Diagnosis of Antifungal Drug Resistance Mechanisms in Fungal Pathogens: Transcriptional Gene Regulation

Defining the frontiers between antifungal resistance, tolerance and the concept of persistence

A restricted number of antifungal agents are available for the therapy of fungal diseases. With the introduction of epidemiological cut-off values for each agent in important fungal pathogens based on the distribution of minimal inhibitory concentration (MIC), the distinction between wild type and drug-resistant populations has been facilitated. Antifungal resistance has been described for all currently available antifungal agents in several pathogens and most of the associated resistance mechanisms have been deciphered at the molecular level. Clinical breakpoints for some agents have been proposed and can have predictive value for the success or failure of therapy. Tolerance to antifungals has been a much more ignored area. By definition, tolerance operates at antifungal concentrations above individual intrinsic inhibitory values. Important is that tolerance to antifungal agents favours the emergence of persister cells, which are able to survive antifungal therapy and can cause relapses. Here we will review the current knowledge on antifungal tolerance, its potential mechanisms and also evaluate the role of antifungal tolerance in the efficacy of drug treatments.

Update on Antifungal Drug Resistance

Invasive fungal infections remain a major source of global morbidity and mortality, especially among patients with underlying immune suppression. Successful patient management requires antifungal therapy. Yet, treatment choices are restricted due to limited classes of antifungal agents and the emergence of antifungal drug resistance. In some settings, the evolution of multidrug-resistant strains insensitive to several classes of antifungal agents is a major concern. The resistance mechanisms responsible for acquired resistance are well characterized and include changes in drug target affinity and abundance, and reduction in the intracellular level of drug by biofilms and efflux pumps. The development of high-level and multidrug resistance occurs through a stepwise evolution of diverse mechanisms. The genetic factors that influence these mechanisms are emerging and they form a complex symphony of cellular interactions that enable the cell to adapt and/or overcome drug-induced stress. Drivers of resistance involve a complex blend of host and microbial factors. Understanding these mechanisms will facilitate development of better diagnostics and therapeutic strategies to overcome and prevent antifungal resistance.

Candida glabrata: Multidrug Resistance and Increased Virulence in a Major Opportunistic Fungal Pathogen

C. glabrata is widely acknowledged to be an important and potentially antifungal resistant cause of hematogenously disseminated candidiasis (HDC). In the United States (US) both the frequency of C. glabrata as a cause of HDC and in vitro resistance to fluconazole has increased steadily since 1992. Although this species is generally considered to be less virulent than C. albicans, recent findings suggest that gain of function (GOF) mutations in the transcriptional regulator CgPdr1p results not only in broad resistance to azole antifungals but also an increase in both fitness and virulence in animal models. Furthermore, case reports and case series suggest the emergence of multidrug resistance (MDR) in this species. Recent data from multicenter surveys conducted in the US have demonstrated the emergence of co-resistance to both azoles and echinocandins in clinical isolates of C. glabrata. These findings are highlighted in an effort to bring attention to this important development.