Date Published: October 18, 2018
Publisher: Public Library of Science
Author(s): Jurgen Wuyts, Patrick Van Dijck, Michelle Holtappels, Tom C. Hobman.
Persister cells are a small subpopulation within fungal biofilms that are highly resistant to high concentrations of antifungals and therefore most likely contribute to the resistance and recalcitrance of biofilm infections. Moreover, this subpopulation is defined as a nongrowing, phenotypic variant of wild-type cells that can survive high doses of antifungals. There are high degrees of heterogeneity and plasticity associated with biofilm formation, resulting in a strong variation in the amount of persister cells. The fraction of these cells in fungal biofilms also appear to be dependent on the type of substrate. The cells can be observed immediately after their adhesion to that substrate, which makes up the initial step of biofilm formation. Thus far, persister cells have primarily been studied in Candida spp. These fungi are the fourth most common cause of nosocomial systemic infections in the United States, with C. albicans being the most prevalent species. Remarkably, persisters exhibit characteristics of a dormant state similar to what is observed in cells deprived of glucose. This dormant state, together with attachment to a substrate, appears to provide the cells with characteristics that help them overcome the challenges with fungicidal drugs such as amphotericin B (AmB). AmB is known to induce apoptosis, and persister cells are able to cope with the increase in reactive oxygen species (ROS) by activating stress response pathways and the accumulation of high amounts of glycogen and trehalose—two known stress-protecting molecules. In this review, we discuss the molecular pathways that are involved in persister cell formation in fungal species and highlight that the eradication of persister cells could lead to a strong reduction of treatment failure in a clinical setting.
The global AIDS crisis, the use of implants, and the higher survival rates of immunocompromised patients have resulted in an increase in invasive fungal infections [1,2]. Candida spp. are the fourth most common cause of bloodstream infections in intensive care units  and are associated with mortality rates of up to 40% . Fungicidal compounds currently on the market are able to completely eradicate fast-growing liquid cultures in vitro but are not always successful in clearing fungal infections in a clinical setting . This is extremely problematic, especially in current medical practice in which immunomodulation and device implantation put more patients at risk for fungal infections . Several phenomena can be responsible for treatment failure (e.g., low patient compliance, a lack of antifungal penetration, etc.), but here we will only focus on how pathogens are able to survive fungicidal drug exposure. In this context, we refer to polyenes, such as amphothericin B (AmB), echinocandins, such as caspofungin, and miconazole, a fungicidal azole antifungal drug. Several factors resulting in treatment failure to these drugs were identified [7–9]. First, resistant isolates are not only able to survive high antifungal drug concentrations but are also able to grow in the presence of the fungicidal drug . Second, fungal cells can display tolerance to an antifungal drug. Tolerance is defined as survival following a transient exposure to high concentrations of a fungicidal agent above the minimum inhibitory concentration (MIC) . As a result, it takes longer for a fungicidal agent to kill the cells. Finally, fungal cells can occur as biofilms that are able to attach to biotic surfaces as well as to implantable medical devices . Notably, biofilms are associated with increased resistance against antifungal agents and host immune factors. They can thus result in treatment failure . Several reasons have been proposed for the high resistance of biofilms to antifungal agents, including drug sequestration by matrix components, the up-regulation of drug efflux pumps, and the presence of multidrug-tolerant persister cells [13–15]. Persister cells are a specialized case of tolerance . They are nongrowing, phenotypic variants of wild-type cells and constitute only a small part of the biofilm population that is able to survive high doses of antifungal treatment (Fig 1). When challenged with an increasing amount of a fungicidal drug, they display a biphasic killing pattern by which a large part of the population is killed and a small proportion of the population is able to survive. Moreover, when the cells are regrown and repeatedly challenged with high fungicidal drug concentrations, they display the same biphasic killing pattern [16,17]. An important aspect to take into consideration is that tolerance against fluconazole, often referred to as “trailing growth,” is also observed for fungi [10,18]. However, this is distinct from persister cells. First, persister cells are only observed in biofilms and fluconazole has a limited efficacy against biofilms. Second, fluconazole is a fungistatic agent. Therefore, all cells will survive antifungal treatment, making the distinction of persister cells impossible.