An effective microorganism refers to any of the predominantly anaerobic organisms blended in commercial agricultural amendments, medicines and nutritional supplements based on the trademarked product originally marketed as EM-1 Microbial Inoculant, aka Effective Microorganisms and EM Technology. These blends include:
- Lactic acid bacteria: Lactobacillus casei
- Photosynthetic bacteria: Rhodopseudomonas palustris
- Yeast: Saccharomyces cerevisiae
- Others: beneficial microorganisms that exist naturally in the environment may thrive in the mixture.
EM Technology is purported to support sustainable practices in farming and to improve and support human health and hygiene, compost and waste management, disaster clean-up (the Bangkok floods of 2011, the Southeast Asia tsunami of 2004, the Kobe earthquake, and Hurricane Katrina remediation projects).
EM has been employed in many agricultural applications, but is also used in the production of several health products in South Africa and the US.
The concept of "friendly microorganisms" was developed by Professor Teruo Higa, from the University of the Ryukyus in Okinawa, Japan. He reported in the 1980s that a combination of approximately 80 different microorganisms is capable of positively influencing decomposing organic matter such that it reverts into a "life promoting" process. Higa invoked a "dominance principle" to explain the effects of his "Effective Microorganisms". He claimed that three groups of microorganisms exist: "positive microorganisms" (regeneration), "negative microorganisms" (decomposition, degeneration), "opportunist microorganisms". In every medium (soil, water, air, the human intestine), the ratio of "positive" and "negative" microorganisms is critical, since the opportunist microorganisms follow the trend to regeneration or degeneration. Therefore, Higa claimed that it is possible to positively influence the given media by supplementing with "positive" microorganisms.
The concept has been challenged and no scientific studies support all of its claims. This was acknowledged by Higa in a 1994 paper co-authored by Higa and soil microbiologist James F Parr. They conclude "the main limitation...is the problem of reproducibility and lack of consistent results.".
Parr and Higa mention soil pH, shading, soil temperature and flooding as factors affecting the interaction of EM with local microorganisms and with each other. The approach that Higa and Parr recommend is maintaining pH and soil temperature within conditions known to be detrimental to negative microorganisms as well as the addition of EM to tip the balance of positive and negative microorganisms in favor of the former.
They dismiss inoculants that include only a single microorganism as generally ineffective due to the uncertainty about the conditions in which a single microorganism would be effective. They cite the acknowledgment by the scientific community that multiple microorganisms (as in the case of Bokashi, invented and marketed by Higa) in coordination with good soil management practices positively influence plant growth and yield.
Lwini and Ranamukhaarachchi published in 2006 a paper that discusses biological controls of bacterial wilt disease and showed that EM and EM Bokashi were most-effective as bio-control agents. Yamada and Xu examined the use of EM in making organic fertilizers. Hui-Lian Xu studied photosynthesis and yield of sweet corn, physiological characteristics in peanuts, and fruit yield and quality of tomato plants. Daiss, et. al., looked at pre-harvest and post-harvest  applications of EM-1.
Many earlier papers were on EM-X Rice Bran Supplement, a product sold for human consumption. Chui, et. al., studied the apoptotic potential of microorganisms. Datla, et. al., and Ke B. examined antioxidant-related microorganisms.
- "Trademark Guidelines". emrojapan.com. 2011. Retrieved 13 November 2011.
- Szymanski, N.; Patterson, R.A. (2003). "Effective Microorganisms (EM) and Wastewater Systems in Future Directions for On-site Systems: Best Management Practice." (PDF). In R.A. and Jones, M.J. (Eds). Proceedings of On-site '03 Conference. Armidale, NSW, Australia: Lanfax Laboratories. pp. 347–354. ISBN 0-9579438-1-4. Retrieved 2006-11-14.
- Higa, Dr. Teruo; Dr. James Parr (1994). "Beneficial and Effective Microorganisms for a Sustainable Agriculture and Environment." (PDF). Atami, Japan: International Nature Farming Research Center. p. 7. Retrieved 21 January 2008.
- MYINT LWIN1 AND S.L. RANAMUKHAARACHCHI. Development of Biological Control of Ralstonia solanacearum Through Antagonistic Microbial Populations. International Journal of Agriculture & Biology. 8(5), 2006. Pp 657–660.
- Yamada, K.; Xu, H. L. (2001). "Properties and Applications of an Organic Fertilizer Inoculated with Effective Microorganisms". Journal of Crop Production 3: 255. doi:10.1300/J144v03n01_21.
- Xu, H. L. (2001). "Effects of a Microbial Inoculant and Organic Fertilizers on the Growth, Photosynthesis and Yield of Sweet Corn". Journal of Crop Production 3: 183. doi:10.1300/J144v03n01_16.
- Pei-Sheng, Y.; Hui-Lian, X. (2002). "Influence of EM Bokashi on Nodulation, Physiological Characters and Yield of Peanut in Nature Farming Fields". Journal of Sustainable Agriculture 19 (4): 105. doi:10.1300/J064v19n04_10.
- Xu, H. L.; Wang, R.; Mridha, M. A. U. (2001). "Effects of Organic Fertilizers and a Microbial Inoculant on Leaf Photosynthesis and Fruit Yield and Quality of Tomato Plants". Journal of Crop Production 3: 173. doi:10.1300/J144v03n01_15.
- Daiss, N.; Lobo, M. G.; Socorro, A. R.; Brückner, U.; Heller, J.; Gonzalez, M. (2007). "The effect of three organic pre-harvest treatments on Swiss chard (Beta vulgaris L. Var. Cycla L.) quality". European Food Research and Technology 226 (3): 345. doi:10.1007/s00217-006-0543-2.
- Daiss, N; Lobo, M. G.; Gonzalez, M (2008). "Changes in postharvest quality of Swiss chard grown using 3 organic preharvest treatments". Journal of food science 73 (6): S314–20. PMID 19241576.
- Chui, C. H.; Hau, D. K.; Lau, F. Y.; Cheng, G. Y.; Wong, R. S.; Gambari, R; Kok, S. H.; Lai, K. B.; Teo, I. T.; Leung, T. W.; Higa, T; Ke, B; Tang, J. C.; Fong, D. W.; Chan, A. S. (2006). "Apoptotic potential of the concentrated effective microorganism fermentation extract on human cancer cells". International journal of molecular medicine 17 (2): 279–84. PMID 16391827.
- Datla, K. P.; Bennett, R. D.; Zbarsky, V; Ke, B; Liang, Y. F.; Higa, T; Bahorun, T; Aruoma, O. I.; Dexter, D. T. (2004). "The antioxidant drink effective microorganism-X (EM-X) pre-treatment attenuates the loss of nigrostriatal dopaminergic neurons in 6-hydroxydopamine-lesion rat model of Parkinson's disease". Journal of Pharmacy and Pharmacology 56 (5): 649–54. doi:10.1211/0022357023222. PMID 15142343.
- Ke, B; Xu, Z; Ling, Y; Qiu, W; Xu, Y; Higa, T; Aruoma, O. I. (2009). "Modulation of experimental osteoporosis in rats by the antioxidant beverage effective microorganism-X (EM-X)". Biomedicine & Pharmacotherapy 63 (2): 114–9. doi:10.1016/j.biopha.2008.03.008. PMID 18930627.