The Genetic Basis of Resistance to 5-Fluorocytosine in Candida Species and Cryptococcus Neoformans

Some of the key principles derived from the genetic study of the pathogenic yeasts Candida albicans and Cryprococcus neoformans may be illustrated by studies of the genetic basis of resistance to the clinical antifungal agent 5-fluorocytosine (5-FC). Genetically determined resistance of fungi to 5-FC, observed soon after introduction of this compound,' limits its clinical usefulness as one of a few antifungal agents suitable for use in systemic mycoses. However, resistance to 5-FC is a particularly useful genetic marker in basic studies of the fungi concerned because it is recessive and selectable. These properties permit detection of infrequent genetic events. The present reviewer will discuss the genetic and biochemical bases of resistance to 5-FC primarily as they relate to basic studies. 11. RESISTANCE DEFINED: C. ALBZCANS An important aspect of resistance to 5-FC in C. albicans is that there exist strains which are neither highly susceptible nor highly resistant, but which are partially (slightly) resistTwo s~rveys,~.~ which differed in terms of the samples of clinical isolates tested and also differed in methodology, provided estimates of the frequencies of the resistance classes. Susceptible strains were in the majority (60 and 57%), partially resistant isolates were a significant minority (36 and 37%), and highly resistant isolates were infrequent (4 and 6%). In their survey, Stiller et al. distinguished two intermediate resistance classes (I1 and III);4 for the present comparison these were taken to constitute one class, equivalent to the partially resistant class of Defever et Resistance may be assessed conveniently by growing cells on a defined minimal agar medium supplemented with 5-FC.3 Partially resistant C. albicans isolates are readily distinguished from susceptible isolates and from highly resistant isolates (Figure 1). This method has an advantage over methods which employ liquid medium or 5-FC-containing discs on solid medium in permitting ready detection of highly resistant variants derived from the partially resistant strains (Figure 1A). It was a striking characteristic of all partially resistant isolates tested in one survey that they gave rise to readily detectable resistant variants.' These variants, which can be shown to be highly resistant in pure culture (similar to Figure lC), may be responsible for the treatment failure which results from the development of secondary resistance. By this notion, if the infecting strain were partially resistant and unstable for resistance, treatment with 5-FC would select for the highly resistant variants derived from the partially resistant strain. Development of secondary resistance would seem much less likely if the infecting strain were susceptible (Figure 1B). However, these problems have not been studied. It would be interesting to determine whether a highly resistant strain which appeared during treatment was a clonal descendant of a partially resistant strain which was present before treatment. Recent developments in strain-typing methods may be useful in testing for clonal descent.6-" Strain typing based on restriction fragment length polymorphism is under development in at least two laboratories.