has emerged as a major cause of bloodstream infection in the United States. Its decreased susceptibility to azole antifungal agents has led to the increased use of the newest antifungal agents, the echinocandins, for standard therapy of C. glabrata
infections, as recommended in the IDSA guidelines for management of candidiasis (24
). In our surveillance, C. glabrata
remained largely susceptible to all three of the echinocandins, with susceptibility rates of 98.6%, 98.9%, and 99.6% for caspofungin, micafungin, and anidulafungin, respectively, assuming the current breakpoint of 2 μg/ml. However, if the breakpoint for susceptibility were lowered to ≤0.125 μg/ml to account for the MIC values for isolates with known FKS
mutations, then 96.9% of our isolates would fall within the susceptible range, regardless of which echinocandin was considered. Among the 16 isolates with FKS
mutations, the highest MIC values were for caspofungin, while the lowest MIC values were for anidulafungin, similar to the results reported by Perlin for C. albicans
isolates with FKS1
). Isolate CAS09-1648 (with R665G mutation) had MIC values of only 0.06 μg/ml for anidulafungin and 0.125 μg/ml for caspofungin, both of which are within the epidemiological cutoff range, but had an MIC of 0.25 μg/ml for micafungin. This mutation would have been missed if anidulafungin or caspofungin alone had been used to screen for mutations. It is quite possible that this mutation confers a response specific to micafungin, but at this time we do not have any information that would allow us to predict the clinical significance of such a mutation and the outcome of usage of any of the three echinocandins for this isolate. The hot spot 1 mutations R631G in Fks1p and R665G in Fks2p, which are structurally homologous mutations in the two Fks proteins, have not been reported previously for clinical C. glabrata
isolates. Because they are not associated with highly elevated MIC values, perhaps patients harboring C. glabrata
isolates with either of these two mutations would be less likely to fail therapy and the MIC values would not be noted because they fall under the current breakpoint for susceptible isolates.
There was no temporal difference in the number of isolates with FKS mutations collected over the 2 years of surveillance: eight isolates were collected in 2008, and eight isolates were collected in 2009. It is interesting that despite receiving isolates from 40 hospitals, 12 of the 16 isolates were clustered in five hospitals, two in Baltimore and three in Atlanta, and 25% of the isolates, although genetically unrelated to one another, came from a single hospital in Atlanta. While in at least two cases in Atlanta we cannot rule out clonality among isolates with identical Fks2p mutations and sequence types, MLST analysis does not support the hypothesis of clonal spread of a single resistance phenotype.
In looking for spontaneously derived caspofungin-resistant isolates of C. albicans
on plates containing 4 μg/ml of caspofungin, Balashov and coworkers (2
) found that the overwhelming majority of mutations (86%) occurred at serine 645 of Fks1p. This is structurally homologous to C. glabrata
serine 663 of Fks2p. Their distribution of 62% of isolates having the S645P mutation and 8% of isolates having the S645F mutation closely parallels our findings for C. glabrata
, with 63% of our high-MIC isolates having the S663P mutation and 6% having the S663F mutation. In the Balashov study, it was also found that 22% of the isolates had the S645Y mutation, one that we did not find in our study. While their study was not designed to detect mutations in Fks2p, we did find one structurally homologous mutation in Fks1p, S629P. The Fks2p S663P mutation in C. glabrata
has previously been reported for at least one patient who failed anidulafungin therapy (14
) and in two other surveillance reports on FKS
). In a collection of random C. glabrata
clinical isolates with FKS
mutations, the majority of the isolates (69%) had mutations in FKS2
). One of the most interesting aspects of the frequency of the S663P mutation in our surveillance is that this mutation is not clonal in origin in our isolates, since it is spread among five different sequence types. Our data and previously published data (2
) indicate that there is strong pressure for mutation at this particular amino acid position in Fks1p of C. albicans
and in Fks2p of C. glabrata
, such that the emergence of this mutation in multiple C. glabrata
clones is not surprising.
One of the more unfortunate aspects of the relative abundance of the S663P mutation is that it is associated with the highest echinocandin MIC values. However, not all of the isolates with the S663P mutation had the same MIC values. This may be a reflection of the expression level of the FKS2
gene compared to that of the FKS1
gene. It has been shown that in C. glabrata
gene is expressed at a higher level than FKS1
in wild-type isolates but that the expression levels change in isolates with FKS
). We have not determined the expression levels of FKS
genes in our isolates, but it will be interesting to do so in the future to assess the possible role of FKS
gene expression levels in MIC values for isolates with identical mutations.
All of our isolates with FKS
mutations for which we had epidemiological data came from patients who had previously been treated with an echinocandin. The clinical significance of elevated MIC values for the echinocandins is unclear. Although there is a clear link between echinocandin therapy, elevated echinocandin MIC values, and treatment failure in a limited number of case reports (7
), many patients with Candida
isolates having elevated echinocandin MIC values respond to echinocandin therapy, and elevated MICs may not be a good predictor of outcome (3
). If the breakpoints for the echinocandins and C. glabrata
were lowered to reflect the epidemiological cutoff value for this organism, our data support the lowered values as being able to distinguish between wild-type isolates and those carrying mutations in their FKS
genes that affect susceptibility.
Echinocandins are now the IDSA-recommended first-line therapy for C. glabrata
). While only a small proportion of the C. glabrata
isolates in this study had elevated echinocandin MIC values, it is likely that these isolates will continue to emerge and increase in frequency as echinocandin usage increases. It is important to continue surveillance for FKS
mutations associated with elevated echinocandin MIC values and to monitor the impact on clinical outcomes.