Antibacterial Activity of Twenty Different Endophytic Fungi Isolated from Calotropis procera and Time Kill Assay

Background: Development of drug resistance in bacteria is a common and alarming problem worldwide and there is continuous and urgent need of antibacterial agents. Endophytes offer a plethora of secondary metabolites with various biological activities. These secondary metabolites may help the host plant in defense from pathogens and insects, growth stimulators also helps the host plant in stress tolerance. Calotropis procera is a well-known medicinal plant, used to cure various health ailments traditionally. So, the endophytic fungi isolated from different tissues (leaf, stem and root) of C. procera were evaluated for their antibacterial potential.

Methods: The antibacterial activity of crude ethyl acetate extracts of 20 different endophytic fungi was evaluated by using agar well diffusion assay against total nine bacterial reference strains. Minimum inhibitory concentration was determined by using microbroth dilution method. Time kill assay study was performed against the Salmonella typhi bacterial strain by using Aspergillus nomius extract.

Results: Out of the total 20 different endophytic fungal strains 7 endophytic fungal extracts showed activity against all tested bacterial strains. The endophytic fungi which belong to Aspergillus and Fusarium genus exhibited good antibacterial activity. Maximum zone of inhibition (17.33 mm) was shown by extracts of Aspergillus nomius, Fusarium solani, Aspergillus oryzae and Curvularia hawaiiensis against S. typhi, S. flexneri, S. typhi and S. marcescens respectively. Extracts of Aspergillus nidulans, Curvularia hawaiiensis, Chaetomium arcuatum and Chaetomium atrobrunneum also exhibited significant antibacterial activity against the tested bacterial strains. The MIC values were ranged between 15.6 μg/well to 250 μg/well. The endophytic fungal extracts were more efficient against the growth of Gram-positive bacteria as compared to Gram-negative. Time kill assay study against the S. typhi showed bacteriostatic effect of Aspergillus nomius strain extract at different concentrations.

Conclusion: Several endophytic fungi inhabit the different tissues of C. procera have capability of producing bioactive secondary metabolites with significant antibacterial activity. Further isolation and identification of these secondary metabolites may provide a new lead for development of novel drug molecules.