Smoking is known to induce a variety of changes in the oral cavity. Cigarette smoke affects both saliva
 and oral microorganisms, including C. albicans, a leading cause of oral candidiasis
. However, the specific effect of cigarette smoke on C.
albicans remains to be elucidated. The first question we addressed in this study was: What is the effect of cigarette smoke on C. albicans adhesion? We demonstrated that CSC promoted C. albicans adhesion, which is in agreement with previously reported studies showing a high rate of oral candida carriage in tobacco smokers compared to non-smokers
[30, 31]. Furthermore, bacteria exposed to CSC were shown to adhere more to epithelial cells compared to non-exposed specimens
, which supports our data with C. albicans. Increased adhesion of C. albicans in the presence of CSC may occur due to changes in the interaction of C. albicans with its environment through the expression of high levels of adhesins, as previously suggested
. Furthermore, it is important to realize that cigarette smoke comprises a high number of individual compounds
, including acetaldehyde, benzene, 1,3-butadiene, and isoprene with an elevated mutagenic potential
. Thus, it is possible that these compounds may have exerted specific effect on C. albicans adhesion, growth and probably biofilm formation. The effect of cigarette smoke promoting cell adhesion was previously reported by Baboni et al., (2009) showing linear dose response adhesion
. The mechanism involved in such effect could involve kinase pathways. These pathways can be promoted by CSC compounds at certain concentration, but inhibited when these compounds are high explaining the decrease of C. albicans adhesion/biofilm formation at 40% and 50% of CSC. Further research is mandatory to shed light on the mechanisms leading to the up-regulation of C. albicans adhesion when exposed to cigarette smoke.
C. albicans adhesion is one of the key events leading to candidiasis
[36, 37]. This adhesion is usually followed by overgrowth and invasion
. Consequently, in promoting C. albicans adhesion, CSC may lead to an over-growth of this yeast. Our study confirms this hypothesis showing a growth increase of C. albicans in the presence of CSC. This concurs with previously published reports showing that smoking can be an important predisposing factor for oral candidiasis
, which may be enhanced by cigarette smoke through an increased secretion of histolytic enzymes by C. albicans, thus contributing to its virulence
. However, the exact pathogenic influence of smoking has yet to be investigated.
C. albicans adhesion and growth are particularly necessary for biofilm formation
[39, 40]. Because CSC significantly increased C. albicans adhesion and growth in the present study, it is suggested that CSC may also promote C. albicans biofilm formation. Using appropriate conditions to form biofilms, our findings indicate that CSC was indeed capable of promoting biofilm formation. Of interest is that a significant increase of biofilm formation was obtained at both tested concentrations, and that this phenomenon was dependent on CSC concentration. These useful data are comparable to those of other studies showing increased microbial biofilm formation with cigarette smoke
[41–43]. By showing the significant stimulatory effect on increasing C. albicans biofilm formation, cigarette smoke can thus be labeled as an infection-promoting agent.
Promoting C. albicans adhesion, growth, and biofilm formation may operate through the modulated expression of certain C. albicans genes
[44, 45], as supported by our study demonstrating that CSC led to a high expression of the EAP1 gene. As a member of the GPI-CWP family in C. albicans, Eap1p was originally identified because of its ability to mediate adhesion to polystyrene when the EAP1 gene was expressed in a flocculin-deficient Saccharomyces cerevisiae strain. EAP1 expression in a C. albicans efg1/efg1 mutant was able to restore C. albicans adhesion to epithelial cells
. Deleting EAP1 in C. albicans was shown to reduce cell adhesion to polystyrene and to epithelial cells in a gene dosage-dependent manner
[12, 46]. Indeed, this suggests that exposure to CSC increases EAP1 expression, which may in turn contribute to increasing C. albicans adhesion, and ultimately, biofilm formation and pathogenesis.
We also demonstrated that CSC increased HWP1 mRNA expression. HWP1 is a downstream component of the cAMP-dependent PKA pathway and is positively regulated by EFG1. The transcript level of HWP1 increased with increasing CSC stimulation, which suggests that CSC did affect cAMP–EFG1 pathway activity, resulting in an increase of C. albicans adhesion and growth with biofilm formation. Further investigations are therefore warranted to gain greater insight into the interaction between cigarette smoke and C. albicans leading to infection.
Candida pathogenesis is associated with the production and secretion of histolytic enzymes
. Secreted aspartyl proteases (Saps) and phospholipases were specifically reported as being directly related to C. albicans virulence
. During infection, Saps are incriminated degrading host proteins involved in tissue barriers and immune defense
[18, 50, 51]. Here, we report that CSC upregulated Sap2 mRNA expression. It is known that Sap gene upregulation contributes to increasing C. albicans transition, and later, its pathogenicity through an augmented secretion of proteinases
. Our study thus establishes, for the first time, a clear link between cigarette smoke and C. albicans pathogenesis through the behavior of key genes such as EAP1, HWP1 and Sap2. These genes are known to be involved in controlling Candida adhesion, growth, and biofilm formation
; however, the precise interactions between these different genes and cigarette smoke during C. albicans pathogenesis have not yet been fully investigated. Data suggest that gene activation can be involved in C. albicans adhesion, growth and biofilm formation promoted by CSC. This may involve kinase pathways contributing to C. albicans adaptation to the CSC environment as previously suggested