Nosema can almost always be found in bees, and is therefore essentially always present. For this reason Nosema can be considered as an organism that simply belongs to bees, just as certain bacteria.
Function of Nosema
This is a symbiosis between insect and fungus, just as in termites. There is a fungus in termites that is capable of decomposing woody material. Nosema is found in the intestinal wall and grows in the intestinal wall cells. The fungus should be considered as a channel for nutrients, just as mycorrhiza fungi in plant roots. Increase of Nosema seems mainly a consequence of a shortage of certain nutrients.
Nosema could have even more functions in the bee system, for example:
- breaking down certain nutrients (e.g. certain species of pollen)
- producing glucose-oxidase (to inhibit bacteria via H2O2, to fabricate chelates such as gluconic acid)
- stimulating reproduction
- stimulating foraging
Difference between N. apis and N. ceranae
During the last 15 to 20 years there has been a shift from Nosema apis to Nosema ceranae. At present Nosema ceranae is by far the most common. Apart from the morphological difference (N. ceranae has smaller spores and a thinner cell wall) the main difference is in the higher expression of manganese transporters (proteins) in N. ceranae. This has been established using genome analyses.
In N. apis a standard/modest supply of manganese can supply sufficient iron from the bee to counter bacterial infections. If, all of a sudden, there is too much manganese, then the fungus reacts by sporulating. This manifests itself as ‘copious Nosema’. If there is too much manganese continually, then the fungus adapts by producing more manganese transporters (N. ceranae), so that the fungus does not have to sporulate.
Nosema ceranae is considered more harmful than N. apis. Yet N. apis occurs more frequently in CCD.
Just as certain bacteria can become dominant, this can also happen with Nosema.