Medical mycology deals with the nature and the causes of the diseases occasioned directly (mycoses and allergies) or indirectly (poisoning) by fungi. Mycotoxins are toxic molecules that are present in various moulds. Reference is made to mycetism when a mycotoxin causes poisoning directly, as is the case for instance with Amanita phalloides (Death Cap). Mycotoxicoses are diseases caused by ingestion of foodstuffs in which toxins are released and therefore involves an indirect form of poisoning. The most well known example is aflatoxicosis caused by aflatoxins produced by Aspergillus flavus. Some moulds can cause allergic syndromes such as extrinsic allergic alveolitis (e.g. cheese washer’s lung, maltworker’s lung). All these conditions differ from mycoses, which are a parasitic type of infection. This text deals only with mycoses.

Beware of the term "mycotic" and "mycosis", which can be misleading, as in "mycotic aneurysms" (in general due to bacteria, e.g. in endocarditis), mycosis fungoides (a neoplastic disease), and bothryomycosis (Staphylococcus aureus infection).

The majority of medical important fungi are land organisms in contrast to some more primitive fungi which have more in common with protozoa and exhibit a motile stage.

Pneumocystis carinii, the taxonomy of which was for a long time unclear, has been reclassified among the moulds as a result of molecular biology.

*

Fungi are eukaryotes. They differ fundamentally in terms of cell structure and organisation from bacteria (prokaryotes) and are not susceptible to most antibacterial antibiotics. They are heterotrophic, in other words they have to obtain their energy from already existing organic molecules. Consequently, many fungi live in association with living plants (often as harmful parasites, but also as beneficial symbionts, cfr mycorhiza) or as free saprophytes on dead organic substances. Like bacteria, they feed by absorption.

*

Moulds do not form true tissues but branching filaments - hyphae or fungal filaments - which grow apically. The cross-walls or septa in these hyphae have a central opening through which cytoplasm and nuclei can pass. The combined hyphae – the mycelium – may be imagined as a sort of network of pipelines.

Ergosterol (and not cholesterol as in animals) is the most important sterol of the cell membrane. It is on this ergosterol that most antifungals act, the polyenes by binding to it, the azoles and allylamines by interfering in its biosynthesis.

*

Two groups are distinguished among the microscopic fungi:

• the yeasts, unicellular organisms that proliferate by budding and identification of which is based predominantly on morphological, as well as biochemical, properties such as oxidative assimilation or fermentation of various sugars.
• the filamentous moulds, which are identified on the basis of morphological characteristics.

*

Reproduction, which is usually associated with considerable proliferation and dissemination, occurs in fungi through spores. The spores are formed either through normal mitosis (asexual spores) or by nuclear fusion followed by meiosis (sexual spores).

The division of fungi into classes, orders, etc., is based on the characteristics of sexual reproduction. However, it is mostly on the basis of the characteristics of asexual sporulation that moulds are identified in the laboratory.

In addition to their potential pathogenicity, fungi have one fundamental characteristic in common: they are first and foremost saprophytes. This means that their existence as a parasite in humans or animals is entirely unnecessary for the completion of their life cycle. They are at most facultative pathogens, which only parasitise if they encounter promoting factors, whether systemic or local. Examples of the former are deep candidiasis and aspergillosis in patients with neutropenia and superficial candidiasis and deep cryptococcosis in AIDS patients. Examples of local promoting factors are skin irritations, which predipose to cutaneous and subcutaneous mycoses.

The majority of the causative agents of mycoses are exosaprophytes. A patient only develops symptoms following exposure to the natural biotopes or ecological niches of the fungi. Knowledge of these possible sources of infection is therefore important.

Some moulds have a limited geographical distribution. An AIDS patient can catch an infection with the cosmopolitan Cryptococcus anywhere in the world, but can only acquire an infection with Penicillium marneffei in Southeast Asia.

Other moulds live in or on humans.

Candida albicans, the main causative agent of candidiasis, is an obligate endosaprophyte. The normal biotope of this yeast is the gastro-intestinal tract and the oral cavity in particular. Local or systemic promoting factors are responsible for the transition from the saprophytic to the parasitic phase.

Malassezia furfur (Pityrosporum ovale) is a lipophilic yeast present in everyone as an episaprophyte on the skin, which in certain circumstances can become pathogenic.

This may explain why, with a few exceptions such as certain dermatophytoses and sporotrichosis, mycoses should not be considered infectious.

Skin scrapings obtained with a scalpel or tweezers, nail fragments and hair are sent to the laboratory in sterile bags (postage bags). Preservatives should not be added to any swabbed material.

Biopsies are best divided into two fragments. One piece is fixed for histological examination and the other is kept in a sterile glycerine solution for culture (1 volume glycerine/4 volumes water).

TAKE CARE with specimens from normally non-sterile sites such as sputum, the results of which must always be interpreted according to the clinical context.

The laboratory diagnosis of a mycosis is based in the first place on direct microscopic examination of the clinical specimen and sometimes on histological examination with the use of specific stains such as silver impregnation of the wall (Grocott stain). Some dimorphic moulds such as Histoplasma have a specific morphology in the parasitic phase.

Very simple nutrient media are usually sufficient for culture, such as Sabouraud agar to which antibiotics such as chloramphenicol are added. Cycloheximide (actidione) can be added to suppress the growth of common moulds. Despite the fact that pathogenic fungi can – by definition – grow at 37°C, the optimal incubation temperature for most moulds is 25-30°C. The saprophytic phase of the mould develops on simple nutrient media incubated at 25°C. The parasitic phase of dimorphic fungi can be observed by incubating them at 37°C in a rich medium such as BHI (Brain Heart Infusion) agar.

Everyone comes into contact with moulds from the environment and can thus form antibodies spontaneously. This makes the interpretation of serological results difficult. The detection of circulating antigens is more promising.