Mucor is dimorphic?!?

Mycology has always been an interesting subject for me. I am fascinated by the different morphologies of moulds that grow on bread and fruit. During class, I loved to draw the morphologies (see my Mycology notes from my PHM training).

My notes about Zygomycetes

I learned that Mucormycetes (or Zygomycetes in the old taxonomy) have coenocytic, aseptate hyphae. The organisms within the Mycormycete group (e.g., Mucor sp. and Rhizopus sp., and the organisms I had sketched, in the picture above) can be differentiated based on their microscopic morphologies. For example, both Mucor sp. and Rhizopus sp. have circular to oval sporangia without apophyses. Mucor sp. is distinguished from Rhizopus sp. by its lack of rhizoids (i.e., roots).

Fungi can take on either multicelled forms (e.g., moulds) or single-celled forms (e.g., yeast-like). Some fungi can take on both! Dimorphic fungi, such as Coccidioides sp. and Blastomyces sp., have been observed in both mould (in the environment) and yeast (in the lungs) forms. Coccidioides immitis is known to cause Valley Fever, commonly in California's Central Valley and in Arizona. Blastomyces dermatitidis causes Gilchrist's disease around the Mississippi and Ohio River valleys. Both diseases infect the respiratory system first and eventually spread all over the body.

One day, the laboratory received a specimen for identification. The paperwork stated that the clinical laboratorians suspected that the organism they had isolated from a patient specimen was a zygomycete. I observed a sample of this organism under the microscope and was baffled to find two colony types: a mould and a yeast. Could there be a second organism that came with the mould?

I rechecked the specimen after one week of incubation. Still, there were two morphological types. There was no sign of yeast colonies underneath all the mycelia that were characteristic of Mucormycetes. I had to decide quickly: send the specimen to the reference lab (California Department of Public Health) because it has the tools appropriate for making an identification, or attempt another identification in a week. I opted to send it out.

Lo and behold, the reference test came back, and the isolate was indeed a Mucormycete. In fact, it was Mucor circinelloides. And here's something I didn't learn in training: this species of Mucor is dimorphic! No wonder I kept observing yeast-like cells in tease mounts. It was a fantastic thing to see. 

Because it is dimorphic, does it cause lung infections as well? Or is the dimorphism brought about by a different environmental condition?