We demonstrated that KSL-W was effective in inhibiting C. albicans growth at short and long culture periods. Although growth inhibition obtained with KSL-W was less than that obtained with amphotericin B, the effects of KSL-W nevertheless remain significant (p < 0.01). The growth inhibition effects of KSL-W are in accordance with previously reported findings  showing a downregulation of C. albicans activity induced by a bacteriocin-like peptide isolated from Lactobacillus pentosus. Furthermore, our results support other findings  reporting the effectiveness of KSL-W in disrupting P. gingivalis-induced hemagglutination and its synergistic interaction with host AMPs engaged in innate defense. The results strongly suggest that KSL-W is also effective against fungal growth and may be suitable for use to control C. albicans infections. Further studies on the possible synergistic effect of amphotericin B and KSL-W against C. albicans growth may provide insight.
C. albicans pathogenesis can also take place through the transition from blastospore to hyphal form [39, 40]. Our results indeed show that KSL-W completely inhibited C. albicans transition with a concentration as low as 5 μg/ml. These data are consistent with those of other studies with naturally occurring antimicrobial peptides (e.g., β-defensins) which were effective in blocking the morphological shift of Candida from yeast to hyphae [41, 42]. Thus KSL-W may possibly contribute to the control of C. albicans infection by reducing cell growth and yeast-hyphae transition. The effect of KSL-W on C. albicans growth can occur either through cytolysis or cell membrane disruption, resulting in cell death similar to what has been demonstrated with histatin-5 [43, 44]. Indeed, it was shown that histatin-5 induces the selective leakage of intracellular ions and ATP from yeast cells. This is caused by the translocation of histatin-5 into the intracellular compartment and accumulates to a critical concentration . Further studies are thus warranted to shed light on the fungicidal mechanism of KSL-W.
C. albicans growth and transition from blastospore to hyphal form are particularly important for biofilm formation and C. albicans virulence because a strain that is genetically manipulated to grow exclusively in the yeast form is greatly hindered in generating biofilms. In addition, a variety of C. albicans mutants known to be unable to form hyphae also show biofilm defects [46, 47]. As KSL-W significantly reduced C. albicans growth and inhibited its transition from yeast to hyphae, this suggests that KSL-W may inhibit C. albicans biofilm formation. Our findings indicate that KSL-W was indeed able to reduce biofilm formation and that its effect was comparable to that obtained with amphotericin B, a well-known antifungal molecule. Also of interest is that a significant inhibition of C. albicans biofilm formation was obtained at a concentration of as low as 25 μg/ml of KSL-W antimicrobial peptide. These useful data are comparable to those of other studies showing the positive action of synthetic peptide in controlling and preventing microbial biofilm formation . Thus, with its significant impact in reducing C. albicans biofilm formation, KSL-W may show potential for several novel applications in the clinical setting. Further investigations will elucidate this effect.
Biofilm formation can be controlled with anti-biofilm molecules prior to its development, although this is not actually the case in clinical applications, as antifungal and microbial molecules cannot be used on a daily basis to prevent biofilm formation. An effective molecule should ideally be able to prevent biofilm formation, but more importantly to disrupt biofilms that are already formed. We therefore questioned whether KSL-W was capable of disrupting mature C. albicans biofilm.
We proceeded to examine the impact of KSL-W on mature biofilm formation and demonstrated a significant disruption of these biofilms following contact with KSL-W, thus suggesting the possible use of this antimicrobial peptide to reduce/eliminate mature biofilms. Further studies should confirm such observations and demonstrate how KSL-W reduces or disrupts C. albicans biofilms.
Once it reaches the cell, KSL-W can potentially act on the cytoplasmic membrane as well as on intracellular targets [49–51]. The action of KSL-W against C. albicans may operate through the modulated expression of certain C. albicans genes that control growth , transition , and biofilm formation . We therefore examined the effect of KSL-W on a number of genes either directly or indirectly involved in phase transition and biofilm formation. EFG1 and NRG1 expression was assessed under hyphae/non-hyphae-inducing conditions. Our results show that KSL-W increased NRG1 mRNA expression twofold under non-hyphae-inducing conditions; however, under hyphae-inducing conditions, KSL-W significantly reduced NRG1 gene expression. These findings contrast with other reports that an increased NRG1 expression contributes to repressing various hypha-specific genes [55, 56]. This confirms that the effect of KSL-W in controlling C. albicans virulence does not take place through NRG1. KSL-W was also able to decrease EFG1 mRNA expression, when C. albicans was maintained under hyphae-inducing conditions.
EFG1p has been found to be a central regulator of C. albicans, as it is required for the development of a true hyphal growth form, and EFG1 is considered to be essential in the interactions between C. albicans and human host cells [7, 8]. The downregulation of this gene by KSL-W points to the singular role of this antifungal peptide. Thus the effect of KSL-W on C. albicans transition can be manifested through a repression of certain genes, such as EFG1 and NRG1.
KSL-W has a significant inhibitory effect on EAP1 mRNA expression. As a member of the GPI-CWP family [5, 57], deleting EAP1 can reduce the adhesion of C. albicans to different surfaces. This suggests that treatment with KSL-W may reduce EAP1 expression, which in turn may contribute to reducing C. albicans adhesion and ultimately, biofilm formation and pathogenesis. KSL-W was also shown to reduce HWP1 mRNA expression, particularly when C. albicans was cultured under hyphae-inducing conditions.
HWP1 is a downstream component of the cAMP-dependent PKA pathway and is positively regulated by EFG1 . The transcript level of HWP1 decreased with the KSL-W treatment at low and high concentrations. These data suggest that KSL-W indeed impacts the activity of the cAMP–EFG1 pathway and leads to an alteration of C. albicans growth and morphogenesis. Further studies are therefore required to investigate the invasion/virulence of KSL-W-treated C. albicans.
It is well known that Candida pathogenesis can be established by virtue of Candida growth and yeast-to-hyphae morphogenesis. Specific SAP genes were found to be preferentially expressed by Candida hyphal forms [10, 15, 59]. Because KSL-W downregulated C. albicans growth and transition, this may have occurred through a modulation of the SAP genes. Our findings confirm that KSL-W is capable of decreasing SAP2, SAP4, SAP5, and SAP6 mRNA expression in C. albicans which may lead to reducing C. albicans virulence [60–62].
Our study thus establishes, for the first time, a clear link between an antimicrobial peptide (KSL-W), hyphae morphogenesis, and hyphae-modulating SAPs 2, 4, 5, and 6. However, the precise interactions between these SAPs and KSL-W during C. albicans pathogenesis remain unclear. Additional studies should focus on identifying the role of SAP subfamilies involved in Candida invasion as well as the role of KSL-W in controlling Candida virulence/pathogenesis in conjunction with host defenses. In conclusion, this study is the first to demonstrate that synthetic antimicrobial peptide KSL-W downregulates C. albicans growth and transition, resulting in a decrease in biofilm formation and a disruption of mature biofilm. Also of interest is that these effects may occur through the modulation of C. albicans genes EFG1, NRG1, EAP1, HWP1, and SAPs. Overall results clearly suggest the potential of KSL-W as an antifungal molecule.