C. albicans is an opportunistic pathogen whose association with human disease is well documented (Ho et al. 2021). Colonization with Candida is common in human, even in premature infants (Bliss et al. 2012). The dietary habits and other environmental exposures can potentially shape microbial composition, and might affect the susceptibility of C. albicans infection in adolescents. C. albicans is widely isolated from different aquatic environments, and has already been advised as a fecal microorganism indicator for aquatic environments (Buck 1977). Candidalysin is a recently described cytolytic peptide toxin exclusively secreted by the pathogenic hyphal forms of C. albicans, contributing to mucosal tissue damage and the establishment of systemic infection (Hanaoka and Domae 2021). Therefore, candidalysin is not yet well investigated and should be further examined for its environmental occurrence, fate in the environment, and its effects to aquatic microbiota.
Environmental chemical exposure is strongly linked to neurodevelopmental impact. Zebrafish (Danio rerio) is one of the most popular vertebrates used in environmental toxicants identification due to its rapid life cycle, small size, rapid development, features like external fertilization and embryonic transparency. The analysis based on the behavior profile of zebrafish at embryonic stage can evaluate the impacts of environmental exposure (Zhang et al. 2021). Here, we use zebrafish larvae exposure model, which might reflect candidalysin’s effect in Candida infection in adolescents.
The exposure of low and high concentration of candidalysin reflects chronic and acute exposures respectively, which are representative factors of the risk of candidalysin. In this study, effects of low and high concentration of candidalysin were both investigated, as low concentrations of candidalysin represented environmentally realistic doses’ effect on embryonic zebrafish (Christen et al. 2010), while high concentrations were chosen as pharmacologically relevant. We demonstrated that exposure of zebrafish embryos to low-concentrations of candidalysin affected physiological end points, including decreased spontaneous contraction, heartbeat rate and hatching rate. Spontaneous contraction of zebrafish is initiated and regulated by the embryonic neural circuity within the spinal cord and normally serves as an important indicator for neural developmental effects. The decreased spontaneous contractions pointed out the potential neurotoxic effects of candidalysin which needs further exploration. The hatching rate decreased significantly while stimulated by relatively low-dose of candidalysin (0.008 mg/L), suggesting that candidalysin is a strong candidate imposing stress on embryonic development at low concentrations. Furthermore, the heartbeat and hatching success showed similar decreases at concentration as low as 0.008 mg/L, indicating that candidalysin could impose stress on embryonic development at relatively low concentrations.
Embryonic zebrafish showed increasing swimming frequency when stimulated by high-concentration of candidalysin at 72 hpf. The swimming behavior (bout per min) of 72 hpf embryos showed a significant increase at 10 mg/L, and displayed dose-dependent pattern (except 20 mg/L). This was possibly because of the imposed stress on embryonic zebrafish by high doses of candidalysin, which affected the fitness of the embryos and consequently behavior and survival of embryos. The embryos were dead when stimulated by 80 mg/L and 160 mg/L of candidalysin, which prompted its ecotoxicological risk. However, this current research provides one perspective on the effectiveness of candidalysin on embryonic zebrafish. There is an urgent need to further analyze the consequences of candidalysin in additional aquatic organisms, and forthcoming studies is needed within this field to elucidate the molecular mechanisms underlying the developmental disruptions.
In the present study, significant effects of candidalysin including decreased spontaneous contraction, hatching rate, heartbeat rate, and increased swimming frequency were observed in embryonic zebrafish. In addition, candidalysin treatment leads to embryo death at relatively high concentrations. Overall, the present study demonstrates that candidalyin can impair the development of embryonic zebrafish and provides insights into unexpected and potentially environmental hazard relevant health impacts of candidalysin on aquatic organisms, but also be important for humans (especially adolescents), as C. albicans is an important human fungal pathogen.