A fungus is far more like you than a bacterium is. That is why there are almost no antifungal drugs

Antibiotic resistance is a phrase almost everyone has heard. Antifungal resistance is not — and yet the underlying problem is considerably worse, because medicine never had many antifungal drugs to lose. Against a group of diseases that recent estimates hold directly responsible for around 2.5 million deaths a year, clinicians work with essentially four classes of drug. The reason is not neglect by chemists. It is a fact of cell biology that makes the whole field hard.

The problem is that fungi are our relatives

Bacteria are prokaryotes. Their cells have no nucleus, they build proteins on distinctly different ribosomes, and they wrap themselves in peptidoglycan, a material human cells do not make at all. Those differences are gifts to a drug designer: penicillin attacks a structure we simply do not have, which is why it can be given in large doses with little effect on the patient.

Fungi are eukaryotes, like us. They have nuclei, mitochondria, familiar ribosomes, and metabolic pathways that look a great deal like ours — closer, in evolutionary terms, to animals than to plants. Almost every process worth attacking in a fungal cell is also running in a human one. A compound that kills the fungus efficiently will very often damage the patient, and the search for selective toxicity — the narrow gap between the two — is what limits the field.

The four things we found to attack

The drugs we do have cluster around the few genuine differences.

• Azoles block the synthesis of ergosterol, the sterol that stiffens a fungal membrane. Human membranes use cholesterol instead, and the enzyme fungi use to make ergosterol is different enough to attack. This is the largest class and the workhorse — fluconazole, itraconazole, voriconazole, posaconazole and their relatives.
• Polyenes, chiefly amphotericin B, bind ergosterol directly, wrecking the membrane. Ferociously effective and famously toxic, especially to the kidneys; liposomal formulations reduce that and cost far more, which is not a trivial detail in Indian practice.
• Echinocandins inhibit the synthesis of β-1,3-glucan, a component of the fungal cell wall. Human cells have no cell wall at all, which makes this the cleanest target in the whole field — and the class is only about twenty years old in clinical use.
• Flucytosine, a pyrimidine analogue that disrupts nucleic acid synthesis, is used almost entirely in combination because resistance emerges quickly when it is used alone.

Four classes. Antibacterial medicine has dozens. And where a bacterial infection is usually confirmed within a day or two, fungal diagnosis is slow: cultures can take days to weeks, blood cultures miss a large fraction of invasive Candida infections, and the antigen and PCR tests that speed things up are not available everywhere. Treatment therefore begins empirically far more often than anyone would like, which is precisely the condition under which resistance is bred.

The resistance that arrives before the first dose

Here is the part of the story that deserves far more attention than it gets.

Azoles are not only medicines. Chemically similar azole fungicides are sprayed in enormous quantities on crops, timber and flower bulbs, because the fungi that rot wheat and grapes are vulnerable to the same enzyme as the fungi that infect people. Aspergillus fumigatus — a mould that lives harmlessly in soil and compost almost everywhere, and whose spores every person inhales daily — is exposed to those sprays in the environment.

The consequence is that azole-resistant A. fumigatus has been recovered from patients who have never taken an azole in their lives, carrying specific resistance signatures first identified in environmental isolates. The selection happened in a field, and the patient inhaled the result. India, with heavy agricultural fungicide use and a large population of susceptible patients, has been an important part of that evidence base.

This is a One Health problem in the exact sense of the phrase: the agricultural and clinical uses cannot be regulated separately, because they are chemically the same intervention on the same organism. It is also why "use antifungals prudently in hospitals" is only half a policy.

Antibiotic stewardship asks doctors to prescribe carefully. Antifungal stewardship has to ask that of a farmer too — the resistant mould reaching a hospital ward may never have met a doctor.

Two things India learned the hard way

Mucormycosis. During the second COVID-19 wave in 2021, India reported tens of thousands of cases of an infection most clinicians had previously seen only rarely. The Mucorales moulds behind it are common in the environment and ordinarily harmless; what changed was the number of people with the right vulnerabilities at once — uncontrolled diabetes, high-dose corticosteroids, and the immune disruption of severe COVID. Treatment meant amphotericin B and, frequently, disfiguring surgery, because the mould invades blood vessels and kills the tissue it grows through, putting it beyond the reach of a drug arriving through the bloodstream. The episode was a demonstration that fungal disease is largely a disease of opportunity: the organism is already present, and it is the host's defences that decide.

Candida auris. First described in 2009, this yeast is unusual in that it spreads between patients in hospitals, survives for weeks on surfaces and equipment, resists routine disinfection, is frequently multidrug-resistant, and was for years misidentified by standard laboratory methods as a harmless relative. It sits in the critical group of the World Health Organization's first fungal priority pathogens list, published in 2022, alongside Aspergillus fumigatus, Cryptococcus neoformans and Candida albicans — a list whose main purpose was to point out how thin the research and diagnostic base for all of them is.

There is a plausible and much-discussed hypothesis about why a heat-tolerant yeast like C. auris should emerge now. Mammalian body temperature has long been an effective barrier against most environmental fungi, which grow best well below 37 °C. If a warming climate selects for fungi that tolerate higher temperatures, that barrier weakens. It remains a hypothesis rather than an established fact, and it is worth stating as such — but it is the kind of question that decides what the next thirty years look like.

What is actually coming

The pipeline is thin but no longer empty. A long-acting echinocandin allowing weekly dosing has reached approval, as has an oral drug that hits the same glucan-synthase target by a different chemical route — useful precisely because it does not share the azoles' resistance mechanisms. Further back sit compounds attacking genuinely new targets: an inhibitor of fungal pyrimidine synthesis with activity against moulds that current drugs handle poorly, and an inhibitor of a protein-anchoring enzyme with an unusually broad spectrum.

Alongside the drugs, the more consequential work may be diagnostic. A therapy that only helps when started early is worth little if confirmation takes ten days, and rapid, affordable identification of both species and resistance would change outcomes more than any single new molecule. There is also, still, no licensed antifungal vaccine.

Why it matters for students and researchers

Mycology is one of the few areas of biology where the basic science is visibly under-built relative to the clinical need — and where a well-run laboratory without extraordinary equipment can contribute. Environmental surveillance for resistant Aspergillus around agricultural land, characterisation of local isolates, antifungal susceptibility work, and studies of the fungal ecology of Indian soils, air and hospital surfaces are all tractable and genuinely under-published from this region, where the exposure is high and the sampling is thin.

The subject reaches well past infection, too. Fungi are the principal recyclers of the terrestrial carbon cycle, the partners in mycorrhizal networks that most plants depend on, the source of a great deal of industrial enzymology and the statins, and one of the more interesting toolkits in modern biotechnology. That whole span — taxonomy, genetics, ecology, physiology, biotechnology and molecular biology of fungi — is the remit of the International Journal of Fungi (ISSN 3049-1509), a peer-reviewed journal launched in 2024. For life sciences, microbiology and agriculture students, it is a good field in which to notice that a hospital problem and a farm problem can turn out to be the same organism.

Frequently asked questions

Why are there so few antifungal drugs?

Because fungi are eukaryotes, like humans, so most of the machinery a drug could attack in a fungal cell also exists in ours. The few drugs we have exploit the small number of real differences, chiefly ergosterol in the fungal membrane and the glucan in the fungal cell wall.

What are the main classes of antifungal drugs?

Azoles, which block ergosterol synthesis; polyenes such as amphotericin B, which bind ergosterol directly; echinocandins, which inhibit cell wall glucan synthesis; and flucytosine, which disrupts nucleic acid synthesis and is used in combination.

How does agricultural fungicide use cause antifungal resistance in patients?

Azole fungicides sprayed on crops are chemically similar to azole medicines and act on the same fungal enzyme. Environmental Aspergillus fumigatus exposed to them can develop resistance, and people who inhale those spores may acquire a resistant infection without ever having taken an antifungal drug.

What is Candida auris?

A yeast first described in 2009 that spreads between hospital patients, persists on surfaces, resists routine disinfection, is often resistant to multiple antifungal classes, and was long misidentified by standard laboratory tests. The WHO places it in the critical group of its fungal priority pathogens list.

Why did mucormycosis surge in India during COVID-19?

The moulds involved are common in the environment and normally harmless. The 2021 surge followed an unusual concentration of vulnerable patients — uncontrolled diabetes, high-dose steroid treatment and severe COVID-19 together — rather than any change in the fungus itself.