Fungal Cell Structure
Fungal cells are eukaryotic: a nucleus bound by a nuclear envelope, mitochondria, endoplasmic reticulum, Golgi apparatus, vacuoles and 80S ribosomes. External to the plasma membrane lies a rigid cell wall that dictates shape, protects against osmotic and immune attack, and carries most of the antigens recognised clinically.
Two features are clinically decisive: the cell wall, which contains polymers absent from human cells, and the cell membrane, in which ergosterol replaces cholesterol as the principal sterol.
The wall is layered. The inner layer is a skeleton of chitin covalently linked to beta 1,3 glucan, with beta 1,6 glucan as cross linker. The outer layer is rich in glycosylated mannoproteins, many anchored through glycosylphosphatidylinositol. The wall is dynamic. It is remodelled during growth, morphological switching and drug exposure, and chitin content rises after echinocandin exposure as a compensatory response. Hyphal walls contain more chitin than yeast cell walls.
| Component | Location | Principal function | Clinical relevance |
|---|---|---|---|
| Chitin | Inner wall; concentrated at septa, bud scars and hyphal tips | Rigidity | Binds calcofluor white, the basis of fluorescence microscopy |
| Beta 1,3 glucan | Inner wall, linked to chitin | Main structural scaffold | Target of echinocandins; released into blood, detected by the beta D glucan assay; ligand for Dectin 1 |
| Beta 1,6 glucan | Cross links between components | Anchors mannoproteins | Structural |
| Alpha 1,3 glucan | Outer layer of Aspergillus, Cryptococcus, and the yeast phase of Histoplasma and Blastomyces | Masks beta glucan from immune recognition | Immune evasion |
| Mannoproteins | Outer layer | Adhesion, antigenicity | Adhesins such as the Als family in Candida species; mannan antigen and antimannan antibody tests |
| Galactomannan | Aspergillus wall | Structural, antigenic | Basis of the galactomannan assay; cross reacts with Histoplasma, Blastomyces, Talaromyces, Penicillium and some Fusarium |
| Galactosaminogalactan | Aspergillus | Adhesion, immune modulation | Virulence factor |
| Melanin | Dematiaceous fungi, Aspergillus conidia, Cryptococcus | Protection from oxidative and phagocytic damage | Pigmentation; Fontana Masson positivity |
Exceptions with direct therapeutic and diagnostic consequences:
- Mucorales walls are rich in chitin and chitosan with little beta glucan. Echinocandins are inactive, and the beta D glucan assay is usually negative.
- Cryptococcus is intrinsically resistant to echinocandins despite possessing the target enzyme, and its beta D glucan level is low.
- Pneumocystis cysts have a beta glucan rich wall. Echinocandins act on the cyst form and are not used as treatment because of poor efficacy against the trophic form, while the beta D glucan assay is strongly positive.
Ergosterol maintains membrane fluidity, permeability and the function of membrane bound enzymes and transporters. It differs from cholesterol by an extra double bond and a methyl group in the side chain, enough to allow limited selectivity.
| Target | Pathway | Drug class | Consequence |
|---|---|---|---|
| Ergosterol in the membrane | Membrane integrity | Polyenes (amphotericin B, nystatin) | Pore formation and sterol sequestration; leakage of ions and cell death |
| Lanosterol 14 alpha demethylase (Erg11, Cyp51) | Ergosterol synthesis | Azoles | Ergosterol depletion and accumulation of toxic methylated sterols; fungistatic against yeasts |
| Squalene epoxidase (Erg1) | Early ergosterol synthesis | Allylamines (terbinafine) | Squalene accumulation, fungicidal for dermatophytes |
| Delta 14 reductase and delta 7 to 8 isomerase | Late ergosterol synthesis | Morpholines (amorolfine) | Topical nail and skin use |
| Beta 1,3 glucan synthase (Fks1, Fks2) | Wall synthesis | Echinocandins | Fungicidal against Candida species, fungistatic against Aspergillus hyphal tips |
| Cytosine permease and cytosine deaminase | Pyrimidine metabolism | Flucytosine | Conversion to 5 fluorouracil inside the fungal cell; human cells lack cytosine deaminase |
| Microtubules | Mitosis | Griseofulvin | Disrupts the mitotic spindle in dermatophytes |
Newer agents act on other targets: ibrexafungerp on glucan synthase at a site distinct from the echinocandins, olorofim on dihydroorotate dehydrogenase (pyrimidine synthesis) and fosmanogepix on Gwt1 (cell wall anchor protein maturation).
Selectivity is incomplete. Amphotericin B binds ergosterol with greater affinity than cholesterol, but cholesterol binding in renal tubular cells and endothelium accounts for nephrotoxicity and infusion reactions. Azoles inhibit human cytochrome P450 enzymes, which accounts for their drug interactions and hepatotoxicity. Echinocandins target an enzyme with no mammalian counterpart and are consequently the least toxic class.
Pneumocystis jirovecii contains cholesterol and lacks ergosterol. Polyenes and azoles are therefore ineffective.
Ploidy shapes how resistance evolves:
- Candida albicans is diploid and has no complete meiosis. Resistance arises by gain of function mutations, loss of heterozygosity and aneuploidy, including duplication of the left arm of chromosome 5, which amplifies ERG11 and the efflux regulator TAC1.
- Candida glabrata, Aspergillus species and Cryptococcus are haploid. A single resistance mutation is expressed immediately.
- Mitochondrial dysfunction in Candida glabrata (petite mutants) can up regulate efflux pumps through PDR1 and confer azole resistance.
Two efflux pump families carry azoles out of the cell: ATP binding cassette transporters (Cdr1, Cdr2) and major facilitator superfamily transporters (Mdr1). Overexpression is a principal route to azole resistance in Candida species.
- Capsule: Cryptococcus is surrounded by a polysaccharide capsule, mainly glucuronoxylomannan with glucuronoxylomannogalactan and mannoproteins, anchored to the wall through alpha 1,3 glucan. Capsule enlarges in response to carbon dioxide, low iron and mammalian serum. It is anti phagocytic, is the antigen detected in cryptococcal antigen testing, excludes India ink, and stains with mucicarmine.
- Biofilm matrix: Adherent Candida communities on catheters and devices embed in an extracellular matrix of polysaccharides, proteins and extracellular DNA. Matrix beta 1,3 glucan binds azoles and reduces their activity, which contributes to the antifungal tolerance of device associated infection.
- Adhesins: the Als family and Hwp1 on the Candida surface mediate attachment to epithelium, endothelium and plastic.
