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Dimorphic Fungi


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Published: December 2012

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Dimorphic fungi cause several common diseases including histoplasmosis, blastomycosis, and coccidioidomycosis. Dr. Roberts discusses the distribution and ecology of these fungi, as well as their pathogenesis and cultural characteristics, in this 5-part presentation.

Presenter: Glenn D. Roberts, PhD

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Welcome to Mayo Medical Laboratories Hot Topics. These presentations provide short discussion of current topics and may be helpful to you in your practice. Our speaker for this program is Dr. Glenn Roberts, a Professor of Laboratory Medicine and Pathology and Microbiology at Mayo Clinic, as well as a consultant in the Division of Clinical Microbiology. Dr. Roberts discusses disease-causing dimorphic fungi and how to identify them in culture. This presentation examines Sporotrichosis. Thank you, Dr. Roberts. Thank you, Sarah, for that introduction.

I have nothing to disclose.

The diseases caused by the dimorphic fungi are as you see here, Histoplasmosis, Blastomycosis, Coccidioidomycosis, Paracoccidioidomycosis, Sporotrichosis and Penicilliosis.  Histoplasmosis, Blastomycosis, Coccidioidomycosis, and Sporotrichosis are found in North America.  The other two are not.  Paracoccidioidomycosis is found in Central and South America and Penicilliosis is found in places like Vietnam and Cambodia and so on and Thailand.

The dimorphic fungi have a number of features that are in common with each other and the growth rate is one of those.  The growth rate depends upon a number of things.  It depends on how much of an organism is present in a culture.  It depends on the culture medium that you use for recovering the organism and it depends upon the individual metabolism of the organism.  Blood enrichment is something that we see there’s a need for with recovering a dimorphic fungi.  Certain of them seem to have a requirement to grow better with blood enrichment and others do not and if they do require blood enrichment, this is an interesting fact that you can recover them better but they don’t sporulate well.  So, you have to subculture them to another medium to allow them to sporulate.  They all have very small delicate hyphae.  Dimorphic fungi all have this particular feature but not all of the fungi that have small delicate hyphae are these dimorphic fungi either.  We do have nucleic acid probes, nucleic acid sequencing and now MALDI-TOF to help us identify these organisms in addition to traditional methods which most of us probably still use and will for a while using a microscopic morphologic features and a few other things to go along with it.

This image shows you small hyphae seen with the dimorphic fungi and it’s hard to see that there is septate.  But they are and they are just very tiny hyphae probably 0.5 microns in size.

The colonial morphologic features of the dimorphic fungi vary a lot depending upon the individual organism, the isolate and also on the medium that is used for recovery because they look a lot different just on three different media, you’ll have three different morphologic features showing on each of those different media.  The colonies of Histoplasma and Blastomyces dermatitidis are indistinguishable and so we have problems with trying to use macroscopic morphology for helping us to identify these two organisms, we rely on the microscopic morphology. 

This presentation will focus on Sporothrix schenckii and this organism is a bit different than all of the others too. 

This organism is dimorphic just like all the others that we have talked about but it has a yeast and a mold form.  But it is an organism that causes generally subcutaneous infection that is a result of direct trauma to an area of the skin and some exposed surface like an extremity where the organism has been introduced, the mold form has been introduced from the environment into the tissue and then it converts inside the host to the yeast form.  It is found world wide, found primarily in the tropical parts of the world but it’s found world wide.  This happens to be a yeast-like colony of the mold form of Sporothrix.  Now, this is something that you see pretty often in this organism.  It starts to grow and you think you are working with yeast and in fact you actually can develop an API profile for the yeast, API Aux system for identifying yeast.  You can have a profile number for this particular organism because it looks like a yeast when it starts off.

And here you see another yeast-like colony of Sporothrix schenckii.  The names Sporothrix schenckii came from the following description.  When you looked at it underneath a microscope you noticed that the spores were produced and they were attached to a central hyphal filament by a thread-like attachment.  So, the spore will refer to spore, thrix refers to hair or a thread-like attachment and then schenckii is the term used to describe the person who first named this organism, Dr. Schenk in Baltimore.  And so this is an example of one of the yeast-like colonies of Sporothrix schenckii, the mold form.

Now here you see some stellate colonies or star-like colonies of Sporothrix.  And all of these things look different.  They are all cultures of Sporothrix schenckii.  And these do not appear to be all that fluffy.

Here you can see a primary isolate from a patient we had had Sporotrichosis and notice the center of the colony is darker than the perimeter.  In time, this culture will begin to form melanin in the center and it will then as the culture matures, the rest of the colony will turn dark.  

And here you see a colony that shows melanin production in there.  And this is common for this particular organism.  The spores turn dark.

Melanized fungus, this is a colony of a melanized fungus.  You can see the dark melanin-like pigment inside of this colony and it’s a yeast-like colony of Sporothrix schenckii.  You can see someone wrote the name down on there when they identified it.  First, they didn’t know what it was, you can see the question mark and once they looked at it, they knew exactly what it was. 

This is an example of a colony that is older, that has become much more mature and it has melanin all throughout the whole colony and it’s turned to kind of leather-like and dry.  It’s totally different from what it started out to be which was yeast-like and this is probably a month or so old and if you were to try and make a mount from this colony, you would have to take a wire and cut a piece out, it’s like cutting a piece of leather out. 

This is really a highly melanized isolate of Sporothrix schenckii, totally dark black not all that common.

This is a primary isolate of Sporothrix schenckii and you can see that it has a fluffy central part of the colony along with the perimeter being a little bit more kind of maybe feather-like.

And this is the same isolate under a different medium. And here it looks almost yeast-like on that medium.  So we have two different media with the same culture growing differently on two different medias.

And here is another one, these are kind of star or stellate colonies on a medium, this is still the same isolate and the same patient on a third medium. 

So, Sporothrix schenckii produces these small delicate sepate hyphae we’ve talked about with all of the others.  This one has probably even smaller septate hyphae that are produced.  And as you look at the culture underneath the microscope, what you will see are conidiophores that will be a long stalk that are very delicate.  They may be short but they are very delicate and at the tip you find these spores connected to the tip of this long conidia form by a little thread-like attachment and that’s where it gets its name Sporothrix schenckii.  As the culture gets dark because pigmented and the conidia begin to grow all the way around the hyphal strand just like a sleeve on your shirt and they call it the sleeve arrangement and when it does this, then the conidia begin to turn dark because of the melanin.

This is a beginning of the production of conidia with Sporothrix schenckii.  If you notice that you will see that the tip of some of those conidiophores, you will see three or four spores beginning to be produced.  Each one of those is connected by a thread-like attachment to that long conidiophore and in time there will be a cluster of those up there that we call a flowerette.

And here’s the flowerette.  And you can see the long stalk that’s a conidiophore and each of those spores is still connected to that conidiophore, that long conidiophore by a thread-like attachment.  And then notice along the sides of some of the hyphae, you will see spores coming off.  That’s the beginning of the sleeve arrangement. 

This is the flowerette I was talking about.  You can see them all over there.  There is one that is almost in to the left and down a little bit to the bottom of the center.  That one shows you the flowerette arrangement of conidia.

This is kind of a good contrasting photo micrograph where you can see those conidia on the tip of a long conidiophore and there may be three or four of them there.  You don’t see a total cluster there but that’s how they are formed, just like that.

This is the sleeve arrangement showing these melanized darkly pigmented spores that are produced and they’re produced after the flowerette arrangement of spores is produced.

And here you can see just how dark these are.  This is all melanin in these things. This is melanized conidia.  These are the spores of a conidia of an older culture. 

And this just shows you a large view of how they are connected but there is a thread-like attachment. And you can see one to the left hand side where it is connected to the hyphal strand.

Sporothrix schenckii is still a different in another way.  And that is that we don’t really have any probes or anything for this organism to identify it so we have to rely upon traditional methods to do that.  And this organism actually is easily converted from the mold form to the yeast form.  It’s done by placing from the mold form on a blood-enriched medium incubated thirty five to thirty seven degrees Centigrade and you might not believe this but you can put it on there during the day and come in tomorrow and the next day and it will be converted from the mold form to the yeast form, that quickly.  You just make a mount of it and there are those yeast cells.

The yeast cells are different from the other things that we see and they are elongated.  They look like cigars.  They are called cigar bodies and they are about the same size as Histoplasma between two to six microns.  Some of them are round, some are oval.  It might look a little bit like Histoplasma but you will still see the elongated ones.  And the yeast colonies that are grown up during the night, are creamy, smooth bacterial-like and this is an example of

The top is the melanized mold form and it converted to that creamy, yeast-like form after being incubated at thirty five to thirty seven degrees Centigrade.

And there are the cigar bodies.  They are an elongated cells and that’s about the only yeast that you see is elongated is Sporothrix. 

And this is a better example here.  You see the elongated cells here and each one of those is capable of producing a bud and it just continues on.