Objectives: This study aims to utilize conductivity as a parameter to comprehend:  
1) Time required for the drug to reach to its optimum activity.
2) Correlation between disc diffusion and conductivity results. 
3) Minimum inhibitory concentration (MIC) through conductivity studies.
Methods: 
Inoculum preparation: C. neoformans culture was added to 0.9% sterile saline solution, and adjusted to 0.5 McFarland. This suspension was further diluted in the ratio of 1:10 with Sabouraud Dextrose Broth (SDB) to obtain the final concentration of 1.0-5x106 CFU/ml.
Drug preparation: Three drugs: bacitracin, cinnamaldehyde and amphotericin B were prepared so as to have final concentration as 1.375, 2.75, 5.5, 11 and 22 mg/ml for bacitracin, 0.390, 0.781, 1.56, 3.12 and 6.25 µl/ml for cinnamaldehyde and 0.0156, 0.03125, 0.0625, 0.125 and 0.25 mg/ml for amphotericin B. 
Incubation and Observations: The incubation tubes containing 3.7 ml of fungal inoculum along with 300 µl of the respective drug concentration were incubated and the readings of both suspension and menstruum were recorded after 24, 48, 72 and 96 hours. 
Results: The conductivity values of suspension and menstruum of different concentrations of bacitracin have been plotted (figure 1).
Here, conductivity of the suspension increases with concentration of the drug and time elapsed. Rise in conductivity of fungal suspension is contingent upon 4 factors: A) Dead fungal cells pose less resistance than the live ones therefore cell death increases conductivity B) Decrease in size of the fungal cells as an action of drug facilitates the ion mobility hence increasing conductivity. C) Leakage of physiological salts due to exosmosis after cell death results in increased conductivity and D) rise in the concentration of the menstruum. In figure 1, conductivity of suspension seizes to increase after 72 hours therefore, indicating the optimum activity of the drug after 72 hours of incubation. Additionally, positive correlation value (R2=0.98) was obtained between the disc diffusion and suspension conductivity values (Figure 2 a).
 MIC is the lowest concentration at which the cell death has taken place or in this case highest conductivity. For cinnamaldehyde and amphotericin B the MIC calculated as 1.56µl/ml and 0.0625 mg/ml respectively lies in the range as determined by broth macrodilution assay. 
Conclusion:
There are numerous methods in order to determine the antifungal susceptibility against a particular micro-organism. Here, we have utilized the property of conductivity to determine the effect of a drug on a particular fungi. By this method we can determine the time required by the drug to reach to its optimum effect and additionally MIC of the drug can also be calculated. The digital multi-meter utilized here reduces the chances of errors and rapidity of the results obtained helps to expedite the process of identification of the best drug against a fungal pathogen.



1 / Conductivity: A novel approach for antifungal susceptibility testing