There is just one essential nutrient that was believed to be absent in the plant kingdom: Vitamin B12. But recent studies are confirming that B12 is present to varying degrees in raw mushrooms, according to a 2009 study published in the Journal of Agricultural and Food Chemistry. At the Centre for Plant and Food Science in Sydney, Australia, researchers concluded that B12 was present to varying degrees in the same form as found in foods traditionally known to be high in B12 such as beef, beef liver, salmon, egg, and milk. It is believed that a healthy bacteria forming on the surface of the mushroom caps results in a higher concentration of B12. The bacteria is believed to be present in the soil which would explain why it is most present on the surface of unwashed mushroom caps.
The George Mateljan Foundation also reports on the presence of B12 in Cremini mushrooms tested from several farms, concluding that B12 varied significantly from farm to farm and depended on growing conditions. However, this study questions the reliability of B12 in mushrooms but nonetheless considers them to be an important source of the nutrient.
According to the late Dr. Kanichi Mori (once head of the Japanese Mushroom Institute), shiitake mushrooms lower serum cholesterol, have strong anti-tumor and anti-viral properties, very low fat, no starch, and more vitamin B12 than milk and fish.1
In any case, it is important for everyone to get B12 in their diet. Many non dairy milks, including soy and almond, are fortified with B12 and other nutrients such as Vitamin D and calcium. Most multivitamin formulas also contain generous sources of B12. For all you need to know from a trusted source, See http://www.veganhealth.org/articles/vitaminb12.
1The Small Farm Program is part of the University of California Division of Agriculture & Natural Resources