Aspergillus

Aspergillus: introduction

Together with Fusarium sp . and Penicillium sp., the genus Aspergillus is the chapter of toxigenic molds: we are talking about miceti microorganisms able to synthesize a fair amount of toxins, whose toxic power is subordinated to the genotype of the producer strain.

In common parlance, the term Aspergillo refers to the genus of belonging of this ubiquitous mold; the "aspergillosis", on the other hand, identify the infections transmitted by this pathogen.

Aspergillus transmits opportunistic mycoses: similarly to Candida albicans, Aspergillus creates damage in the host (man) exclusively in favorable conditions. In fact, the Aspergillus is one of the commensal microorganisms that, only in certain precise circumstances, turn into opportunistic pathogens. Decreased host immune defenses, asthma and bronchial cysts are important predisposing factors.

Unlike candidiasis, aspergillosis is contracted following inhalation of Aspergillus spores.

Generality

The microorganisms belonging to the genus Aspergillus are cosmopolitan, ubiquitous, found in soil, organic plant materials and animals. Aspergillus commonly grow on substrates rich in polysaccharides (eg amylose) and carbon (eg glucose and other monosaccharides). Starchy foods, such as potatoes and cereals, are the best reservoirs for the growth of many Aspergillus species. However, these molds can grow even in the total absence of essential nutrients: as an example, Aspergillus niger grows freely on damp walls.

Conidia propagation (spores) occurs through the air: it is estimated that man daily inhabits a number of fungal cells equal to 6 x 107, of which 8% is constituted by Aspergillus. In closed environments or in areas where vegetation is particularly generous, it is assumed that the proportion of fungal cells inhaled increases up to 6 x 108 per day!

Considering that such large amounts of Aspergillus are inhaled, it is clear that these fungi cannot be called pathogens in all respects. For this reason, Aspergillus are considered apatogens in the healthy host, components of the normal human commensal flora and common saprophytes in nature.

In sheep and cattle, Aspergillus infections can induce abortion; this microorganism is a bird parasite, where it can cause deadly lung infections.

Curious and particular is the etymological analysis: the name of these fungi derives from the Latin phrase "asperges, aspergillus", which means aspersorio (an instrument used by the priest to sprinkle the people with holy water). In the far 1729 the priest PA Micheli cataloged these microorganisms for the first time: after having carefully observed them under a microscope, he documented the peculiar "aspersorio" structure of Aspergillus, giving the name to a new genus of microorganisms still recognized as such.

Microbiological description

The genus Aspergillus is composed of several hundred different species, found everywhere: it is believed that Aspergillus is the most populated genus of microorganisms in terms of toxogenic species.

Aspergillus excellently tolerate high temperatures, up to 50 ° C. They are defined as non-dimorphic mushrooms, of which only the mycelial form is known.

Aspergillus reproduction follows a typically conidial mode. Although the vast majority of aspergillus do not reproduce sexually, some species have been documented that can form sexed structures called cleist eye chambers.

Under the microscope, the aspergillus has these characteristics:

1. Conidal head, consisting of conidia, fialides and metules
2. The terminating jamb is called conidophore (with conidophoric hyphae), having a swelling called vesicle

Toxins are the virulence products secreted by Aspergillus: among these, aflatoxin confers hepatotoxicity and (it seems) carcinogenicity and teratogenicity. Glycotoxin, on the other hand, is useful for aspergillus to inhibit phagocytosis by macrophages and activation of B lymphocytes.

The most important enzymes are elastase, protease and catalase.

Classification

The various species of Aspergillus are classified according to the morphology and color of the colonies (from the macroscopic point of view), and as a function of the morphology of the components of each single microorganism, therefore of the heads, of the phialides, of the vesicle, etc. (from the microscopic point of view).

The main species of medical interest belonging to the genus Aspergillus include:

Aspergillus flavus : of great pathological interest, this aspergillus produces aflatoxins, toxic toxic mycotoxins for humans and animals. The "preferred" substrate from this species appears to be flour. Forms yellow colonies, from the net margin. It generally presents spherical vesicles, fialides are arranged in double series (only rarely unique) and wrinkled conidiophores.

Aspergillus fumigates : the most common and most widespread species in nature (90% of Aspergillus). Form green colonies bounded by a white outline; the margin of the colonies is clear and have a granular surface. The vesicles of this aspergillus are clavate, the fialides are arranged in a single series and the conidiophores are smooth.

Aspergillus niger : the colony takes on a whitish appearance, with a peculiar black punctuation (which corresponds to the aspergillate head). The margins are irregular, the spherical vesicles, the phialides arranged in single or double series and smooth conidiophores. Aspergillus niger are exploited by man to produce citric acid and some enzymes.

Aspergillus versicolor : the name of the species recalls the particular polychromy of the colonies: green, pink, yellow

Aspergillus clavatus and Aspergillus nidulans are also widespread species of Aspergillus, although less known than A. flavus, A. fumigates and A. niger .

Risk factors

We have analyzed that Aspergillus infections appear only under certain predisposing conditions. Medical statistics show that patients with a compromised immune system are considered more at risk of Aspergillus infections. However, immunosuppressed patients are not the only targets of Aspergillus infections: asthmatics, transplant recipients, leukemics, patients receiving chemotherapy, patients on long-term steroids and cystic fibrosis patients, HIV or COPD are susceptible to infections in general, including those from Aspergillus.

Aspergillus infections

The (relative) rarity of Aspergillus infections is justified by the typically opportunistic nature of aspergillosis. Despite what has been said, it seems that the incidence of invasive forms has dramatically increased over the past twenty years. Aspergillosis can occur in several forms:

1. primary / secondary invasive form
2. NON-invasive primary form
3. dermatosis
4. nose-orbital infections
5. otomycosis and onychomycosis
6. Secondary invasive bronchopulmonary form (or Aspergillus allergic bronchopulmonary infection)

Considering the importance and heterogeneity of the infections transmitted by Aspergillus, the topic will be analyzed in depth in the next article.