Not So Fun-gi
On 17th May 2020 a new species of parasitic fungi was discovered on Twitter! This feat was achieved by Ana Sofia Reboleira from the University of Copenhagen in Denmark, who was casually scrolling through her twitter feed when she came across a picture of an American millipede, which had dots around its head which they usually don’t. This intrigued her a lot, and as she kept digging further into the picture, a new parasitic fungi called Troglomyces twitteri was discovered.
Parasitic fungi are in fact really interesting, they can be found almost anywhere, plants, insects and humans. But of course, they end up doing more harm than good. But for now, we will be focusing on their interaction with insects, humans and other animals. The fungi which make insects their hosts are called Entomopathogens, and over a 1000 species are said to exist in about 90 different genera.
These are dispersed as spores like every other fungi, but these spores need to land on the cuticle of the insect host to stick to it till it germinates. Entomopathogen spores are adhesive, and different fungi have different ways to keep sticking to the body of the host. For example, Entomophthera have mucilagenous coatings, which allows it to stick to the host. For germination, the spores require warm temperatures, humidity, and a chemically favourable environment. There are some spores which can also tolerate the toxins present on the body and as they start growing, the hyphae enters the host. Some invade the host body immediately, whereas some do it gradually and slowly.
As they enter the body of the insect, they’re initially dominant around the region where they had entered from and only colonise the rest of the body during the exploitation phase. Successful entry into the host does not ensure colonisation since insects have immune systems that can respond to invasion by eliminating or confining the pathogen. Exploitation is usually necrotrophic, with host death resulting from fungal toxins (e.g. destruxins), toxic proteases, or chronic disruption of host physiology following extensive mycelial development. Zygomycetes tend to colonise as mycelia whereas ascomycetes form budding, blastospores that colonise the haemocoel (body cavity), and then other tissues by mycelial spread. Some pathogens only colonise certain tissues, for example, Entomophthora erupta is confined to the abdomen of the green apple bug (Lygus communis).
Now, the fungi usually entering human and other animal bodies cause fungal infections which are all classified as mycoses. The fungi end up entering the body through open wounds on the skin. However it is extremely difficult for them to attack on the body of a healthy and immunised person, due to the presence of a sophisticated immune system. Their entrance depends on 4 major factors, resistivity to high temperatures(above 37 Degrees Celsius), them being able to reach the human muscles and tissues, their ability to digest components of the tissues and resistance to the human immune system.
Out of the millions of fungi species, only a few hundred affect humans causing diseases like athlete’s foot, ringworms, candidiasis(yeast infection) etc. If we were to look into a specific kind of fungi, then let’s focus on this one called pathogenic onygenales, which belongs to the phylum ascomycota. These were initially soil-inhabiting members of the ascomycete order Onygenales have evolved to parasitize mammals and cause systemic infection. The disease caused by them usually begins in the lungs and spreads like influenza or pneumonia. They can disseminate to distant sites, can persist and reactivate, with different organ predilections among different genera: Paracoccidioides in oral and respiratory mucous membranes, Blastomyces in bones, joints, and skin, and Histoplasma in multiple organs including the gastrointestinal tract and adrenals, as well as bones and skin.
Coevolution with animal hosts and human beings, this type of fungi has developed mechanisms to evade the host immune system. They reduce their interaction with the host immune cells as much as possible to increase the effectiveness of the infection.
– Shrijeet Kolley