Chapter 12 - Eukaryotes
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I. Eukaryotic organisms
A. Major groups of Eukaryotes we will be concerned with
1. Fungi (Kingdom Fungi)
2. Protozoa (Kingdom Protist)
3. Helminths (Kingdom Animalia)
B. Eukaryotes differ significantly from the Bacteria in their internal cell structure and metabolism, meaning that drugs directed against Bacteria do not typically phase eukaryotic pathogens.
C. Eukaryotic metabolism has a number of similarities to human metabolism (DNA replication, transcription, cell membrane synthesis) and as a consequence drugs aimed at eukaryotic pathogens typically cause harm to the human host.
II. Fungi
A. All are members of the Kingdom Fungi
B. While there are beneficial fungi in our everyday lives (i.e. cheese, beer, wine, etc.), the incidence of severe fungal infections in humans has sharply risen in the last decade.
C. Fungal infections are most common as nosocomial infections or in people with compromised immune systems.
D. Characteristics of fungi
1. Physical Characteristics
a. Fungal colonies are referred to as vegetative, since they are actively undergoing growth and metabolism.
i. Molds and fleshy fungi –
a. The main body (thallus) of the fungus is composed of numerous long filaments called hyphae (singular: hypha).
b. Hyphae can be physically separated into distinct cells, by cross walls called septa (singular: septum), this is a physical characteristic that can be useful for identification.
c. The mat of hyphae is called the mycelium and is the nutrient obtaining segment of the growth.
d. Growth above the surface is called the aerial, or reproductive, hyphae and is the site of spore formation.
ii. Yeasts –
a. Nonfilamentous fungi that typically spherical or ovoid.
b. Budding yeasts are those that divide unevenly with a daughter cell “budding” off the parent.
c. Fission yeasts are those that divide evenly, after elongation of the original parent cell.
iii. Dimorphic Fungi –
a. Most common in pathogenic species
b. Temperature or CO2 controlled switch between yeast phase and filamentous fungi phase.
2. Life Cycle of Fungi
A. Fungi can reproduce asexually through the fragmentation of their hyphae.
B. Fungi can reproduce both asexually and sexually through the formation of spores.
C. Fungal spores are quite different from bacterial endospores
i. Fungal spores are a means of dispersal and reproduction not strictly survival
ii. Though they can tolerate extended hot or dry environments they do not exhibit the extreme tolerance of endospores.
D. Asexual Fungal Spores
i. Produced by an individual fungus through mitosis
ii. Conidia (singular: conidium) are specialized spores produced at the ends of specialized hyphae called conidiophores.
iii. Arthroconidia (singular: arthroconidium) are spores produced from the fragmentation and slight thickening of the hyphae.
iv. Blastoconidia (singular: blastoconidium) spores produced in certain types of yeasts.
v. Chlamydoconidia thick walled spores developed from rounding off of terminal hyphae.
vi. Sporangiospore profuse small spores produced by the Zygomycota
3. Nutritional Adaptations
a. Tend towards acidic environment (around pH 5), which does not support most bacteria. (though Lactobacillus species survive at even lower pH)
b. Almost all molds are aerobic and yeasts are facultative anaerobes.
c. Most fungi can grow in high sugar and salt concentration (survive relatively high osmotic pressure).
d. Xerophilic, can grow in areas of low moisture content
e. Fungi have a lower nitrogen requirement
f. Fungi are capable of digesting very complex carbohydrates (such as wood)
g. Notice how these allow fungi to exploit environmental niches that are unique and separate from competition, such as bacteria.
D. Medically Important Phyla of Fungi
1. Zygomycota – “Conjugation Fungi”
a. A common example of this is the grey furry mold that arises on your strawberries, a mold called Rhizopus.
b. Typically arise as tissue destroying infections in the sinuses, very uncommon excepting in immune compromised individuals. There are also a statistically high number of cases in postmenopausal women
2. Ascomycota – “Sac Fungi”
3. Basidomycota – “Club Fungi”
4. Deuteromycota – “Imperfect Fungi”
a. A holding category left-over from classical taxonomy, representing the fungi that do not have a determined sexual state.
b. Current molecular techniques are enabling mycologists to assign these organisms into their actual phyla.
5. Teleomorph vs. Anamorph
a. Teleomorph – fungi that produce asexual and sexual spores
b. Anamorph – fungi that only have an observed asexual cycle (Deuteromycota).
E. Fungal Diseases
1. Mycosis – a fungal infection
2. Subclasses of Mycosis
a. Systematic Mycosis
i. fungal infections within the body, but not restricted to a particular region or organ.
ii. Usually initiated by inhalation of spores, and start in the lungs.
iii. Examples – histoplasmosis (Ajellomyces capsulatum) and coccidiomycosis
b. Subcutaneous Mycosis
i. Fungal infections beneath the skin by saprophytic fungi
ii. Infection occurs due to implantation of spores or mycelium into a wound.
c. Cutaneous Mycosis
i. Grown only on the skin, hair, or nails.
ii. Fungi that cause these are termed dermatophytes
iii. These fungi are able to digest keratin (a major protein in the skin, hair, and nails)
d. Most Fungal diseases are opportunistic pathogens which are organisms that are typically harmless, but can cause disease in compromised individuals.
III. Protozoa
A. Unicellular, eukaryotic chemoheterotrophs
B. Many Protozoa can undergo encystment, the production of a protective capsule called a cyst. These are survival structures for adverse conditions, or transfer to a new host.
C. Medically Important Phyla
1. Archaezoa
a. Eukaryotic organisms that lack a mitochondria (though they have a vestigial organelle called a mitosome)
b. Many live as symbionts in the digestive tracts of animals.
c. Example: Trichomonas vaginalis – a common STD
d. Example: Giardia lamblia – intestinal pathogen that is excreted in feces and transmitted by fecal to oral transmission.
2. Microspora
a. Unusual organisms that do not possess mitochondria or microtubules
b. Obligate intracellular pathogens
c. Cause conditions such as chronic diarrhea, most notably in AIDS patients.
3. Amoebozoa
a. Protozoa capable of extending pseudopods
b. Entamoeba histolytica
i. Colonizes as many as 10% of the human population
ii. Causes “Traveler’s Diarrhea”
iii. Passage by fecal to oral route.
4. Apicomplexa
a. Obligate intracellular pathogens
b. Plasmodium – genus responsible for malaria (the 4th leading cause of death in the world)
c. Complex life cycle (Plasmodium example)
i. Anopheles mosquitoes (the definitive host, the host that harbors sexual reproduction) carries the infective stage, the sporozoite, which is passed to a human during feeding. The sporozite travels through the blood to the liver.
ii. Sporozite undergo asexual reproduction in the liver to produce thousands of merozoites
iii. Merozoites enter the bloodstream and infect red blood cells (RBC).
iv. Repeated asexual reproduction within the RBC until the RBC ruptures and releases the merozoites to infect new RBC (this synchronized release also releases waste products that cause the recurrent fever and chills that are hallmarks of malaria).
v. Some merozoites develop into gametocysts (sexual forms) which can be taken in by a feeding Anopheles mosquito.
vi. The ingested gametocysts enter the mosquito intestinal system and begin a sexual cycle.
vii. Male and female gametocysts unite and form a zygote.
viii. The zygote forms an oocyst that forms asexual sporozites
ix. New sporozites migrate to the salivary glands of the mosquito to restart the cycle.
d. Humans are the intermediate hosts for Plasmodium, since only asexual reproduction occurs in them.
e. Other notable pathogens in this phylum
i. Babesia microti
ii. Toxoplasma gondii
iii. Cryptosporidium
a. A newly recognized pathogen of humans
b. Waterborne transmission from fecal contaminated sources.
c. Difficult to remove from water systems, and typically is controlled at the source versus in the system.
5. Euglenozoa
a. Flagellated, with disk shaped mitochondria and an absence of sexual reproduction
b. A subgroup called the hemoflagellates (blood parasites) are transmitted by the bites of blood-feeding insects.
i. Trypanosomes – African Sleeping Sickness
IV. Helminths
A. Helminths are the “worms” parasitic helminths are divided between two phyla.
1. Platyhelminthes contain the parasitic flatworms
2. Nematodes contain the parasitic roundworms
B. The Life Cycle of the Helminths are designed for parasitism
1. Adults stages exist in the definitive host
2. Each larval stage requires an intermediate host
C. Platyhelminthes have a number of pathogens; the most recognized of them is the tapeworm
1. Humans serve as the definitive host for beef tapeworms and the pork tapeworm
2. Passage is typically through contaminated meat
3. The use of tapeworms for weight loss is a recurrent scam where the powdered eggs are sold as a “diet supplement”. The unsuspecting victims typically do lose weight, at the cost of their own health.
D. Nematodes can infect through eggs or as mature worms, most control is due to sanitary and food prep considerations.