Mucormycosis

Mucormycosis is a rare but serious fungal infection that rapidly attacks and usually kills its untreated victims, who are often acidotic, 'immunocompromised', or receiving desferrioxamine. Many reported cases have been poorly controlled diabetics. 'Synonyms' are zygomycosis or phycomycosis.

The presumed way in which the fungus attacks compromised individuals is fascinating. Iron availability limits the growth of many microorganisms, and this seems to be the case for the phycomycete fungi that cause mucormycosis. Normally, the fungal hyphae produce a substance called rhizoferrin, which binds iron avidly. The iron-rhizoferrin complex is then taken back into the fungus, and the iron becomes available for vital intracellular processes. Human resistance to fungal infection seems to depend to a large degree on non-immune factors. One of these defence strategies is to limit iron availability to invading microorganisms, by binding the iron to proteins such as apotransferrin. It is clear that a defect in the body's ability to hide iron from invading fungi will predispose to overwhelming fungal attack.

A convincing experiment by Artis and colleagues in 1982 supported this hypothesis. They took serum from seven patients with diabetic ketoacidosis, and showed that four of the seven sera supported profuse growth of Rhizopus oryzae, a common cause of mucormycosis, at a pH under 7. 3, which pH one might expect in the blood and tissues of a ketoacidotic diabetic. This breach in the body's defences seemed to be related to a decreased ability of the sera to bind iron at low pH! (Diabetes 1982 Dec;31(12):1109-14) Such defects were not seen in normal sera.

Phycomycetes can grow amazingly fast. This has been shown in experiments which looked at Rhizopus as a potential source of protein - it has been estimated that 450kg of fungus, provided with adequate substrate, can produce up to 40kg of (dry weight) protein per hour! (Appl Environ Microbiol 1976 Sep;32(3):381-7) It is thus not surprising that given the right conditions, mucormycosis can within a day destroy the sinuses and invade the brain of a susceptible person.