As I work in microbiology I am often asked questions about the microbial world, as well as what specific types of things actually are, with that in mind I thought it might be beneficial to talk about what Fungi are specifically as this is something I am very often asked about. This post is a combination of my thoughts, information and some rambling but should be considered as a bit of a primer, as always if you want more information you will no doubt find significant information around the web which goes into a greater level of detail.

When we talk about Fungi, it is useful to understand where these sit in terms of the classification of ‘life'. Within the scientific field, we like to try and name and classify things through a process called taxonomy which is essentially the science of naming and classifying organisms. We can classify things at a very low level to say that something is species X or belongs to Genus Y, but at a higher level, we can also classify things to group them into a kingdom.

When we classify things, we have multiple levels which vary from Life, Kingdom, Phylum, Class, Order, Family, Genus and Species (occasionally subspecies). The first level (Life) is where we classify things as life or minerals (not really that significant in this context) but at the next level (Kingdom) we start to group based on the type of microbes.

Within the level Kingdom (based on the Catalogue of Life), there are multiple kingdoms which include: Animalia, Archaea, Bacteria, Chromista, Fungi, Plantae, and Protozoa. There is a separate group for Viruses as these are neither living nor dead hence they form their own group. In this case, we are specifically looking at Fungi, which have their own kingdom.

What are Fungi

Fungi are a type of eukaryotic (cells have a nucleus within a nuclear envelope) organism that includes yeasts, moulds as well as mushrooms. Estimates suggest that the kingdom fungi may have between 2.2 to 3.8 million species however, only around 148,000 species have been described.

These Fungi are ubiquitous (found in most environments) however they do not produce their own food, requiring energy from other sources which can include dead organic matter, and in some cases, fungi can be parasites and feed on plants and animals or live on or within their hosts for part of their lifecycle.

Fungi can exist in many forms, and often have a microscopic and/or macroscopic structure. Most fungi grow as hyphae which are thread-like structures around 2 – 10um in diameter and several centimetres in length. These hyphae grow at their tips and new hyphae can form through a process called branching which can lead to multiple growing (even parallel) hyphae. Depending on the species these hyphae can often grow inside other materials (i.e., trees or plant structures).

The term mycelium is often used, this refers to a root-like structure of the fungi that has a mass of branching hyphae.

Fungi can also produce macroscopic structures, for example, fungal hyphae growing on damp walls or spoiled food (commonly referred to as moulds) can be visible. Mould can be grown on an appropriate growth agar media, when this occurs the growth is referred to as a colony, these can have different shapes and colors and this information (along with what they look like including microscopically) can be used to aid in their identification.

Beyond the visible mould, fungi can also produce a number of macroscopic structures which are more well known, an example of this is the fruiting body we know as a ‘mushroom' but it should be noted that this is only part of the fungi and there are more structures which are not visible.

How do Fungi Reproduce?

The reproduction of fungi is fairly complex but in general, can be broken down into two main forms. The first form is asexual reproduction whilst the second form is through sexual reproduction.

• In Asexual reproduction, the reproduction can occur through vegetative spores (conidia) are produced which are in essence generated through a cellular process of mitosis (where a cell replicates) or when the fungal mycelium (hyphae) breaks into pieces and each component grows into a separate mycelium.
• In sexual reproduction, there are multiple types of species that reproduce slightly differently. Some species are only able to reproduce with compatible individuals, whilst other species can reproduce with many species including themselves.
  • In general, though, there are three stages to sexual reproduction,
    • The first stage is where two hyphae fuse (called plasmogamy) which results in a dikaryotic stage where there are two haploid (single set of chromosomes) nuclei in the cell.
    • In the second stage (karyogamy) the two haploid (with a single set of chromosomes) nuclei fuse to form a diploid (two complete sets of chromosomes) zygote nucleus.
    • In the third stage cellular division (meiosis) takes place where spores of different mating types are generated and disseminated into the environment.

In both cases, fungi produce spores that disperse from the parent organism by floating or getting a ride on an animal (they are smaller and lighter than plant seeds) and can then germinate at a different location when conditions allow (assuming viability).

Do Fungi play an important part in nature?

Fungi are given many different names, but one of the most common is nature's recyclers. In reality, fungi are essential for life to flourish. As fungi are not able to produce their own food they source this from other materials, excluding the symbionts and parasites many fungi break down dead organic matter into simpler blocks that can be used by other organisms. An example is the wood rot fungi which are responsible for breaking down the cellulose and lignin in trees, these polymers are very difficult to be broken down by other species so when the wood rot fungi break them down it leads to the decay of that material producing a range of metabolites and nutrients which can be recycled back into the ecosystem.

The decay of trees is not the only area where fungi recycle though, in many cases fungi form symbiotic relationships (beneficial relationships) with other organisms such as trees, the fungi help provide resources to the tree such as nitrogen and phosphorus whilst the tree in return provides carbohydrates allowing the fungi to grow, this relationship is not to the detriment of either party and in many cases results in increased growth of the tree.

Do Fungi play an important part in industrial processes?

There are many types of fungi and some are very commonly used in industrial processes, an example is the production of beer which requires a type of fungi known as yeast. In beer production, typically the strain Saccharomyces is used which (simplified) breaks down the sugar into alcohol and carbon dioxide gas (carbonation).

Fungi are responsible for more than just making beer, they are also essential for making bread, the production of some types of cheeses, the creation of many antibiotics and vitamins, and more. Many of the first antibiotics we used (i.e., Penicillium) were discovered from fungi, and continue to be created from them.

Do Fungi have an impact on human health?

As there are thousands of types of fungi a blanket statement can't be made to say that all fungi have an impact on human health however, many species are known to cause issues in humans and other animals.

When we look at Fungi, there are primarily three ways that they can impact the health of an individual, these can vary based on the species and the level of risk but they can include:

• Causing allergies as the body reacts to the fungal structures (spores, hyphal fragments et al), this can be an issue even if the fungi itself is actually dead.
• Producing toxins known as mycotoxins, which are usually naturally occurring metabolites (chemicals) that are produced by fungi and can cause death and disease in humans and other animals. Different species of fungi can produce different toxins, or multiple species can produce the same toxin. An example of a mycotoxin is Aflatoxin B1 which is a potent carcinogen linked to diseases such as liver cancer in some animal species.
• Causing infection itself, fungi can be either a pathogen or what is known as an opportunistic pathogen which means in healthy people they don't typically cause infection but in people who are immune suppressed (i.e., those undergoing cancer treatment) they can cause infection.