Paradoxical growth effect of caspofungin observed on biofilms and planktonic cells of five different Candida species

The paradoxical growth (PG) of Candida sp. biofilms in the presence of high caspofungin (CAS) concentrations was previously unknown. We sought to characterize the PG at supra-MICs of CAS among clinical Candida sp. isolates grown as biofilms in 96-well polystyrene microtiter plates. The MICs of CAS were determined for 30 clinical Candida sp. isolates (4 Candida albicans, 6 C tropicalis, 7 C. parapsilosis, 8 C. orthopsilosis, and 5 C. metapsilosis isolates) when they were grown as planktonic cells and biofilms and were defined as the lowest drug concentrations that resulted in a prominent decrease in growth and a 50% reduction in metabolic activity, respectively. PG was defined as a resurgence of growth (>50% of that in the drug-free growth control well) at drug concentrations above the MIC. With the exception of C. tropicalis, all isolates displayed PG more frequently when they were grown as biofilms than when they grown as planktonic cells. PG was undetectable among C. metapsilosis isolates in planktonic cell MIC tests but was present in 100% of the isolates in biofilm MIC tests. The drug concentration and the number of drug dilutions supporting PG were higher for biofilms than for planktonic cells. Microscopic changes in cell morphology were observed among both planktonic and biofilm cells with PG. Specifically, the accumulation of enlarged, globose cells was associated with PG, and we hypothesize that CAS-induced changes in the cell wall composition may be the explanation.