Trna Phe Yeast
6.1 Fungi #
6.1.1 Form, wall, and spores #
Fungi are absorptive chemoorganoheterotrophs. The wall is chitin, ฮฒ-1,4-linked N-acetylglucosamine, plus glucans. Echinocandins block glucan synthesis because human cells lack that wall. A yeast is unicellular: Saccharomyces buds, Schizosaccharomyces divides by fission. A mould grows hyphae, tip-extending tubes whose mass is a mycelium. Zygomycete hyphae are coenocytic; ascomycete and basidiomycete hyphae are septate but perforated. A mycelium can feed a growing tip from a distant part of the colony.
Classical thermal dimorphs such as Histoplasma are moulds in the environment and yeasts in tissue. Candida albicans is a commensal yeast that makes hyphae when it invades. Fungal spores are reproductive and numerous. A bacterial endospore is one dormant copy of one cell.
| Group | Sexual spore | Asexual spore | Remember |
|---|---|---|---|
| Chytrids | after a motile stage | flagellated zoospore | the only fungi that swim |
| Zygomycete moulds (a grade, not one clade) | zygospore | sporangiospore | coenocytic; Mucor, Rhizopus |
| Ascomycota | ascospore in an ascus | conidium | yeasts, Aspergillus, Penicillium, Candida |
| Basidiomycota | basidiospore on a basidium | often inconspicuous | mushrooms; Cryptococcus |
| Glomeromycota | none classical | large soil spores | arbuscular mycorrhizae |
“Deuteromycetes” meant “sexual stage not seen”. It is not a clade. Most such fungi are ascomycetes once a sequence is known.
6.1.2 Decomposers, symbionts, and pathogens #
External enzymes let fungi, with bacteria, decompose cellulose and lignin. White-rot fungi attack lignin; brown-rot fungi prefer cellulose. The polymers are plant biology and biochemistry. Without that rot, wood would accumulate. Ectomycorrhizae wrap roots; arbuscular mycorrhizae enter cells and trade minerals, especially phosphorus, for plant carbon. Most land plants use one or the other. The plant side is plant physiology. A lichen is a fungus plus a green alga or a cyanobacterium, not a single species.
Dermatophytes grow on keratin. Candida and capsulated Cryptococcus are opportunists, the second a cause of meningitis when cell-mediated immunity fails. Invasive aspergillosis is a disease of neutropenia, not of every inhaled spore. Aspergillus flavus on grain secretes aflatoxin, so food can poison after the mould is gone. No dose is stated. The category is a preformed metabolite. Ergosterol, where humans have cholesterol, is the other selective antifungal target (Chapter 7).
6.2 Protozoa #
6.2.1 A functional survey #
“Protozoa” is not a clade. The survey is functional. Some lack classical mitochondria: Giardia has mitosomes and Trichomonas hydrogenosomes, both reduced mitochondria, not a primitive absence. Flagellates swim with axonemes, not rotary bacterial flagella. Giardia transmits as a cyst and feeds in the small intestine as a trophozoite. Trypanosoma brucei replaces its variant surface glycoprotein so that yesterday’s antibody misses today’s coat. Trichomonas has no cyst. Amoebae use pseudopodia. Entamoeba histolytica transmits as a cyst and invades as a trophozoite. Free-living Acanthamoeba and Naegleria occasionally infect. For gut protozoa, killing trophozoites in water is not enough if cysts remain.
Apicomplexans invade using an apical complex of rhoptries and micronemes. Toxoplasma completes sex in cats and encysts in many intermediate hosts. Cryptosporidium passes an oocyst that is immediately infectious and resists ordinary chlorination. Ciliates have a polyploid macronucleus for everyday expression and a diploid micronucleus for meiosis and conjugation. That conjugation exchanges nuclei; it is not plasmid transfer. Balantidium is the ciliate tied to human dysentery. Most ciliates graze bacteria, including in wastewater and the rumen.
6.2.2 Plasmodium #
P. falciparum, P. vivax, and P. ovale have an erythrocytic cycle of about 48 hours (tertian: fever counted on day one and day three). P. malariae takes about 72 hours. P. knowlesi takes about 24 hours. Sex occurs in the female Anopheles, which is therefore the definitive host. The human is the intermediate host. Male mosquitoes do not transmit.
Sporozoites from the salivary glands enter hepatocytes. A liver schizont releases merozoites. In P. vivax and P. ovale, some liver parasites become hypnozoites and can relapse months or years later. P. falciparum has none; returning illness is blood-stage recrudescence. An agent active only against blood stages cannot, by this biology, touch a hypnozoite. No drug and no dose are named.
In red cells the cycle is ring, trophozoite, schizont, rupture. P. falciparum reaches high density, and a protein on the infected red cell sticks it to endothelium, out of the spleen and into microvessels. Cerebral malaria is that sequestration in the brain. Some intracellular parasites become gametocytes instead of schizonts. They do not multiply in the human. In the mosquito they become gametes, the zygote becomes a motile ookinete, and an oocyst on the gut wall produces the next sporozoites. The apicoplast, a non-photosynthetic plastid of algal ancestry, is absent from human cells and is therefore a selective target in principle, as peptidoglycan is. Stopping sporozoites, blood schizonts, or gametocytes blocks different parts of the cycle. One measure is not the others.
6.3 Algae and oomycetes #
6.3.1 A microbial alga is not a plant #
Algae, traditionally, are oxygenic photosynthetic eukaryotes other than land plants. Greens are related to plants. Diatoms, brown algae, and dinoflagellates got chloroplasts by secondary endosymbiosis. A land plant has an embryo nourished by the parent, true tissues, and a cuticle. Chlamydomonas โ haploid, two flagella, one chloroplast โ is a microbial alga. Diatoms build a silica frustule of two valves. Some dinoflagellates are the zooxanthellae of corals; bleaching is their loss. Some, and some cyanobacteria, make toxins that shellfish concentrate. Cyanobacteria are bacteria, so a “blue-green algal” bloom is a bacterial bloom. Saxitoxin blocks voltage-gated sodium channels. Kelp is large and is still a brown alga, a stramenopile, not an embryophyte. Size was never the criterion.
6.3.2 Oomycetes are not fungi #
Phytophthora infestans, cause of potato late blight, and Pythium grow hyphae, which is why they were called fungi. Walls are cellulose and glucan, not chitin. Vegetative hyphae are diploid. Zoospores have two unequal flagella, the stramenopile pattern of diatoms and brown algae, and sequences agree. A chitin-directed drug has nothing to bind.
6.4 Key terms, summary, and self-check #
Key terms. Yeast; hypha; chitin; conidium; ascospore; basidiospore; mycorrhiza; lichen; cyst; trophozoite; sporozoite; merozoite; hypnozoite; gametocyte; ookinete; definitive host; apicoplast; oomycete.
Summary. Fungi absorb through a chitin wall, as yeasts or mycelia, and disperse by reproductive spores. They rot plant polymers, form mycorrhizae and lichens, and sometimes infect or leave a toxin in food. Protozoa in this survey are flagellates, amoebae, apicomplexans, and ciliates. Plasmodium has sex in the mosquito; sporozoites infect liver, merozoites cycle in blood, hypnozoites of P. vivax and P. ovale wait, and gametocytes return the parasite for the oocyst. A microbial alga is not an embryophyte. An oomycete is a stramenopile with a cellulose wall.
Self-check.
1. Why is a conidium not a Bacillus endospore?
2. What wall polymer do fungi have and oomycetes lack, and where do oomycetes belong?
3. Which host is definitive for Plasmodium, and what happens only there?
4. What is a hypnozoite, which human species form it, and why is clearing the blood not the end of their biology?
5. Why is kelp not a land plant, and a cyanobacterial bloom not an algal one?
