Fungal and non-fungal moulds, and pathogenic protozoa – Microbiology and Biotechnology (Senior 4 Biology) | Ganzaa

Microbiology and Biotechnology

Lessons

Senior 4 · Biology

Microbiology and Biotechnology

5 lessons

Lesson 2 of 5

Fungal and non-fungal moulds, and pathogenic protozoa

25 min

Mould on the bread, blight in the field

Leave a slice of bread in a warm, damp kitchen for a few days and fluffy white threads appear, then tiny black dots. Store maize that is not properly dried and a greenish powder may grow on the grains. Walk past a potato field in cool, wet weather and you may see leaves turning brown and slimy within days. All three are caused by "moulds", yet only the first two are true fungi. The potato blight organism, Phytophthora infestans, is a water mould, more closely related to brown algae than to fungi.

This lesson compares fungal and non-fungal moulds, then turns to the single-celled eukaryotes (protozoa) that cause some of Rwanda's most serious infections, following their life cycles in detail.

Definition

Mould: a general name for any organism that grows as a fuzzy mass of fine threads on food or other material.

Hypha (plural hyphae): one thread of a mould; a mass of hyphae is a mycelium.

Saprotroph (saprophyte): an organism that feeds on dead organic matter by secreting enzymes onto it and absorbing the products (extracellular digestion).

Sporangium: a structure in which spores are made; conidia: spores formed in chains at the tips of special hyphae, not inside a sporangium.

Protozoa: single-celled, heterotrophic eukaryotes, many of which move by flagella, cilia or pseudopodia.

Cyst: a resting stage with a tough wall that survives outside the host. Trophozoite: the active, feeding stage.

Fungal moulds

Fungal moulds have walls of chitin, store food as glycogen and oil, and reproduce by large numbers of light, haploid spores. Three are important in the home and in industry:

Rhizopus (black bread mould): hyphae without cross-walls; spores made inside black sporangia. It rots bread, fruit and sweet potatoes.
Penicillium: hyphae with cross-walls (septa); spores (conidia) in brush-like chains. Blue-green moulds on oranges and bread; one species makes the antibiotic penicillin (Lesson 5).
Aspergillus: conidia on a swollen head. A. flavus and A. parasiticus growing on badly stored maize, sorghum and groundnuts make aflatoxins, which WHO describes as among the most poisonous mycotoxins: they damage DNA, can cause liver cancer, and, like most mycotoxins, survive cooking and food processing.

Rhizopus reproduces asexually when food is plentiful: each sporangium releases thousands of genetically identical spores that blow to new food. When two mating strains (+ and −) meet, often as food runs out, they reproduce sexually, forming a resistant zygospore; meiosis when it germinates produces new combinations of alleles.

Penicillium under a light microscope (scale bar 100 µm). The numbers mark (1) a hypha, (2) an upright conidiophore, (3) phialides, the bottle-shaped cells that pinch off spores, (4) chains of conidia (spores) and (5) septa, the cross-walls of the hypha. Photo: Y_tambe, CC BY-SA 3.0, via Wikimedia Commons

Non-fungal moulds

Feature	Fungal moulds (Rhizopus, Penicillium)	Water moulds (oomycetes, e.g. Phytophthora)	Slime moulds (e.g. Physarum)
Kingdom	Fungi	Protoctista (related to brown algae)	Protoctista (related to amoebae)
Cell wall	Chitin	Cellulose	None in the feeding stage
Hyphae	Haploid	Diploid	No hyphae: a creeping mass of cytoplasm (plasmodium) or separate amoeba-like cells
Spores	Non-motile, spread by air	Swimming zoospores with two flagella, spread in water films	Spores from stalked fruiting bodies
Feeding	Extracellular digestion and absorption	Absorption; many are plant parasites	Engulf bacteria and organic particles

Because water moulds spread by swimming spores, Phytophthora infestans (potato late blight) spreads fastest in cool, wet weather. It caused the Irish potato famine of the 1840s, and farmers today control it with resistant varieties, clean seed tubers and fungicide sprays timed to wet weather.

Pathogenic protozoa

Protozoan	Movement	Disease	Transmission
Plasmodium (sporozoan)	No organelles of movement in the blood stages	Malaria	Bite of a female Anopheles mosquito
Entamoeba histolytica (amoeba)	Pseudopodia	Amoebic dysentery	Faecal–oral: cysts in contaminated water or food
Trypanosoma brucei (flagellate)	One flagellum along an undulating membrane	African trypanosomiasis (sleeping sickness)	Bite of a tsetse fly
Giardia (flagellate)	Several flagella	Giardiasis (diarrhoea)	Faecal–oral: cysts in water

Life cycle of Plasmodium

Plasmodium needs two hosts: it reproduces asexually in humans and sexually in the mosquito, so the mosquito is both the vector and the host in which the parasite's sexual reproduction takes place.

The bursting of red cells releases waste and toxins together, which triggers the cycles of chills and high fever; the destruction of red cells causes anaemia. In P. falciparum, the most dangerous species, infected red cells stick to the walls of small blood vessels, blocking blood flow to the brain and other organs (severe malaria). Each stage is a target: nets and spraying stop the bites; artemisinin drugs kill the blood stages; and some drugs kill the gametocytes so that a treated person cannot infect mosquitoes.

Life cycle of Entamoeba histolytica

A person swallows cysts (each with four nuclei) in water or food contaminated with faeces. In the small intestine each cyst releases amoebae, which divide and become trophozoites in the large intestine. There they feed on bacteria and may invade the gut wall, causing ulcers and bloody diarrhoea (amoebic dysentery); sometimes they reach the liver and form an abscess. In the lower colon, trophozoites round up and form new cysts, which pass out in the faeces and survive for weeks in moist surroundings. Only one host is needed, so the cycle is broken by safe water, latrines and handwashing.

A stained blood smear from a patient with African trypanosomiasis (sleeping sickness). The long, wavy, dark-stained cells are trypanosomes, each with a flagellum running along a rippled undulating membrane; the round pale cells around them are red blood cells, about 7 µm across. Photo: CDC/Dr. Myron G. Schultz, public domain, via Wikimedia Commons

Worked example 1: how fast do malaria parasites build up?

Suppose $10^{5}$ merozoites leave the liver and enter the blood, and the number of parasites multiplies by 8 in each 48-hour blood cycle (illustrative figures). An adult has about $2.5 \times 10^{13}$ red blood cells. (a) How many parasites are there after 4 cycles, and what percentage of red cells is infected (assume one parasite per cell)? (b) After how many cycles will 2 % of red cells be infected?

(a) $N = 10^{5} \times 8^{4} = 10^{5} \times 4096 = 4.1 \times 10^{8}$ parasites. Percentage infected: $\frac{4.1 \times 10^{8}}{2.5 \times 10^{13}} \times 100\,\% = 0.0016\,\%$ Too few to detect easily, and the person may feel well.
(b) 2 % of the red cells is $0.02 \times 2.5 \times 10^{13} = 5 \times 10^{11}$ . We need $10^{5} \times 8^{n} \ge 5 \times 10^{11}$ , so $8^{n} \ge 5 \times 10^{6}$ : $n \ge \frac{\log (5 \times 10^{6})}{\log 8} = \frac{6.70}{0.903} = 7.4$ So 8 cycles, about 16 days. Exponential growth explains why malaria can turn from mild to severe within days, and why early testing and treatment matter.

Worked example 2: fungus or water mould?

A student finds a white, thread-like growth on diseased potato leaves. Under the microscope the threads have no cross-walls, and in a drop of water some spores are seen swimming. A cell-wall test shows cellulose but no chitin. Is the organism a fungus? Explain.

Hyphae without cross-walls are found in both Rhizopus and water moulds, so this feature alone does not decide.
Swimming spores (zoospores with flagella) are not made by true fungi of the bread-mould type; they are typical of water moulds.
A cellulose wall with no chitin rules out a true fungus, whose walls contain chitin.
Conclusion: it is a water mould (an oomycete), almost certainly Phytophthora infestans, the cause of potato late blight, and so a protoctist, not a fungus.

HHMI BioInteractive: "Malaria Life Cycle Animation: Mosquito Host". Watch the stages inside the mosquito: gametes fusing in the gut, the ookinete crossing the gut wall, and the oocyst releasing sporozoites that travel to the salivary glands.

Warning

Biosafety and common mistakes

Do not sniff or open containers of mouldy food: spores of Aspergillus and other moulds can cause allergies and lung infections. Seal mouldy samples in clear bags or Petri dishes taped shut, and never eat food from a mould investigation. Throw away mouldy maize and groundnuts: aflatoxins survive cooking.
Calling Phytophthora and slime moulds fungi. They are protoctists, despite looking like moulds.
Saying the mosquito is "the cause" of malaria or only a "carrier". The cause is Plasmodium; the mosquito is the vector and also the host of its sexual stage.
Thinking amoebic dysentery spreads by mosquitoes. It spreads by the faecal–oral route, through cysts.

Tip

Memory hook. Malaria in the human: "Saliva → Liver → Red cells → Gametocytes" (Some Like Red Gravy). In the mosquito: "Gut → Ookinete → Oocyst → Salivary glands".

✦

Practice: moulds and pathogenic protozoa

Give three differences between Rhizopus and Penicillium.
Describe how Rhizopus obtains its food from bread.
Give three ways in which water moulds differ from true fungi.
Explain why aflatoxin contamination is a public health problem, and suggest two ways farmers can prevent it.
Explain why the mosquito is called the definitive host of Plasmodium and the human the intermediate host.
Explain why a person with malaria has fever every two days or so.
Explain how each of these breaks the life cycle of Entamoeba histolytica: (a) boiling drinking water, (b) using latrines.
If parasite numbers multiply by 10 every 48 hours, starting from $2 \times 10^{4}$ , how many parasites will there be after 6 days?

Show answersHide answers

Any three: Rhizopus hyphae have no cross-walls, Penicillium hyphae have septa; Rhizopus makes spores inside sporangia, Penicillium makes chains of conidia on conidiophores; Rhizopus has stolons and rhizoids; Rhizopus reproduces sexually by zygospores; Penicillium is blue-green, Rhizopus white then black.
It is a saprotroph: its rhizoids and hyphae secrete digestive enzymes (amylases, proteases) onto the bread; these break starch and proteins into sugars and amino acids, which are absorbed through the hyphal walls and used for respiration and growth (extracellular digestion).
Any three: water moulds have cellulose cell walls (fungi have chitin); their hyphae are diploid (fungal hyphae are haploid); they make swimming zoospores with flagella; they belong to the Protoctista, related to brown algae.
Aflatoxins made by Aspergillus flavus and A. parasiticus on stored maize, sorghum and groundnuts are very poisonous, damage DNA and can cause liver cancer; they cannot be seen or cooked out, so whole families can be exposed over years. Prevention: dry grain quickly and fully after harvest; store it off the ground in dry, airy stores; sort out damaged or mouldy grains.
Sexual reproduction (fusion of gametes) happens in the mosquito, so it is the definitive host; in the human the parasite reproduces only asexually, so the human is the intermediate host.
Merozoites multiply inside red blood cells in step, and the cells burst together at the end of each cycle (about 48 hours in some species), releasing merozoites, waste and toxins into the blood at once; this triggers chills and fever each time.
(a) Boiling kills the cysts in water, so they cannot be swallowed alive. (b) Latrines keep faeces containing cysts away from water sources, food and hands, breaking the faecal–oral route.
6 days = 3 cycles: $2 \times 10^{4} \times 10^{3} = 2 \times 10^{7}$ parasites.

Quick check

Put the stages of Plasmodium in the human in order.

Tap the cards in the right order.

Key points

Fungal moulds (Rhizopus, Penicillium, Aspergillus) have chitin walls and haploid hyphae, feed saprotrophically by extracellular digestion, and spread by spores; Aspergillus on stored grain makes cancer-causing aflatoxins.
Non-fungal moulds are protoctists: water moulds (cellulose walls, diploid hyphae, swimming zoospores, e.g. potato blight) and slime moulds (no walls when feeding).
Plasmodium: asexual stages in human liver and red cells (fever every cycle), sexual stage in the Anopheles mosquito; numbers rise exponentially.
Entamoeba histolytica: one host; cysts spread faecal–orally; trophozoites cause amoebic dysentery. Trypanosoma is spread by tsetse flies.

Sources and credits

5.1: Unicellular Eukaryotic Microorganisms and 5.3: Fungi, from Microbiology by OpenStax (Nina Parker, Mark Schneegurt, Anh-Hue Thi Tu, Philip Lister, Brian M. Forster), via LibreTexts Biology, licensed under CC BY 4.0. Used for moulds, water and slime moulds, and the protozoa and their life cycles. Adapted by Ganzaa.
World Health Organization, Mycotoxins fact sheet (2 October 2023).
Figures 1 and 2, the tables, worked examples (the parasite numbers are illustrative), practice and quick check: written by Ganzaa.
Photos: Y_tambe, CC BY-SA 3.0; CDC/Dr. Myron G. Schultz, public domain; both via Wikimedia Commons.
Video: "Malaria Life Cycle Animation: Mosquito Host", HHMI BioInteractive, embedded from YouTube.

1Mould on the bread, blight in the field

2Fungal moulds

3Non-fungal moulds

4Pathogenic protozoa

5Life cycle of Plasmodium

6Life cycle of Entamoeba histolytica