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The Eukaryotes: Fungi, Algae, Protozoa, and Helminths

Fungi

 

  1. Mycology is the study of fungi. 
  2. The number of serious fungal infections is increasing. 
  3. Fungi are aerobic or facultatively anaerobic chemoheterotrophs. 
  4. Most fungi are decomposers, and a few are parasites of plants and animals.

 

Characteristics of Fungi

 

  1. Molds and fleshy fungi consist of long filaments of cells joined together (hyphae).
    1. A fungal thallus (body) consists of hyphae; a mass of hyphae is called a mycelium.

                                                    i.     Septate hyphae contain cross-walls (septa), which divide them into uninuclear cell-like units.

1.     Septa usually have openings that allow the cytoplasm of adjacent “cells” to be continuous.

                                                   ii.     Coenocytic hyphae have no cross-walls and look like one long, filamentous, multinucleated cell.

  1. Yeasts are unicellular fungi.  To reproduce, fission yeasts divide symmetrically, whereas budding yeasts divide asymmetrically. 
    1. Buds that do not separate from the mother cell form pseudohyphae. 
  2. Pathogenic dimorphic fungi are yeast like at 37 degrees Celsius and moldlike at 25 degrees Celsius. 
  3. Fungi reproduce asexually by fragmentation or production of asexual spores.
    1. These spores can be produced asexually:
    2. Conidium (conidia) – a unicellular or multicellular spore that is not enclosed in a sac.

                                                    i.     Produced in a chain at the end of aerial hyphae called conidiophores.

                                                   ii.     Arthrospores – Formed by fragmentation of septate hyphae into single cells.

1.     Coccidioides immitis

                                                 iii.     Blastoconidia (blastospores) – Buds from parent cell, found in some yeasts.

1.     Candida albicans, Cryptococcus

    1. Chlamydospores – Thick-walled spore, formed by rounding and enlargement within a hyphael segment.

                                                    i.     C. albicans

    1. Sporangiopores – formed within a sporangium (sac).

                                                    i.     Sporangia are formed at the end of aerial hyphae called sporangiophores. 

  1. Fungi are taxonomically classified according to the type of sexual spore that they form, but clinically, since most only exhibit asexual spores in the lab, are classified by asexual spore type. 
  2. Sexual spores are usually produced in response to special circumstances, often changes in the environment.
  3. Phases of sexual reproduction:
    1. Plasmogamy – the haploid nucleus of a donor cell (+) penetrates the cytoplasm of a recipient cell (-).
    2. Karyogamy – nuclei fuse to form diploid zygote nucleus.
    3. Meiosis – forms haploid nuclei (sexual spores) that are genetic recombinants.
  4. Nutritional Adaptations:
    1. Fungi grow in aerobic environments; molds are aerobic, most yeasts are facultative anaerobes.
    2.   pH optima of about 5, too low for most bacteria.
    3. Can grow in low moisture conditions, which don’t support most bacterial growth.
    4. Can grow in relatively high sugar or salt concentrations (resist osmotic pressure gradients).
    5. Usually require less nitrogen than bacteria.
    6. They are able to metabolize complex carbohydrates, can grow on substrates bacteria cannot use. 
    7. Culture media:

                                                    i.     Sabouraud’s agar - A culture medium for fungi containing neopeptone or polypeptone agar and glucose, with final pH 5.6; it is the standard, most universally used medium in mycology and is the international reference. Modified Sabouraud's agar (Emmons modification) with less glucose is better for pigment development in the colonies.   Synonym: French proof agar.

                                                   ii.     More

 

Medically Important Phyla of Fungi

 

  1. The Zygomycota have coenocytic hyphae.
    1. Sexual spores are zygospores. 
    2. Asexual spores are sporangiospores.
    3. Examples:

                                                    i.     Rhizopus

                                                   ii.     Mucor

  1. The Ascomycota have septate hyphae.
    1. Sexual spores are ascospores produced in a sac-like structure called an ascus.
    2. Asexual spores are usually conidiospores.
    3. Examples:

                                                    i.     Penicillium 

                                                   ii.     Aspergillus 

                                                 iii.     Blastomyces dermatitidis

                                                 iv.     Histoplasma capsulatm

                                                  v.     Microsporum 

                                                 vi.     Trichophyton 

  1. Basidiomycota (club fungi) have septate hyphae and includes fungi that produce fruiting structures called mushrooms.
    1. Sexual spores are basidiospores formed externally on a base pedestal called a basidium.
    2. Asexual spores in some are conidiospores.
    3. Examples: 

                                                    i.     Cryptococcus neoformans

                                                   ii.     Amanita phalloides (Death Angel)

                                                 iii.     Psilocybe mexicana

  1. Deuteromycota – formerly used as a holding category for fungi without a known sexual spore type.
    1. Ribosomal RNA sequencing identifies most as anamorphic phases of Ascomycota, a few are basidiomycetes.
    2. Examples:

                                                    i.     Pneumocystis carinii (tentative placement)

                                                   ii.     Epidermophyton

                                                 iii.     Sporothrix schenckii

                                                 iv.     Coccidioides immitis

                                                  v.     Candida albicans

 

  1. Telemorphic fungi produce sexual and asexual spores; anamorphic fungi produce asexual spores only. 

 

Fungal Diseases

 

  1. Systemic mycoses are fungal infections deep within the body that affect many tissues and organs.
    1. Coccidiodomycosis – Coccidioides immitis
    2. Histoplasmosis – Histoplasma capsulatum
    3. Blastomycosis – Blastomyces dermatitidis
  2. Subcutaneous mycoses are fungal infections beneath the skin.
    1. Sporotrichosis – Sporothrix schenckii
    2. Chromomycosis – Fonsecaea, Phialophora, Cladosporium
  3. Cutaneous mycoses affect keratin-containing tissues such as hair, nails, and skin.
    1. Dermatophytoses (ringworm)

                                                    i.     Tinea capitis – ringworm of the scalp

                                                   ii.     Tinea cruris – ringworm of the groin (jock itch)

                                                 iii.     Tinea pedis – ringworm of the feet (athlete’s foot)

1.     Epidermophyton – affects only skin and nails

2.     Trichophyton – can affect hair, skin, or nails

3.     Microsporum – usually affects only hair or skin

    1. Dermatomycoses – caused by other fungi, most often Candida
  1. Superficial mycoses are localized on hair shafts and superficial skin cells.
    1. Tinea versicolor – Malassezia furfur
    2. Tinea nigra – Cladosporium werneckii
  2. Opportunistic mycoses are caused by normal microbiota or fungi that are not usually pathogenic. 
    1. Opportunistic mycosis can infect any tissues.  However, they are usually systemic.
    2. AIDS patients susceptible to Cryptococcus, Pneumocystis, and Penicillium infections.
    3. Mucormycosis - caused by some Rhizopus and Mucor.
    4. Aspergillosis, caused by Aspergillus.
    5. Candidiasis, caused by C. albicans, can be dermal, oral (thrush) or vaginal. 

 

Economical Effects of Fungi

 

  1. Food production
    1. Aspergillus niger – citric acid for foods and beverages
    2. Saccharomyces – bread, wine, beer, and used as a protein supplement and vitamin B source
    3. Torulposis –protein supplement
    4. Trichoderma – produces cellulase, removes cell walls to produce clear fruit juice
  2. Biological control of pests
    1. Candida oleophilia can grow on harvested fruit and prevent the growth of spoilage fungi. 
  3. Mold spoilage of fruits, grains, and vegetables is more common than bacterial spoilage of theses products. 
  4. Many fungi cause diseases in plants.
    1. Phytophthora infestans – Irish potato famine, infects soybeans and cocoa also.
    2. Cryphonectria parasitica – killed off most of the chestnut trees
    3. Ceratocystis ulmi – Dutch elm disease
  5. Medical
    1. Taxomyces produces taxol 

 

Lichens

 

  1. A lichen is a mutualistic combination of an alga (or a Cyanobacteria) and a fungus. 
  2. The alga photosynthesizes, providing carbohydrates for the lichen; the fungus provides a holdfast. 
  3. Lichens colonize habitats that are unsuitable for either the alga or the fungus alone. 
  4. Lichens may be classified on the basis of morphology as crustose, foliose, or fruticose. 
  5. Lichens are used for their pigments and as air quality indicators.

 

Algae

 

  1. Algae are unicellular, filamentous, or multicellular (thallic). 
  2. Most algae live in aquatic environments. 

 

Characteristics of Algae

 

  1. All algae are eukaryotic photoautotrophs that produce oxygen. 
  2. The thallus (body) of multicellular algae usually consists of a stipe, a holdfast, and blades.
  3. Algae reproduce asexually by cell division and fragmentation. 
  4. Many algae reproduce sexually. 
  5. Algae are classified according to their structures and pigments.

 

Selected Divisions of Algae

 

  1. Brown algae (kelp) may be harvested for algin. 
  2. Red algae grow deeper in the ocean than other algae because their red pigments can absorb the blue light that penetrates to deeper levels.
    1. Irish moss (Chondrus crispus) produces carrageenan, used as a food thickener
    2. Gracilaria can be used for food but some species are toxic. 
  3. Green algae have cellulose and chlorophyll a and b and store starch. 
  4. Diatoms are unicellular and have pectin and silica cell walls: some produce a neurotoxin (domoic acid, concentrates in mussels). 
  5. Dinoflagellates produce a neurotoxin that cause paralytic shellfish poisoning and ciguatera.
    1. Alexandrium – produces the saxitoxins responsible for PSP.  Large concentrations cause red tides
    2. Gambierdiscus toxicus concentrates in reef fish that aren’t normally toxic, causes ciguatera (symptoms include tingling around the lips, nausea, vomiting, weakness, and numbness).  

 

Roles of Algae in Nature

 

  1. Algae are the primary producers in aquatic food chains. 
  2. Planktonic algae produce most of the molecular oxygen in the earth’s atmosphere. 
  3. Most petroleum is the fossilized remains of planktonic algae. 
  4. Unicellular algae are symbionts in such animals as Tridacna (giant clam).

 

 

Protozoa  

 

  1. Protozoa are unicellular, eukaryotic chemoheterotrophs. 
  2. Protozoa are found in soil and water and as normal microbiota in animals. 

 

Characteristics of Protozoa

 

  1. The vegetative form is called a trophozote. 
  2. Asexual reproduction is by fission, budding, or schizogomy (multiple fission events followed by separation of the resulting multinucleated cell into multiple daughter cells). 
  3. Sexual reproduction is by conjugation. 
  4. During ciliate conjugation, two haploid nuclei fuse to produce a zygote. 
  5. Some protozoa can produce a cyst, which provides protection during adverse environmental conditions. 
  6. Protozoa have complex cells with a pellicle (protective outer covering), a cytostome (mouthlike opening), and an anal pore. 

 

Medically Important Phyla of Protozoa

 

  1. Archaezoa lack mitochondria and have flagella
    1. Trichomonas vaginalis – no cysts, causes vaginitis; usually nonsymptomatic in males but may cause nongonococcal urethritis
    2. Giardia lamblia – forms cysts, causes giardiasis.
  2. Microporidia lack mitochondria and microtubules, obligate intracellular parasites.
    1. Microsporans (including Nosema species), are implicated in causing diease in immunocompromised patients
    2. Nosema species cause disease in honey bees (and grasshoppers, but who cares?)
  3. Rhizopoda are amoeba, move by extension of pseudopodia.
    1. Entamoeba histolytica – amoebic dysentery.
    2. Acanthamoeba corneal infections (keratitis) leading to blindness (can grow in tap water, loves to see you rinse your contacts with nonsterile tap water).
    3. Naegleria fowleri – meningocephalitis (usually classified as a free-living amoeba but has flagellated form as well).
  4. Apicomplexa have apical organelles for penetrating host tissue; nonmotile in mature forms, obligate intracellular parasites.
    1. Plasmodium vivax – malaria

                                                    i.     Definitive host (sexual reproduction): Anopheles mosquito

1.     Gametocytes unite to form zygote, which develops into an oocyst

2.     Cell division within the oocyst produces asexual sporozoites

3.     Sporozoites migrate to the mosquito’s salivary glands

4.     Injected into human by mosquito bite

                                                   ii.     Intermediate host (asexual reproduction): Humans

1.     Travel to the liver in humans

2.     Undergo schizogony to produce merozoites

3.     Merozoites enter blood and infect RBCs

4.     Ring stage; grows and produces more merozoites

5.     RBCs are lysed, releasing merozoites and waste products (cause fever and chills)

6.     Merozoites may:

a.     Infect more RBCs and continue asexual development.

b.     Develop into male and female gametocytes and be picked up by another mosquito.

c.     Release occurs on a 24 hour cycle regulated by body temperature change in the host – insures gametocytes are mature at night when mosquitos are feeding.

    1. Babesia microti – babesiosis, fever and anemia in immunosuppressed patients.
    2. Toxoplasma gondii - toxoplasmosis
    3. Cryptosporidium – respiratory and gallbladder infections in immunocompromised patients.
    4. Cyclospora cayetanensis – raspberry diarrhea 
  1. Ciliophora move by means of cilia.
    1. Balantidium coli – dysentery.
  2. Euglenozoa move by means of flagella and lack sexual reproduction.
    1. Euglenoids – photoautotrophs
    2. Hemoflagellates (blood parasites) – transmitted by blood-feeding insects (defecation into wound during feeding); have undulating membranes. 
    3. Trypanosoma brucei gambiense, T.b. rhodesiense – African sleeping sickness, transmited by tsetse fly.
    4. T. cruzi – Chagas’ disease, transmitted by Triatoma (“kissing bug”).
    5. Leishmania – leishmaniasis, transmitted by the sand fly Phlebotomus

 

Slime Molds

 

  1. Cellular slime molds resemble amoebas and ingest bacteria by phagocytosis. 
  2. Plasmodial slime molds consist of a multinucleated mass of protoplasm that engulfs organic debris and bacteria as it moves. 

 

Helminths

 

  1. Parasitic flatworms belong to the phylum Platyhelminthes. 
  2. Parasitic roundworms belong to the phylum Nematoda. 

 

Characteristics of Helminths

 

  1. Helminths are multicellular animals; a few are parasites of humans. 
  2. The anatomy and life cycle of parasitic helminths are modified for parasitism. 
  3. The adult stage of a parasitic helminth is found in the definitive host. 
  4. Each larval stage of a parasitic helminth requires an intermediate host. 
  5. Helminths can be monoecious (both male and female sex organs in the same individual; hermaphroditic) or dioecious (two separate sexes, adults are either male or female)
    1. Hermaphrodites usually fertilize other adults, rarely self-fertilize.

 

Platyhelminths

 

  1. Flatworms are dorsoventrally flattened animals; parasitic flatworms may lack a digestive system. 
  2. Trematodes (flukes)
    1. Adult trematodes have an oral and ventral sucker with which they attach to host tissue. 
    2. Eggs of trematodes hatch into free-swimming miracidida that enter the first intermediate host; two generations of rediae develop in the first intermediate host; the rediae become cercariae that bore out of the first intermediate host and penetrate the second intermediate host; cercariae encyst as metacercariae in the second intermediate host; after they are ingested by the definitive host, the metacercariae develop into adults.
    3. Clonorchis sinensis – Asian liver fluke
    4. Paragonimus westermani – lung fluke

                                                    i.     Miracidia hatch from eggs in water, enter snail

                                                   ii.     Miracidium develops into redia in snail

                                                 iii.     Redia reproduces asexually to produce more redia

                                                 iv.     Redia produce several cercaria

                                                  v.     Cercaria bore out of snail, enter crayfish

                                                 vi.     Cercaria encyst to produce metacercaria, taken in by ingestion when host eats crayfish

                                                vii.     Metacercaria bores out of intestine and travels to lungs, develops into adult.

                                              viii.     Eggs are released into sputum, swallowed and excreted in feces.

    1. Shistosoma – blood fluke

                                                    i.     Cercariae   burrow through skin enter circulation

                                                   ii.     Adults found in intestinal blood vessels

                                                 iii.     Eggs stimulate granuloma formation

  1. Cestodes (tapeworms)
    1. A cestode consists of a scolex (head) and proglottids.
    2. Absorb nutrients from hosts (don’t eat host tissue).
    3. Proglottids produce eggs, released in feces.
    4. Larvae hatch in intestine of intermediate host and migrate to site where they encyst.
    5. Cysts are ingested by definitive host, scolex is released and attaches. 
    6. Taenia saginata – beef tapeworm

                                                    i.     Humans serve as the definitive host and cattle are the intermediate host. 

    1. Taenia solium – pork tapeworm

                                                    i.     Humans serve as the definitive host and can be an intermediate host.

                                                   ii.     Larvae encyst in pig muscle, but in humans larvae encyst in brain and other places (cysticercosis).

                                                 iii.     Primarily transmitted from human to human in the U.S. (fecal-oral) 

    1. Echinococcus granulosus

                                                    i.      Humans serve as the intermediate host; the definitive hosts are dogs, wolves, and foxes.

                                                   ii.     Larvae migrate to lungs or liver and encyst (hydatid cyst) forming brood capsules from which thousands of scoleces might be produced. 

 

Nematodes

 

  1. Roundworms have a complete digestive system. 
  2. The nematodes that infect humans with their eggs are:
    1. Enterobius vermicularis (pinworm)

                                                    i.     Fecal oral route of infection

    1. Ascaris lumbricoides

                                                    i.     Fecal –oral infection

                                                   ii.     Eggs hatch in intestines, migrate to lungs to mature, and then migrate to intestines 

  1. The Nematodes that infect humans with their larvae are:
    1. Necator americanus (hookworm)

                                                    i.     Larvae burrow through skin, travels to lungs, carried to intestine.

    1. Trichinella spiralis

                                                    i.     Larvae acquired by eating encysted larvae in undercooked pork or game animals

                                                   ii.     Mature into adults in intestine, sexually reproduce, eggs develop in female

                                                 iii.     Femal gives birth to live larvae, travel to muscles and other tissues and encyst.

    1. Anisakine worms
    2. Transmitted to humans from infected fish and squid.

 

Arthropods as Vectors

 

  1. Jointed-legged animals, including ticks and insects, belong to the phylum Arthropoda. 
  2. Arthropods that carry disease are called vectors. 
  3. Elimination of vector borne disease is best done by the control or eradication of the vectors.