47 Introduction to fungi

Fungi constitute a distinct kingdom of life that is evolutionarily separate from plants, animals, and bacteria. All fungi are eukaryotic organisms, meaning their cells possess a true nucleus and membrane‑bound organelles. This fundamental cellular organisation underpins their complex metabolic processes, reproductive strategies, and ecological roles. Unlike prokaryotic microorganisms such as bacteria, fungi exhibit a higher level of cellular organisation that contributes to their adaptability across diverse environments.

The Structure of Fungi

A defining structural feature of fungi is the presence of a rigid cell wall composed primarily of chitin, a polysaccharide also found in the exoskeletons of arthropods. This rigid cell wall renders fungi non‑motile, as it prevents the formation of locomotory structures such as flagella or cilia (with rare exceptions only in primitive fungal lineages).  Fungi exist in both unicellular and multicellular forms: unicellular fungi are commonly referred to as yeasts, while multicellular fungi form filamentous networks known as moulds, composed of thread‑like hyphae that collectively form a mycelium.

As an example, an Aspergillus‑like mould is shown below, highlighting structures including the nutrient‑absorbing vegetative structures and aerial reproductive structures (Figure 9.35).

Hyphae – Individual thread‑like filaments that grow through the substrate and absorb nutrients.

Mycelium (branched hyphae) – A network of interconnected hyphae forming the main vegetative body of the fungus.

Conidiophore – An upright, specialised hyphal stalk that supports the asexual reproductive structures above the substrate.

Conidiophore head (vesicle) – The swollen tip of the conidiophore from which spore‑producing cells arise.

Phialides / conidiogenous cells – Flask‑shaped cells radiating from the vesicle that produce asexual spores.

Conidia – Asexual spores formed externally in chains at the tips of phialides, responsible for dispersal.

Sclerotia – Dense, compact masses of hyphae that function as resistant survival structures in unfavourable conditions.

Soil (substrate) – The growth medium providing physical support and nutrients for the fungal mycelium.

Reproduction of Fungi

Fungi exhibit remarkably diverse and complex reproductive strategies. They can reproduce asexually, sexually, or through combinations of both processes. Asexual reproduction often involves the production of spores by mitosis, allowing rapid population expansion and efficient dispersal. Sexual reproduction, by contrast, involves the fusion of compatible mating types followed by meiosis, generating genetically diversity. Importantly, fungal classification traditionally relies on the type of sexual spores produced.

The Structure and Replication of Yeast Cells

The components of a fungal yeast cells (Figure 9.36) are:

Cell wall – The fungal cell wall is a rigid outer layer composed mainly of chitin and glucans that provides structural support, protection, and resistance to osmotic stress.

Cell membrane – The cell membrane lies beneath the cell wall and regulates the movement of substances into and out of the fungal cell, with ergosterol as a key sterol component.

Cytoplasm – The cytoplasm is the aqueous intracellular matrix that suspends organelles and serves as the site of many metabolic reactions.

Nucleus – The nucleus contains the fungal genome and controls gene expression, cell growth, and division.

Nucleolus – The nucleolus is a dense region within the nucleus responsible for ribosomal RNA synthesis and ribosome subunit assembly.

Ribosomes – Ribosomes are responsible for protein synthesis, translating messenger RNA into functional polypeptides.

Endoplasmic reticulum (ER) – The rough endoplasmic reticulum, studded with ribosomes, is involved in the synthesis and initial folding of proteins destined for membranes or secretion.

Smooth ER – The smooth endoplasmic reticulum is involved in lipid synthesis, membrane production, and cellular detoxification processes.

Golgi apparatus – The Golgi apparatus modifies, sorts, and packages proteins and lipids for transport to their final cellular destinations.

Vesicle – Vesicles are small membrane‑bound sacs that transport proteins, enzymes, and other molecules within the cell or to the cell surface.

Vacuole – The vacuole is a large storage and recycling compartment involved in maintaining ion balance, degrading macromolecules, and regulating cell volume.

Lysosome – Lysosome‑like compartments contain hydrolytic enzymes that break down macromolecules and recycle cellular components.

Peroxisome – Peroxisomes carry out oxidative reactions, including fatty acid metabolism and detoxification of reactive oxygen species.

Mitochondrion – The mitochondrion is the site of aerobic respiration and ATP production, supplying energy for fungal growth and metabolism.

Septum – The septum is the dividing structure between the parent cell and bud that regulates cytoplasmic continuity during budding.

Bud – The bud is a daughter cell that forms during asexual reproduction in yeasts through an asymmetric budding process.

Bud scar – The bud scar is a chitin‑rich remnant on the parent cell wall that marks previous sites of budding and indicates replicative age.

Note, some pathogenic yeasts possess an additional polysaccharide capsule, which can enhance survival in host tissues by reducing immune recognition.

Budding is a form of asexual reproduction characteristic of many yeasts (e.g. Saccharomyces species), in which a new individual arises as an outgrowth of the parent cell rather than by equal division (Figure 9.37). The process begins with a parent yeast cell that is metabolically active and sufficiently resourced, as indicated in the image by the presence of storage granules and a prominent vacuole, which support growth and division. A small protrusion called a bud forms at a specific site on the parent cell surface, where localised weakening of the cell wall allows outward growth driven by cytoplasmic expansion and membrane synthesis. As the bud enlarges, the nucleus of the parent cell undergoes mitosis, ensuring that the genetic material is duplicated before separation. One of the resulting daughter nuclei migrates into the developing bud, while the other remains in the parent cell, establishing genetic independence between the two compartments. A septum forms at the narrow neck between the parent cell and the bud, partitioning the cytoplasm and regulating the final stages of cell separation. The new yeast cell continues to grow, synthesising its own organelles and cell wall components while still attached to the parent cell. Upon complete separation, the daughter cell detaches, leaving behind a bud scar on the parent cell wall, which marks a previous budding site and reflects the replicative age of the parent cell.

Fungal Nutrition and Metabolism

Fungi are heterotrophic absorptive organisms, meaning they cannot synthesise their own food through photosynthesis (but neither can we!). Instead, fungi obtain nutrients via a distinctive strategy often summarised as “digest first, then ingest.” They secrete extracellular (exo-) enzymes into their surroundings, which break down complex organic macromolecules, such as proteins, polysaccharides, and lipids, into smaller, soluble units that can then be absorbed across the cell wall and plasma membrane. This mode of nutrition allows fungi to exploit a wide range of organic substrates, including materials resistant to degradation by other organisms.

Most fungi are strict aerobes, relying on oxygen for efficient energy production via oxidative metabolism. However, some species, particularly yeasts, are facultative anaerobes, capable of surviving in low‑oxygen environments by switching to fermentative metabolic pathways (hence their ability to produce things like beer via fermentation!). This metabolic flexibility is especially important in niches such as deep tissues, fermenting substrates, or oxygen‑poor environments.

Some fungi produce biologically active secondary metabolites known as mycotoxins. These compounds are not required for basic growth or reproduction but can have profound effects on other organisms, including humans and livestock. Mycotoxins may contaminate food and feed, leading to acute or chronic toxicity. While harmful in some contexts, fungal secondary metabolites have also been harnessed for beneficial purposes, including pharmaceutical applications.

Case study

A 3‑year‑old domestic short‑haired cat, Nala, presents with focal alopecia (hair loss), erythema, and mild scaling on the forelimb and face. The lesion has gradually enlarged over three weeks. The referring veterinarian has already prescribed a broad‑spectrum antibacterial antibiotic, but no clinical improvement is observed. Physical examination and history raise suspicion of a fungal infection, most likely dermatophytosis (ringworm). Fungal culture and microscopic examination confirm the presence of Microsporum species. Topical antifungal therapy, combined with environmental decontamination, leads to gradual clinical resolution.

 

A critical practical distinction between fungi and bacteria is that fungi are insensitive to antibacterial antibiotics. Most antibacterial drugs target bacterial‑specific structures or pathways, such as peptidoglycan cell wall synthesis or prokaryotic ribosomes, which are absent in fungi. As a result, fungal infections require antifungal agents that selectively target fungal cellular components, such as ergosterol in fungal membranes.