In vitro and comparative study on the extracellular enzyme activity of molds isolated from keratomycosis and soil

To isolate and identify the molds involved in mycotic keratitis; to isolate corresponding species from soil samples; to compare the extracellular enzyme activity indices of the molds isolated from keratitis cases and the corresponding soil isolates.

The specimens were collected from the target patients attending the microbiology laboratory of tertiary eye hospital in Coimbatore, Tamilnadu state, India. The isolates were subjected for identification based on the growth on solid media, direct microscopy and lacto phenol cotton blue wet mount preparation. Extracellular enzymes such as lipase, deoxyribonuclease (DNase), α-amylase, protease, cellulase and pectinase produced by the fungal isolates were screened on solid media supplemented with the corresponding substrates. Based on growth and zone diameter, the enzyme activity indices were calculated and were compared with that of the soil fungal isolates.

A total of 108 clinical samples were collected from a tertiary eye care hospital and out of which 60 fungal isolates were obtained. Among these, Fusarium spp. (n=30), non sporulating molds (n=9), Aspergillus flavus (n=6), Bipolaris spp. (n=6), Exserohilum spp. (n=4), Curvularia spp. (n=3), Alternaria spp. (n=1) and Exophiala spp. (n=1) were identified and designated as FS1-30, NSM1-9, AF1-6, BS1-6, ES1-4, CS1-3, AS1 and EX1, respectively. For comparative analysis, soil samples were also collected from which, one isolate of each Fusarium spp., Aspergillus flavus, Bipolaris spp., Exserohilum spp., and Curvularia spp., respectively were selected. Highest lipase activity was seen in corneal isolate NSM2 (EAI= 2.14). The DNase activity was higher in NSM9 (EAI=1.88). In case of protease, Fusarium spp. (FS9) had prominent enzyme activity index of 1.38; α-amylase activity was also superior in corneal isolate FS13 with EAI of 1.63 when compared to other isolates. The enzyme activity index for cellulase was also noted to be higher in corneal isolates i.e. NSM7 with EAI of 1.98 when compared to other corneal and soil isolates. The pectinase activity index was also prominent for corneal isolate NSM5 versus the soil isolates, SAF1, SFS1, SES1, SBS1 and SCS 1 as 1.76 versus 1.47, 1.38, 1.16, 1.11 and 1.14, respectively.

The most common isolate was Fusarium spp. followed by Aspergillus, Curvularia, Exserohilum, Bipolaris, Exophiala and Alternaria species. Enzyme activity indices (EAI) of the enzymes analysed varied with the clinical and soil isolates with respect to protease and cellulase (P=0.01). Of all the strains compared it was noted that mean EAI was greater in many clinical fusarial isolates followed by non sporulating molds.