| Summary: | Background: A foodborne bacterium, Bacillus cereus is one of the common food contaminants that could cause food poising with several sickness symptoms to humans. Objective: This study was undertaken to evaluate the effect of ethyl acetate extract of endophytic fungus, Lasiodiplodia pseudotheobromae Industrial Biotechnology Research Laboratory (IBRL) OS-64 towards a test bacterium, B. cereus with emphasis on bacterial growth reduction and cell morphological changes. Materials and Methods: The ethyl acetate (fermentative broth) and methanolic (fungal biomass) extracts were screened for their antibacterial activity against test bacteria via agar disc diffusion assay. Results: The results demonstrated that there is a significant antibacterial activity of the ethyl acetate extract with a diameter zone of inhibition in the value of 22.0 ± 0.6 mm. However, no antibacterial activity was observed from the methanolic extract. In broth microdilution assay, the minimum inhibitory concentration and minimum bactericidal concentration values of the extract were 250 and 500 µg/mL, respectively. The results revealed that the extract exhibited a bactericidal effect on the test bacterium. A time-kill study was then performed to determine the efficacy of the extract and the results showed that the antibacterial activity of the extract was dose and time-dependent. The morphological changes of the bacterial cells exposed to the extract were observed under a scanning electron microscope and transmission electron microscope. The results revealed the abnormalities of the treated bacterial cells compared to the control and thus led to ultimate death beyond repair. Conclusion: The present study suggested the potency of the ethyl acetate extract from an endophytic fungus, L. pseudotheobromae IBRL OS-64 could be further used as an antibacterial agent. © 2021, Faculty of Pharmaceutical Sciences, Chulalongkorn University. All rights reserved.
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