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Aims: Quercus infectoria (QI) gall extract is known to have broad spectrum anti-microbial activity in vitro. However, its mechanism of microbial growth inhibition is not well understood. The objectives of this study were to determine the antimicrobial effect of methanolic QI gall extract on bacteria and yeast and changes to their cell morphology. Methodology and results: The minimum inhibitory concentrations (MICs) of methanolic QI gall extract against Streptococcus agalactiae (ATCC 13813), Proteus vulgaris (ATCC 49312) and Candida albicans (ATCC 10231) were determined using two fold serial microdilution technique at concentrations ranging from 0.01 mg/mL to 5.00 mg/mL (for bacteria) and 0.02 mg/mL to 12.00 mg/mL (for yeast). Minimum bactericidal concentration (MBC) and minimum fungicidal concentration (MFC) were determined by subculturing the broth from the microtitre wells which showed no apparent growth or turbidity onto the nutrient agar plates. The scanning electron microscope (SEM) was used to analyze the morphology of bacteria and yeast cells treated with 1× MIC and 4× MIC of the extract. Substantial antimicrobial activity was observed against ATCC strains of S. agalactiae, P. vulgaris and C. albicans in this study. The MBC/MFC to MIC ratio (≤4) indicated the methanolic QI gall extract was bactericidal and fungicidal against all the tested strains. Changes to the cell morphology were more obvious at higher extract concentration (4× MIC). Conclusion, significance and impact of study: This study showed that QI gall extract has antimicrobial effects on the bacterial and yeast cell morphology and thus, provides scientific information suggesting its possible antimicrobial mechanisms on the cell wall and membrane integrity.