ation, at which the turbidity in the well was 5080% get Orange Yellow S lesser than in the control well. MIC and Fractional Inhibitory Concentration index of each drug combination was calculated to assess drug interaction in each isolate. FIC index is the sum of FIC of each drug, which in turn is defined as the MIC of each drug when used in combination divided by the MIC of the drug when it is used alone. Killing Assay An overnight culture of wild type C. neoformans grown in YPD at 30uC was harvested, washed in sterile distilled water and adjusted spectrophotometrically to an OD600 = 1.0. An appropriate volume of the cell suspension was diluted with 120 mL of AM3 to obtain an inoculum of 16105 CFU/mL. The cell suspensions were divided equally into 12 conical tubes with one serving as control without any drugs. 30 mg/mL MPA was added alone and in combination with graded concentrations of AmB to prepare the test solutions. Growth controls and test solutions were then incubated in a shaker at 37uC. Samples were withdrawn after 6 and 24 hours of drug exposure, serially 
diluted and 200 mL of the suspension was plated onto duplicate sets of YPD agar. The plates were incubated at 30uC for 48 hours and colony counts were determined. Capsule Detection Assay Wild type, ura2 mutant and complement strains were grown in YPD broth till mid-log phase. India ink preparations of the three strains prior to capsule induction were done by mixing 5 mL of cells with 3 mL of stain on a slide and observed under Differential Interference Contrast. Cells were then harvested, washed twice, resuspended in sterile distilled water and adjusted to an OD600 = 1.0 200 mL of this cell suspension was inoculated for capsule induction in 5 mL 1X RPMI or 1:10 diluted YPD, media similar to Sabauraud Dextrose medium used for the same purpose. Medium was either inoculated alone, or in combination with uracil and uridine, in a tissue culture plate or incubated at 37uC, in 5% CO2 for 4872 hours. India ink preparations were made as mentioned earlier and size of capsule was reported by measuring the radius of the dye excluding region between the cell wall and the India ink dye front. Measurements were performed using LAS-AF software. Survival Assay ura2 mutant cells from capsule detection assay were diluted 100 times and 100 mL of this suspension was plated out on YPD agar plates Nucleotide Biosynthesis and Amphotericin B following 0, 24 and 48 hours of capsule induction. Plates were incubated for 48 hours at 30uC following which CFUs were enumerated. Statistical analysis. For killing assays and capsule measurements, a one way ANOVA was used to assess significance. Pairwise analyses were conducted using Tukey’s t-test post-hoc. Results and Discussion Inhibition of Pyrimidine de novo Synthesis Increases Susceptibility of C. neoformans to AmB The combination of AmB and 5-FC exhibited synergy in vitro and enhanced early fungicidal activity in human studies against C. neoformans. PubMed ID:http://www.ncbi.nlm.nih.gov/pubmed/19649022 To determine if this effect was specific to 5-FC, or if general perturbation of pyrimidine biosynthesis could affect AmB susceptibility, we tested the AmB susceptibility of a ura2 mutant of C. neoformans by Etest. As demonstrated in a larger zone of inhibition in the ura2 strain as compared to wild type and complemented strain that correlated to a significant decrease in the MIC of ura2 mutant. This was reverted in the complemented strain with an MIC value comparable to that of wild type. We know that in the absence of de novo s
