Beijerinck war neben anderen Tätigkeiten seit 1895 Professor am Polytechnischen Institut von Delft. He established the Delft School of Microbiology. His studies of agricultural microbiology and industrial microbiology yielded fundamental discoveries in the field of biology. His achievements have been perhaps unfairly overshadowed by those of his contemporaries Robert Koch and Louis Pasteur, because unlike them Beijerinck never studied human disease.
He is most famous as the founder of Virologie. He discovered viruses, by proving in filtration experiments that the tobacco mosaic disease is caused by something smaller than a bacterium. He named that new pathogen virus. Er gewann die Leeuwenhoek-Medaille im Jahre 1905.
Beijerinck also discovered nitrogen fixation, the process by which diatomic nitrogen gas is converted to ammonium and becomes available to plants. Bacteria perform nitrogen fixation, dwelling inside root nodules of certain plants (legumes). In addition to having discovered a biochemial reaction vital to soil fertility and agriculture, Beijerinck revealed this archetypical example of symbiosis between plants and bacteria.
Beijerinck discovered the phenomenon of bacterial sulfate reduction, a form of anaerobic respiration. He learned that bacteria could use sulfate as an electron acceptor, instead of oxygen. This discovery has had an important impact on our current understanding of biogeochemical cycles. Spirillum desulfuricans was the first known sulfate-reducing bacterium, isolated and described by Beijerinck.
Like too many brilliant scientists, Beijerinck was a socially awkward figure. He was verbally abusive to students, never married, and had few professional collaborations. He was also known for his ascetic lifestyle and his view of science and marriage being incompatible. His low popularity with his students periodically depressed him, as he very much loved spreading his enthusiasm for biology in the classroom.