candida* sp. found were *C. albicans*, *C. parapsilosis*, and *C. guilliermondii*, followed by *C. lipolytica*, *C. catenulate*, and *C. famata*. In another study conducted at the Umm Al-Qura University, Makkah, Saudi Arabia from June 2011 to June 2012, the antimicrobial susceptibility of the most common bacterial and fungal infections among infected diabetic patients (foot infections) was determined. All *Candida* sp. showed susceptibility to amphotericin B, econazole, fluconazole, ketoconazole, and nystatin (100% each) [340]. Cases with *Candida* sp. co-infection were also observed in patients with fungal nail infections—both cutaneous and nail infections [337]. Lugo-Somolinos et al. [334] performed a cross-sectional study in Japan and revealed that 51.3% of patients with DM had onychomycosis of the toenails. In this particular case, *C. albicans* was more prevalent in the control group (24% vs. 15% in the DM patients) and nail thickness was significantly associated with an elevated HbA1c value [334]. Gupta et al. [341] reported that there was a 2.77-fold greater risk for diabetic subjects than for healthy individuals to have toenail onychomycosis. In the same year, a previous study in Taiwan reported that 60.5% of onychomycosis was caused by dermatophytes, 31.5% by *Candida* sp., and 8% by non-dermatophyte molds [342]. A total of 20 patients with onychomycosis had concomitant DM. Regarding gender, in diabetic males, the most common pathogens were dermatophytes (58.3%), while in diabetic females, *Candida* sp. was more prevalent (87.5%) [342]. However, on the contrary, Dogra and colleagues found that in Indian diabetics, yeasts were the most common isolate (48.1%), followed by dermatophytes (37%) and non-dermatophyte molds (14.8%) [336]. The authors concluded that diabetics had a 2.5 times greater probability of having onychomycosis when compared to the controls [336]. Chang et al. [331] studied 1245 Taiwanese patients with DM. Among them, 30.76% were reported to suffer from onychomycosis. In this investigation, the diagnosis of onychomycosis was limited to a general histopathologic examination (KOH stain) of the toenails. Therefore, the patients may have been affected by *Candida* sp. and by other fungi. Another study performed by Pierard et al. [335] investigated onychomycosis in 100 DM patients on chronic hemodialysis, showing that 39% of participants had a nail disease. *Candida* sp. was the second most prevalent pathogen (15%), and the authors concluded that diabetics on hemodialysis had about an 88% greater probability of acquiring onychomycosis than non-diabetics [335]. Another report by Wijesuriya and colleagues [338] described the etiological agents causing superficial fungal foot infections (SFFI) in patients with type 2 DM for one year. Their results demonstrated that 295 patients had SFFI and that, among patients with diabetes, more than 10 showed a prevalence of SFFI of 98%. *Aspergillus niger* was the most common pathogen, followed by *C. albicans*. Aging, gender, the duration of diabetes, and less-controlled glycemic levels were significantly associated with SFFIs [338].

A 2018 study explored the differential expression of toll-like receptor 2 (TLR2) and interleukin (IL)-8 secretion by keratinocytes in diabetic patients when challenged with *C. albicans*. Wang et al. [343] determined that the expression levels of both TLR2 and IL-8 increased and then decreased in the control and the diabetic groups, but in different dynamics. The observations revealed that TLR2 and IL-8 act on the keratinocytes interacting with *C. albicans*, and high glucose status can distress the function of HaCaT cells by reducing the secretion of IL-8 and TLR2. The study clearly supports the immunosuppression state that diabetic patients live in [343].

Adherence to the vascular endothelium, neutrophil chemotaxis, phagocytosis and opsonization, intracellular bactericidal activity, and cell-mediated immunity are all decreased in DM patients with hyperglycemia [344,345]. Regarding this matter, Souza et al. [346] treated diabetic rats with aminoguanidine (AMG, an inhibitor of protein glycation) and evaluated neutrophil reactive oxygen species (ROS) generation and *C. albicans* killing ability in order to evaluate the effects of hyperglycemia and the glycation of proteins on the NOX2 (phagocyte NADPH oxidase) activity of neutrophils and its implications for cellular physiology. The authors indicated that AMG increased the NOX2 response and microbicidal activity by neutrophils of the diabetic status. AMG seems to be a promising therapeutic answer for these patients [346].

In another interesting recent in vivo report, the interference of diabetic conditions in diabetic mice and the relation to the progress of *C. albicans* infection and anti-inflammatory response was evaluated. Compared to non-diabetic mice, diabetic mice indicated a significantly lower density of F4/80 and M2 macrophages, higher fungal burden, and deficiency in cytokine responses. *C. albicans* also increased tissue injury, highlighting significant deviations in diabetic animal responses to *C. albicans* infection that may be critical to the pathophysiological processes supporting cutaneous candidiasis in diabetic patients [347].

Several other pathologies related to *Candida* sp. have also been linked to a DM predisposition. Researchers often recognize the importance of DM in the development of the pathology due to immunosuppression issues [348]. The reports are summarized in Table 5.

**Table 5.** Physiopathology and etiology related to the occurrence of other diseases linked to *Candida* sp. in diabetics.


#### **4. Diabetes Mellitus In Vivo Models**

Diabetes mellitus is a serious epidemic disease, and the research for new therapies is becoming critical. Thus, the correct choice of the animal models is of vital importance for the validity of the reported studies.

In DM type 1, the choices range from chemical ablation of the pancreatic β-cells to animals with spontaneously developing autoimmune diabetes. In DM type 2, the animal models can be both obese and non-obese animals with varying degrees of insulin resistance and beta cell failure [376]. The animal models (e.g., species, strain, gender, genetic) of DM type 1 and type 2 have diverse purposes, and the choice of a model depends on the purpose of the study (e.g., applied for pharmacological or genetics studies and understanding disease mechanisms) [376–381]. Table 6 summarizes the main animal models used in diabetes mellitus in vivo studies.


#### **Table 6.** Main animal models used in diabetes mellitus studies [376].

\* can be also used as a DM type 2 model.
