Premed · Premed · Microbiology
Lecture 4: Eukaryotic Microbes -- Fungi and Protozoa
Microbiology
Learning Objectives
By the end of this lecture, students will be able to:
- Describe the general characteristics that distinguish eukaryotic cells from prokaryotic cells
- Classify fungi into major groups and describe their morphology, reproduction, and clinical significance
- Differentiate between yeasts, molds, and dimorphic fungi
- Classify protozoa by motility type and describe representative pathogens
- Explain the life cycles of medically important protozoa
- Describe the basic features of algae and their relevance to microbiology
Lecture Content
I. Eukaryotic Cell Features Relevant to Microbiology
Eukaryotic cells are defined by a membrane-bound nucleus housing linear chromosomes associated with histones. They contain membrane-bound organelles including mitochondria, endoplasmic reticulum, Golgi apparatus, and lysosomes. Their cytoplasmic ribosomes are 80S (composed of 60S and 40S subunits), though 70S ribosomes are found within mitochondria and chloroplasts -- a remnant consistent with the endosymbiotic theory, which proposes that mitochondria descended from engulfed alpha-proteobacteria and chloroplasts from cyanobacteria. Eukaryotic cells possess a well-developed cytoskeleton of actin microfilaments, microtubules, and intermediate filaments, and they divide by mitosis and meiosis rather than by binary fission.
II. Fungi -- General Characteristics
Fungi are eukaryotic organisms that obtain nutrition by absorption: they secrete exoenzymes into their environment and absorb the resulting digested nutrients, a strategy classified as heterotrophic absorptive nutrition. Their cell walls are composed primarily of chitin (an N-acetylglucosamine polymer) and glucans, and their cell membranes contain ergosterol instead of the cholesterol found in animal cells. Ergosterol is a critical target of antifungal drugs. Most fungi are aerobic or facultatively anaerobic. They are ubiquitous in soil and decaying matter and also exist as commensals on human skin and mucosa. Fungi play numerous ecological and economic roles as decomposers, in food production (bread, beer, cheese), as sources of antibiotics (penicillin), and as pathogens of both plants and humans.
A. Morphological Forms
Yeasts are unicellular fungi with round to oval cells that reproduce by budding (producing daughter cells called blastoconidia) or by fission. On agar they form smooth, creamy colonies. Important examples include Saccharomyces cerevisiae, Candida albicans, and Cryptococcus neoformans.
Molds (filamentous fungi) are multicellular organisms that grow as branching filaments called hyphae. Hyphae may be septate, with cross-walls (septa) that divide the filament into cells connected by pores allowing cytoplasmic flow, or coenocytic (aseptate), lacking septa and thus multinucleated. A mass of hyphae is called a mycelium. The vegetative mycelium grows into the nutrient substrate, while the aerial (reproductive) mycelium projects above the surface and produces spores. Mold colonies typically have a fuzzy or cottony appearance. Representative genera include Aspergillus, Penicillium, Mucor, and Rhizopus.
Dimorphic fungi are particularly important in medical mycology because they exist as molds in the environment at 25 degrees C and convert to yeasts within the host at 37 degrees C -- a phenomenon captured by the mnemonic "mold in the cold, yeast in the beast." Medically important dimorphic fungi include Histoplasma capsulatum (histoplasmosis, associated with bird and bat droppings in the Ohio and Mississippi River valleys), Blastomyces dermatitidis (blastomycosis, in the Great Lakes and Ohio/Mississippi River valleys), Coccidioides immitis (coccidioidomycosis or valley fever, in the southwestern US deserts), Paracoccidioides brasiliensis (paracoccidioidomycosis, in Latin America), Talaromyces (Penicillium) marneffei (in Southeast Asia, primarily in immunocompromised patients), and Sporothrix schenckii (sporotrichosis, or rose gardener's disease).
B. Fungal Reproduction
Asexual reproduction is the form most commonly observed in laboratory and clinical settings. Yeasts reproduce by budding, while filamentous fungi produce conidia (asexual spores) on specialized structures called conidiophores. Varieties of asexual spores include arthrospores (formed by fragmentation of hyphae), chlamydospores (thick-walled survival spores), blastospores (buds from a parent cell), and sporangiospores (produced inside a sac called a sporangium, as in Mucor and Rhizopus). Aspergillus and Penicillium bear conidia on specialized structures known as phialides.
Sexual reproduction involves the mating of compatible mating types, followed by karyogamy and meiosis, and produces sexual spores such as zygospores, ascospores, and basidiospores. The type of sexual spore produced forms the basis for the major taxonomic classification of fungi.
C. Major Fungal Groups (Phyla)
Zygomycota are characterized by coenocytic hyphae and the production of zygospores. Mucor and Rhizopus are representative members, with Rhizopus known as bread mold and both genera capable of causing mucormycosis. Ascomycota (sac fungi) have septate hyphae and produce ascospores within sac-like structures called asci; this phylum includes Aspergillus, Penicillium, and Saccharomyces. Basidiomycota (club fungi) have septate hyphae and produce basidiospores on club-shaped basidia; Cryptococcus neoformans and mushrooms belong to this group. Deuteromycota (Fungi Imperfecti) is a historical grouping for fungi with no known sexual stage, classified instead by asexual morphology; many clinically important species were historically placed here.
<image>A composite figure of fungal morphology. Panel A: Microscopic view of yeast cells showing budding (blastoconidia) and pseudohyphae of Candida albicans. Panel B: Septate hyphae of Aspergillus with a conidiophore bearing a vesicle and chains of conidia (labeled). Panel C: Coenocytic hyphae of Rhizopus showing sporangium atop a sporangiophore, with rhizoids at the base. Panel D: Dimorphic transition diagram showing mold form at 25 degrees C (branching hyphae with conidia) and yeast form at 37 degrees C (oval budding cells) connected by a double-headed arrow, for Histoplasma capsulatum.</image>
III. Protozoa -- General Characteristics
Protozoa are unicellular eukaryotic organisms. Most are motile and heterotrophic, obtaining nutrients by ingestion or absorption. They typically exist in two forms: trophozoites, the active, feeding, motile form, and cysts, the dormant, environmentally resistant form. Protozoa are found in aquatic environments, soil, and as parasites of animals and humans. Traditionally they have been classified by their mode of locomotion, though molecular phylogenetics now supplements morphological criteria.
A. Classification by Motility
Sarcodina (amoebae) move by extending pseudopodia (cytoplasmic projections). Entamoeba histolytica causes amoebic dysentery and liver abscess. Naegleria fowleri is a free-living amoeba that causes rapidly fatal primary amoebic meningoencephalitis following freshwater exposure. Acanthamoeba can cause keratitis, particularly in contact lens wearers, and granulomatous amoebic encephalitis.
Mastigophora (flagellates) move using flagella. Giardia lamblia causes giardiasis, characterized by watery diarrhea and malabsorption, and is transmitted by the fecal-oral route through contaminated water. Trichomonas vaginalis causes trichomoniasis, a sexually transmitted infection notable for having no cyst stage. Trypanosoma brucei causes African sleeping sickness (transmitted by the tsetse fly), Trypanosoma cruzi causes Chagas disease (transmitted by the triatomine or kissing bug), and Leishmania species cause leishmaniasis in its cutaneous and visceral forms (transmitted by sandflies).
Ciliophora (ciliates) move using cilia. Balantidium coli is the only ciliated protozoan known to be pathogenic in humans, causing dysentery, with pigs serving as the primary reservoir.
Apicomplexa (sporozoans) are non-motile in their adult form and are obligate intracellular parasites that possess an apical complex for host cell invasion. Plasmodium species cause malaria, with P. falciparum, P. vivax, P. ovale, P. malariae, and P. knowlesi all capable of human infection and Anopheles mosquitoes serving as the vector. Toxoplasma gondii causes toxoplasmosis, with cats as the definitive host and particular risk to immunocompromised patients and pregnant women. Cryptosporidium parvum causes watery diarrhea through resistant oocysts in water. Babesia species cause babesiosis, a tick-borne infection of red blood cells.
<image>A four-panel figure illustrating protozoan diversity by motility type. Panel A: Amoeba (Entamoeba histolytica) showing pseudopodia, nucleus, ingested red blood cells in trophozoite form, and a round cyst with chromatoid bars. Panel B: Flagellate (Giardia lamblia) showing the characteristic pear-shaped trophozoite with two nuclei, ventral sucking disc, and four pairs of flagella; adjacent oval cyst form. Panel C: Ciliate (Balantidium coli) large trophozoite covered in cilia with macronucleus and micronucleus labeled. Panel D: Apicomplexan (Plasmodium) showing ring-form trophozoite inside a red blood cell, with labeled apical complex structures (rhoptries, micronemes, polar ring) in an invasive merozoite.</image>
IV. Life Cycle of Plasmodium (Malaria) -- Key Example
The malaria life cycle alternates between a mosquito stage and human stages. During the mosquito stage (sporogony), a female Anopheles mosquito ingests gametocytes during a blood meal. These gametocytes fuse in the mosquito gut to form a zygote, which develops into an ookinete and then an oocyst. When the oocyst ruptures, it releases sporozoites that migrate to the salivary glands, ready for injection into the next human host.
In the human liver stage (exoerythrocytic schizogony), sporozoites injected during a mosquito bite travel through the bloodstream to the liver, where they invade hepatocytes and multiply asexually to produce thousands of merozoites. Importantly, P. vivax and P. ovale can form dormant hypnozoites in the liver, which are responsible for relapses months or years after the initial infection.
During the human blood stage (erythrocytic schizogony), merozoites released from the liver invade red blood cells. Inside each red blood cell, the parasite progresses through ring, trophozoite, and schizont stages. The schizont then ruptures the red blood cell, releasing a new generation of merozoites and triggering the cyclic fevers characteristic of malaria. Some merozoites differentiate into gametocytes, which circulate in the blood and can be taken up by the next mosquito to complete the cycle.
V. Algae -- Brief Overview
Algae are photosynthetic eukaryotic microorganisms that contain chloroplasts. Their cell walls often consist of cellulose or silica (in the case of diatoms). Found in aquatic and moist terrestrial environments, algae are ecologically important as primary producers and contribute a significant proportion of atmospheric oxygen. Some algae, however, produce potent toxins. Harmful algal blooms (HABs), caused by dinoflagellates, generate neurotoxins such as saxitoxin and brevetoxin, leading to conditions like paralytic shellfish poisoning. Algae are not typically human pathogens, although Prototheca, an achlorophyllous alga, can cause rare infections.
VI. Helminths -- Brief Mention
Helminths are multicellular eukaryotic parasites studied within the frameworks of medical microbiology and parasitology. They include nematodes (roundworms), cestodes (tapeworms), and trematodes (flukes). Although the adult worms are macroscopic, their microscopic stages -- eggs and larvae -- are routinely identified in clinical microbiology laboratories. A more comprehensive treatment of helminthic diseases appears in Lecture 26.
<image>A life cycle diagram of Plasmodium falciparum (malaria). Left side: mosquito stages -- gametocytes ingested, zygote formation in midgut, oocyst development, sporozoite migration to salivary glands. Right side: human stages -- sporozoite injection by mosquito bite, hepatocyte invasion and exoerythrocytic schizogony (with hypnozoites noted for P. vivax/P. ovale), merozoite release into blood, erythrocytic cycle (ring stage, trophozoite, schizont, merozoite release causing RBC lysis and fever), and gametocyte formation. Arrows connect the cycle. Key clinical features (cyclic fevers, anemia, splenomegaly) annotated at the erythrocytic stage.</image>
