Yeasts are microorganisms commonly found in nature. They are present in the normal flora (in moist places like the intestinal system, mouth etc.) in the human body. The dimorphic yeast Candida albicans (C. albicans) is the most prevalent opportunistic pathogen in humans. Candidiasis ranges from superficial and mucosal infections to life-threatening septicaemic mycosis in debilitated patients [1-3], and this fungus is known to be the fourth leading cause of nosocomial infections [4, 5]. Over the last few decades, the incidence of candidal infections due to C. albicans has been increasing, paralleling the growing numbers of immune-compromised patients [4-7]. To the patients, disseminated candidiasis is sometimes a serious disease that often results in death [8]. In addition, the fungus causes local infections such as vaginitis and thrush. For medical treatment of fungal infections, amphotericin B has been considered as the drug of choice [9, 10], and the azoles are mainly used in common clinical situations. However, the toxicity of and the resistance to these antifungal drugs are major problems [4, 5]. In the case of amphotericin B, an increased amount of amphotericin B must be administered to patients due to its poor permeability across the membrane [11], which can result in severe side effects, for example, renal damage [12, 13]. To overcome these problems of side-effects, natural products have been considered to be promising antifungal agents with less profound adverse effects. The antifungal activities of several phytochemicals including polyphenols, phenolics, terpenoids, and alkaloids have been reported [14].

Bee venom (BV) from honey bees (Apis mellifera L.) has been utilized for centuries as a pain reliever, anti-coagulant and anti-inflammatory agent for the treatment of chronic diseases, such as arthritis, rheumatism, tendonitis, bursitis, fibrosis and multiple sclerosis [15-18]. In Korea, apitherapy, which uses the stings of live honey bees, has shown therapeutic value for treating piglets, calves and dairy cows with several respiratory diseases [19]. BV, called apitoxin, is a mixture of proteins: melittin (main component 52%), apamin, adolapin, phospholipase A2, hyaluronidase, histamine, dopamine and protease inhibitor [20]. Two major components of BV, melittin and phospholipase A2, are generally thought to play an important role in the induction of the irritation and the allergic reactions associated with bee stings [16]. Also, sweet bee venom (SBV), in which hyper-molecular materials, such as enzymes and histamines that act as allergens in BV pharmacopuncture, have been removed, has been developed [21]. Melittin, a 26 amino acid polypeptide, is known to have anti-bacterial effects [22-24]. However, information on the antifungal efficacies of BV and SBV is not available.

This study was performed to investigate the antifungal activities of BV and SBV against C. albicans clinical isolates cultured from blood and the vagina.

The BV and the SBV used in this study were produced at the Korean Pharmacopuncture Institute (Seoul, Korea). Lyophilized whole BV and SBV were dissolved in distilled water at different concentrations, and were then used in this experiment. To examine the antifungal activities of BV and SBV, C. albicans (KCTC 7965) as a standard strain from the Korean Collection for Type Cultures (KCTC) at the Korea Research Institute of Bioscience and Biotechnology (KRIBB), Daejeon, Korea; the 10 C. albicans clinical isolates used as test organisms were furnished from the Korean Collection of Medical Fungi (KCMF) at the Konyang Institute of Medical Mycology (KIMM), Daejeon, Korea. Fungal cells were cultured on sabouraud dextrose agar (SDA; BD, Difco, Sparks, MD, USA) plates, and then incubated for 48 hours at 35°C. Amphotericin B (Sigma, St. Louis, MO, USA) and fluconazole (Sigma, St. Louis, MO, USA) were dissolved in 2% dimethyl sulfoxide (DMSO) as a stock solution, and then subsequently diluted.

Disk diffusion was carried out with Mueller-Hinton agar glucose methylene blue (MH-GMB; BD, Difco, Detroit, MI, USA). The inoculum was prepared by using 24-hour plate cultures of C. albicans on SDA. The 0.5 McFarland inoculum was swabbed in three directions on the MH-GMB agar plate. A disk (Advantec filter paper No. 26, size 6 mm) was placed on the surface agar, and a 20 μL sample (10 mg/mL each of BV and SBV) was loaded. Amphotericin B (100 μg) and fluconazole (100 μg) were used as a positive control. The plates were incubated for 24 hours at 35°C. The diameters (cm) of the zone of inhibition were measured by using a metric ruler.

A minimun inhibitory concentration (MIC) assay using a broth microdilution method was performed as described in M27-A2 (CLSI) with modifications [25]. The medium used was Roswell Park Memorial Institute (RPMI) 1640 medium (Sigma Chemical Co., Missouri, USA) with L-glutamine buffered to pH 7.0 by using 0.165 mol/L of morpholino- propanesulfonic acid (MOPS), supplemented with 2% glucose. The strain suspension was prepared in 0.85% saline, with an optical density equivalent to a 0.5 McFarland standard, and was diluted to 1 : 100 in RPMI 1640 medium to obtain a final concentration of 1 − 5 × 10⁴ colony-forming units per milliliter (CFU/mL). This suspension was inoculated into each well of a microtiter plate previously prepared with BV and SBV at concentrations from 500 μg/mL down to 0.9 μg/mL by using RPMI medium. The 96-well microtiter plate was incubated with agitation for 24 hours at 35°C. The control drug for each strain was amphotericin B and fluconazole diluted in DMSO; the concentrations tested ranged from 50 μg/mL down to 0.09 μg/mL in RPMI 1640 medium. The MIC was defined as the lowest concentration at which the optical density (OD) was reduced to 50% of the OD in the growth control well as measured by using an enzyme-linked immunosorbent assay (ELISA) microplate reader (Biotek).

The kinetics of the antifungal actions of BV and SBV against C. albicans cells were evaluated by using killing curve assays. Log-phased fungal cells (2 × 10⁴ CFU/mL) in RPMI 1640 medium were incubated with 125 μg/mL of BV and 62.5 μg/mL of SBV or with 0.1 μg/mL of amphotericin B (at the MIC), which was used as a positive control. The culture was obtained and spread on a SDA agar plate, then, the CFUs were counted after incubation for 24 hours at 35°C [26]. The values represented the averages of measurements conducted in triplicate of three independent assays.

All data were taken in triplicate, and statistical analyses were performed by using the student’s t-test, with (P < 0.05) being considered significant.

The antifungal efficacies of BV and SBV against C. albicans (KCTC 7965) as a standard strain and 10 C. albicans clinical isolates were evaluated by the disk diffusion method with MH-GMB medium, and the results are shown in Table 1 and Fig. 1. The results showed that the diameters of the inhibition zones for C. albicans blood isolates and C. albicans vaginal isolates were 9 − 12 mm and 10 − 12 mm, respectively, for BV and 12 − 18 mm and 12 − 19 mm, respectively, for SBV. The diameters of the inhibition zones for BV and SBV for the C. albicans standard strain were measured as 11 mm and 15 mm, respectively. In the blood isolates and the vaginal isolates, the diameters of the inhibition zones for BV and SBV were not significantly different (P > 0.05). However, the inhibition zone diameters of SBV compared with that of BV in clinical isolates were significantly different (P < 0.05).

For the clinical isolates, the MIC values, which were obtained by using the broth microdilution method, varied from 62.5 μg/mL to 125 μg/mL for BV and from 15.63 μg/ mL to 62.5 μg/mL for SBV. The MIC values for BV and SBV for the C. albicans standard strain were 125 μg/mL and 62.5 μg/mL, respectively. The MIC values for BV and SBV for all clinical isolates were similar to those for the standard strain (Table 2).

A killing-curve assay against C. albicans KCTC 7965 cells was conducted to assess the killing potencies of BV and SBV. The results showed that BV and SBV exhibited antifungal activities through fungicidal effects. C. albicans cells treated with 50 μg/mL of SBV rapidly decreased in number in a manner similar to the decrease observed with 1 μg/mL of amphotericin B after incubation for 4 hours, however, for 100 μg/mL of BV, the decrease in the number of C. albicans was slow (Fig. 2). Thus, the fungicidal effect of SBV was higher than that of BV.

BV, called apitoxin, is a mixture of proteins: melittin (main component 52%), apamin, adolapin, phospholipase A2, hyaluronidase, histamine, dopamine and protease inhibitor [20]. Two major components of BV, melittin and phospholipase A2, are generally thought to play important roles in the induction of the irritation and the allergic reactions associated with bee stings [16]. Also, SBV which has hyper-molecular materials, such the enzymes and the histamines that act as allergens in BV pharmacopuncture, removed, has been developed [21]. The frequent uses of anti-microbial and antifungal agents such as anti-biotics enable many pathogens to acquire resistances to multiple drugs [27]. The emergence of anti-bacterial-resistant strains of animal pathogens and their potential health risks to humans have captured great attention [28, 29]. Therefore, new anti-bacterial or antifungal agents with fewer adverse effects must be developed for the successful treatment of dermatophytes infections [30].

The anti-bacterial properties of BV as a natural antibacterial agent have been extensively studied, and BV therapy has been suggested for use as an alternative to anti-biotic therapy [31-33]. Strong antibacterial activities of BV against both Gram negative and Gram positive bacteria have been reported [34, 35]. Nakatuji et al [36] reported that BV could control the growth of Staphylococcus aureus, which plays an important role in the pathogenesis of inflamed lesions in the case of acne vulgaris. Moreover, BV exhibited antibacterial activities against skin bacteria such as Propionibacterium acnes, Staphylococcus epidermidis, and Streptococcus pyogenes [37].

BV and SBV anti-candidal activities were observed by using the disk diffusion method and the broth microdilution method, confirming that the compounds have a potential for use as anti-candidal agents, although their activities are less efficient as compared with amphotericin B and fluconazole, also, in the disk diffusion assay of C. albicans isolates from blood and the vagina, the antifungal activites of BV and SBV were similar (Tables 1,2, Fig. 1). However, Yu et al [38] reported that the antifungal activities of BV against Trichophyton mentagrophytes and Trichophyton rubrum were much stronger than those of fluconazole, one of the commercial antifungal drugs that is currently being used. In the current study, amphotericin B was used as a positive control: amphotericin B is a fungicidal agent that is widely used to treat serious systemic infections [39]. According to the MIC values, the anti-candidal activities of BV and SBV were less potent than those of amphotericin B and fluconazole as a positive control (Table 2). Most anti- biotics show an anti-microbial effect caused by a cidal or a static action. SBV exhibited an effect similar to that of amphotericin B, which is known a cidal agent for fungi; the anti-candidal activities of SBV are due to its killing action, as described by the time killing curve assay with C. albicans (Fig. 2).

In this study, the MIC values for C. albicans blood and viginal isolates, as obtained by using the broth microdilution method, varied from 62.5 μg/mL to 125 μg/mL for BV and from 15.63 μg/mL to 62.5 μg/mL for SBV (Table 2). Seoung [39] reported that methanol extracts from Galla rhois showed antifungal activity against C. albicans isolated from patients with recurrent vaginal candidiasis and the MIC value was 50 mg/mL for C. albicans. Also, dichloromethane extract from Paeonia japonica showed antifungal activity against C. albicans, and the MIC value was 50 mg/mL for C. albicans [40]. Thus, the anti-candidal activities of BV and SBV are thought to be much stronger than those of extracts from Galla rhois and Paeonia japonica. We demonstrated that SBV had very strong antifungal activity, the antifungal activity of SBV being much stronger than that of BV. Moreover, the anti-candidal activities of BV and SBV were much higher than those of phytochemicals, judging by their effective anti-candidal concentration ranges. This study raises the possibility that BV and SBV can be used as an alternative strategy for treating fungal pathogens, which would reduce anti-biotic use. BV and SBV may be candidates for new antifungal agents against C. albicans clinical isolates. Although this study, demonstrated that BV and SBV had considerable antifungal activites, further experiments for evaluating the in vivo efficacies of BV and SBV for clinical applications are warranted.

The antifungal activities of BV and SBV against 10 clinical isolates of C. albicans that were cultured from blood and the vagina were determined by using the disk diffusion assay, the broth microdilution assay and the killing-curve assay. BV and SBV showed antifungal activities against those clinical isolates of C. albicans, and the antifungal activites of BV and SBV showed similar results on the disk diffusion assay. The MIC values obtained for clinical isolates by using the broth microdilution method varied from 62.5 μg/ mL to 125 μg/mL for BV and from 15.63 μg/mL to 62.5 μg/ mL for SBV. In the killing-curve assay, SBV behaved as amphotericin B, which was used as positive control, did. The antifungal efficacy of SBV was much higher than that of BV.

Fig. 1

Fig. 2

BV, bee venom; SBV, sweet bee venom; KCTC, Korean Collection for Type Cultures; CFUs, colony forming units; SD, standard deviation.