Definition, Characteristics, Types and Morphology
What is Aspergillus?
Aspergillus (Plural Aspergilli) is a genus of
fungi that consists of about 300 identified species of mold (mould).
Aspergillus can be found in a variety of environments throughout the world
given that their growth is largely determined by the availability of water.
The rate at which they grow is largely determined by the temperature
range in the environment they grow in. Regardless, studies have shown
Aspergillus to be able to tolerate extreme conditions only if all of the
other conditions are ideal.
Given that a majority reproduce asexually, they are often described as conidial fungi. However, studies are yet
to determine how a good number of others reproduce while some have
been shown to reproduce sexually.
A majority have been shown to be terricolous, which means that they are largely found in soil/land. Since their discovery in the 1720s, they have become increasingly important in human health, agriculture as well as in biological sciences among others.
* conidial fungi are fungi that reproduce asexually through the production of spores from the fungi hyphae. The spore produced is referred to as conidium.
General Characteristics of Aspergillus
Aspergilli can be found throughout nature with
their spores being abundant in air. In addition to largely being saprophytes
that obtain their nutrition from dead and decaying matter, they can also be
pathogenic to human beings and animals with some also affecting and damaging
Because they lack chlorophyll, and are
therefore unable to produce their own food, they are dependent on other
material in their surroundings for nutrition. Here, it is worth noting that
Aspergilli are incapable of absorbing organic matter in their surroundings.
For this reason, they release different types of enzymes such as
amylase capable of breaking down these materials into
simpler compounds that can be absorbed through the vegetative hyphae.
release of high amounts of these enzymes results in enhanced decay of all
organic matter in their immediate environment and consequently the availability
of more food source necessary for reproduction and growth.
For most part, they reproduce asexually
through the production of spores known as conidium (fungi spore). Once the
spore lands on a favorable environment (with moisture, warmth and nutrients)
they start germinating where they create numerous hyphae that form the mycelium.
The hyphae allows them to grow, spread and continue reproducing
across the surface of the substrate.
The life cycle of
Aspergillus may therefore be summarized as follows:
* Aspergilli's hyphae develops into vegetative
and reproductive hyphae. The vegetative hyphae serves to absorb nutrients while
the reproductive hyphae develop further to produce spores.
Types of Aspergillus
There are several types of species that fall
under the genus Aspergillus. The following are some of the most common fungi
that belong to this genus:
Aspergillus niger largely exist as saprophytes,
which means that they obtain their nutrition from a variety of dead and
decaying material such as leaves, fruits and other vegetation. As such, they
also contribute to the delay of various food products. given that their source
of vegetation are readily available virtually everywhere.
A. niger are
widely distributed and common in many geographical areas. However, they are known to be particularly prevalent in areas with higher temperatures.
* Research has show that the black spores of
A. niger help protect them from the sun's radiation, which in turn
allows this species to thrive in warm areas.
Morphology of Aspergillus Niger
Like others, Aspergillus niger
are filamentous fungi, which means that they tend to form filaments (hyphae)
and thus resemble the structure of a plant. When viewed under the microscope,
A. niger consists of a smooth and colorless conidiophores and spores.
A closer look will reveal the conidial heads of the organism to be globose and
dark brown in color that have been shown to divide into a number of columns as
the A. niger continues to age.
Compared to the other types, A. niger produces dark or dark brown spores from their
conidial heads (biserite). This is a characteristic that has only been seen on
A. niger and none of the others.
* In microscopy, the carbon black/dark brown
color of the spores (as well as the conidia) is used to distinguish A. niger from other species in the same genus.
Aspergillus niger is an ascomycetous fungus.
This means that it is classified under the phylum Ascomycota, also commonly referred
to as Sac Fungi. As such, spores are produced within the sacs (Asci)
Like Aspergillus nidulans, colonies of A. niger that have
been exposed to air (in suitable conditions for growth) tend to form vegetative
and reproductive hyphae. Whereas the vegetative hyphae absorb nutrients from
the dead and decaying matter, the spores (conidium) are produced from the tip
of the reproductive hyphae (that looks like a vesicle).
Like A. niger, Aspergillus flavus are
saprophytes that can be found in soil samples where they obtain their nutrition
from dead and decaying matter. In particular, A. flavus are a nuisance
to farmers given that they tend to infect and contaminate crops (seeds).
A. flavus are divided into two major
groups based on their morphology. These include:
S Strain - The S strain of Aspergillus flavus,
also referred to as Group I strain has a sclerotia (hardened mass of mycelium)
that is less than 400 mm in size, which is its defining characteristic. This
strain has been shown to produce a consistently high content of aflatoxin that
affects crops and animals alike.
L Strain - Compared to the S strains, the L
strain of Aspergillus flavus has a sclerotia that is larger than 400 mm in
diameter. While this strain also produces aflatoxins, the amount produced vary
from high to low and thus not consistent as is the case with the S strain.
* In human beings, aflatoxins produced by
Aspergillus tend to cause serious intestinal problems as well as liver cancers.
* A. flavus is the second leading cause
of Aspergillosis among patients with weak immunity. This is an infection of the
lungs by the fungi (fungi balls)
Like other types, the
A. flavus also produces hyphae, this includes the vegetative and
reproductive hyphae. In the vegetative hyphae (mycelium) A. flavus produces
enzymes that are responsible for the breakdown of food material into simpler
forms that can be absorbed.
While the organism has been shown to produce spores (conidiospores) asexually, studies have shown the genetic diversity of
this species to be as a result of sexual reproduction where ascospores are
produced within the sclerotia of A. flavus.
* ascospore - spores that are produced in the
Morphology of Aspergillus Flavus
Some of the main parts of the A. flavus
- The stipe
When viewed under the microscope, A. flavus
appear to have radiating conidial heads while the conidiophores will appear
Aspergillus fumigatus is one of the most prevalent
Aspergillus found in most environments. One of the most unique characteristics
that separates A. fumigatus from the rest of the Aspergillus is that it can
survive very high temperatures (it is thermotolerant) which is one of the
reasons as to why it is more prevalent.
A. fumigatus also exists as a
saprophyte that plays an important role in the cycle of carbon and nitrogen in
nature. Because it is very prevalent, the spores of A. fumigatus are
also in high concentration in air, which presents a serious health issues for
those with poor immunity. Compared to other species in this genus, A. fumigatus has been shown to be the leading infectious pathogen among those with
* The fungi can survive at 37 degree Celsius while
the conidia can survive temperatures of up to 70 degree Celsius
Aspergillus fumigatus is a filamentous fungi
that uses both sexual and asexual means of reproduction. However, the sexual
means of reproduction are yet to be understood.
In asexual reproduction,
conidia are produced through mitotic division. Here, mitotic division take
place within the conidophore vesicle and are released by any disturbance of the
immediate environment. This causes the spores to be in high concentration in
air where they can be easily inhaled (causing health problems for those with
compromised immunity) - asexual reproduction in Aspergillus fumigatus takes
place when the mycelia does not interact with other haploid mycelia.
When the mycelia comes in contact with the
hyploid mycelia, then sexual reproduction occurs. According to studies, this
means of reproduction in A. fumigatus often occurs when there is a low supply
of nutrients particularly when the growing season comes to the end. This is
therefore a beneficial means of production in that it allows the A. fumigatus
to continue reproducing in unfavorable conditions.
Here, the haploid hyphae
come in contact allowing their nuclei to fuse. This is then followed by meiotic
division of the diploid ascus to form ascospores (four of them) that are then
released in air to continue the cycle.
Morphology of Aspergillus Fumigatus
The following are some of the morphological characteristics of Aspergillus Fumigatus:
- They produce spores of between 200 and 400 mm
- The color of the stipes is gray around the apex
- They have a smooth surface
- They have a small, columnous globuse
- The surface of the conidia is either smooth or spinose
Aspergillus nidulans (Emericella nidulans) is
also one of the most studied Aspergillus. One of the main reasons as to why it
has become one of the most studied fungi is because it has been found to be
very closely related to a majority of other species of Aspergillus in addition
to a well organized genetic system. As such, it has proved to be particularly
important in medical and industrial research.
While it is the subject of many
research studies, the means of reproduction of A. nidulans is yet to
be well understood. However, a number of studies have shown the organism to use
both the sexual and asexual means.
Unlike the others that cause food
spoilage by enhancing decay, Emericella nidulans does not necessary cause such spoilage.
It can grow at relatively high temperatures and has been shown to produce a
toxin (mycotoxin) that can result in health complications among those with
Importance of Aspergillus
- Carbon and nitrogen cycle - In response to its need for nutrition, studies have shown A. fumigatus to be
able to sense the presence of nitrogen and carbon material resulting in the
Aspergillus breaking down this material (particularly non-woody plants) to
obtain nutrients and for absorption. This has been shown to be an important process in the cycle of
carbon and nitrogen in nature.
- Enzymes - Aspergillus
obtain their nutrients by releasing enzymes that break down food material into
smaller constituents that can be easily absorbed. This mechanism has proved
particularly beneficial in various industries where these organisms are being used
for their enzymes to break down various proteins and other compounds. One of
the most commonly used organism is A. oryzae, which is largely used for
fermentation of various products.
Some of the other important uses of Aspergillus
- Food preservation - Due to
its capacity to decompose, A. niger is used to preserve various food products
- Citric acid - A. niger is
used in industry to produce citric acid
While Aspergillus have a number of benefits,
they have various disadvantages that include:
Food spoilage - Aflatoxins produced by some spoil various crop seeds. These toxins are dangerous and can cause
serious health issues and even death when consumed. On the other hand, enzymes
produced by Aspergillus enhance the decay of fruits and other foods.
Human and animal infections - Aflatoxins,
Aspergillus and their spores can have negative health consequences when
inhaled. Whereas aflatoxins can affect both healthy and unhealthy animals and
humans, Aspergillus and their spores can have grave negative consequences on
the health of those with poor immunity.
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Wang. Inhibition of secretion by asexual reproduction in Aspergillus niger.
Keller. Aspergillus flavus.
Hohl. Aspergillus fumigatus: Principles of Pathogenesis and Host Defense.
Article in Eukaryotic Cell · December 2007.