Pathogenic Fungi

3.2. Pathogenic Fungi

It is known universally that fungi essentially comprise of moulds, yeasts, and higher fungi. It has been observed that almost all fungi are eukaryotic in nature, and do possess sterols but not the least peptidoglycan in their cell membrane. These belong to the class of chemoheteromorphs*, and a majority of them are aerobic in nature. Besides, quite a few fungi are also saprophytes** and found in soil and water, and eventually acquire their food and nutrition via absorption. These fungi produce characteristically both sexual and asexual spores. More than one hundred thousand species of fungi have been duly isolated, identified, and hence recognized across the globe ; of which nearly 100 cause infections to the humans.

Moulds are also collectively referred to as a ‘mycelium’ which essentially composed of a number of microscopic branching hyphae. Yeasts represent invariably the unicellular organisms that are found to be either spherical or ovoid in shape and appearance. Yeasts do undergo replication by the phenomenon of budding rather than binary fission. The exact mechanism of budding in yeasts may be explained whereby the cyto- plasm and dividing nucleus from the parent cell is initially a continum with the bud or the daughter yeast,

* Derivating from the normal type by chemical means.

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before a new cell wall gets deposited so as to make a distinct separation of the ensuing two cells. However, in certain specific instances these ‘buds’ do not undergo detachment completely, and thereby may give rise to the formation of short-chain of cells usually known as ‘pseudohypha’. Mostly, yeasts are single

cells and invariably generate rather smooth bacteria-like colonies when grown on the laboratory agar media ; but unlike bacteria they do possess an altogether distinct nucleus, and, therefore, these are essen- tially eukaryotes.

Example : Saccharomyces cerevisiae is a typical example of yeast which solely gives rise to the production of ethanol under anaerobic fermentative conditions.

Various Diseases Caused due to Pathogenic Fungi : In fact, there are several fungal infec-

tions or mycoses that are exclusively categorized under different heads based upon the degree of tissue involvement and mode of entry right into the respective host.

Examples : Following are certain typical examples of fungal infections that are usually observed in the humans, namely :

(a) Superficial—i.e., found to be solely localized to the skin, nails, and the hair. (b) Subcutaneous—i.e., infections are exclusively confined to the dermis, subcutaneous tissue,

and the adjacent areas. (c) Systemic—i.e., deep rooted infections confined to the various internal organs in the humans. (d) Opportunistic—i.e., infections that are caused exclusively in the immunocompromised

subjects.

There are two main types of fungi which are prone to cause dreadful diseases in the human beings, namely :

(i) Pathogenic Fungi, and (ii) Opportunistic Fungi. These two types of fungi shall be discussed briefly in the sections that follows : Pathogenic Fungi : The primary pathogenic fungi cause inections that usually take place in

previsouly healthy humans, and eventually come into being via the ensuing respiratory route. Examples : Histoplasmosis, blastomycosis, coccidio-mycosis, and paracoceidiodomycosis. Histoplasma capsulatum is the causative fungi for histoplasmosis. The organism happens to be

dimorphic* in nature, and is more or less endemic in several parts of the globe. It is invariably found in the soil and its growth could be enhanced considerably due to the very presence of the excreta of birds and bats.

Lungs represent the main site of infection ; however dissemination right into various other inter- nal organs, such as : liver, heart, CN-system can also occur abundantly.

Opportunistic Fungi : These fungi specifically, attack the immunocompromised subject usu- ally having a history of serious immune or metabolic defect or have undergone a recent surgery.

Examples :

(a) Aspergillosis. i.e., the different diseases caused by the mould Aspergillus sp., and exerts its action due to the formation of a large number of spores.

Aspergillus fumigatus invariably effect the lungs, sunuses, inner ear, and rarely the eyes. * A mould that may convert to a yeast form.

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(b) Candiosis. i.e., mostly occur in immunocompromised patients. viz., those undergoing the chemotherapeutic treatment.

Candida albicans mostly proliferates and disseminates throughout the entire body. (c) Cryptococcosis. i.e., largly a systemic infection caused by the yeast Cryptococcus

neoformans. The most common manifestation caused by this organism is a subacute or chronic type of meningitis emenating from the inhalation of the organism.