A method for preventing, treating, or ameliorating a microbial infection can include administering thymoquinone or a pharmaceutical composition comprising thymoquinone to a patient in need thereof. The patient may be suffering from a microbial infection caused by gram-negative bacteria, gram-positive bacteria, or fungi. The microbial infection may be caused by gram negative bacteria. The gram-negative bacteria may include Acinetobacter baumannii. The gram-negative bacteria may include Pseudomonas aeruginosa. The microbial infection may be caused by antimicrobial sensitive Acinetobacter baumannii or antimicrobial resistant Acinetobacter baumannii.