Introduction to Eukaryotic Microorganisms

Core Concepts of Eukaryotic Cells in Microbiology

• Contextual Overview: While human anatomy and physiology courses focus on eukaryotic cells because humans are eukaryotic, microbiology focuses on eukaryotic microorganisms, specifically fungi and protozoa.

• Comparison to Prokaryotes:

  • Prokaryotic Cells: These cells, such as bacteria, lack a nucleus. Their genetic material is located in a nucleoid region within the cytoplasm. They also lack membrane-bound organelles.

  • Eukaryotic Cells: These cells possess a true nucleus where DNA is housed. They contain numerous membrane-bound organelles, including the Golgi apparatus, mitochondria, lysosomes, vacuoles, and the endoplasmic reticulum.

  • Complexity: Eukaryotic cells are much more internally complex than prokaryotic cells. Prokaryotic cells are relatively "hollow" internally, containing mainly genetic material, ribosomes, and cytoplasm.

• Standard Organelles: Certain structures are found in all eukaryotic cells (humans, animals, plants, fungi, and protozoa) as a baseline, including the nucleus, lysosomes, and the endoplasmic reticulum.

• Cell Wall Presence:

  • Nearly all bacteria possess a cell wall containing peptidoglycan.

  • In eukaryotes, cell wall presence varies. Humans lack them, while plants and fungi have them. These walls do not contain peptidoglycan but use other chemicals for structure.

External Appendages: Flagella and Cilia

• Flagella: Both prokaryotic and eukaryotic cells can have flagella for movement through aqueous environments. Both undergo chemotaxis—moving toward (positive) or away from (negative) chemicals.

• Key Differences in Flagella:

  • Complexity/Thickness: Prokaryotic flagella are thin, less complex, and consist of a single long filament. Eukaryotic flagella are much thicker and more complex, consisting of $11$ filaments called microtubules arranged in a specific $9+2$ microtubule arrangement.

  • Quantity: Prokaryotic cells typically have more flagella, ranging from one (monotrichous) to thousands (peritrichous). Eukaryotic cells typically have only one or two.

  • Motion: Prokaryotic flagella move in a circular motion (clockwise/counterclockwise sequences of runs and tumbles). Eukaryotic flagella move in a whip-like motion.

• Cilia: These are motor appendages exclusive to eukaryotic cells; bacteria do not have them.

  • Structure: Cilia have the same $9+2$ microtubule arrangement as flagella but are shorter and much more numerous (typically up to $2,000$ per cell).

  • Function: They move in a back-and-forth, door-like motion to propel organisms like protozoa through water.

  • Human Application: Cilia create a "ciliary escalator" in the respiratory system to push debris and microorganisms up toward the throat to be expelled or swallowed. Smoking, illness, and aging can damage or destroy these cilia, leading to a mechanical "smoker's cough" or higher frequency of coughing in the elderly. Cilia are also found in the female fallopian tubes to move eggs.

Surface and Boundary Structures

• Glycocalyx: Both cell types can have a glycocalyx consisting of a slime layer or a capsule.

  • Slime Layer: Loosely associated; protects against dehydration.

  • Capsule: Tightly bound; protects against phagocytosis.

• Cell Wall: Provided for shape, structure, and support.

  • Bacterial Composition: Contains peptidoglycan.

  • Eukaryotic Composition: Does not contain peptidoglycan. Fungi cell walls are made of chitin ($C-H-I-T-I-N$). Plants have cell walls made of cellulose.

• Cell Membrane (Cytoplasmic/Plasma Membrane):

  • Function: Involved in transport and selective permeability (controlling what enters and leaves).

  • Structure: Composed of a double layer of phospholipids with embedded proteins.

  • Key Eukaryotic Difference: Eukaryotic membranes contain sterols, such as cholesterol. These provide rigidity and stability, especially in cells like human cells that lack a cell wall to prevent them from caving in.

Internal Organelles and the Biological Assembly Line

• Nucleus: The site for housing genetic material. It is surrounded by a nuclear envelope with nuclear pores that allow ribosomes or RNA to exit.

• Endoplasmic Reticulum (ER): Primarily involved in transport and storage. It extends directly off the nucleus.

  • Rough ER: Studded with ribosomes (giving a rough texture). It is the site of protein synthesis, storage, and transport.

  • Smooth ER: Lacks ribosomes. Involved in the storage and transport of non-protein molecules, such as lipids used for cell membranes.

• Golgi Apparatus: A processing factory made of membrane-bound organelles. It receives products from the ER, chemically modifies/finalizes them, and packages them into vesicles for their final destination.

• Vesicle Transport:

  • Transitional Vesicles: Run from the endoplasmic reticulum to the Golgi apparatus.

  • Condensing Vesicles: Run from the Golgi apparatus to the final location (e.g., cell membrane for excretion or to a lysosome).

• The Assembly Line:

  1. DNA in the nucleus contains instructions (recipes) for proteins.

  2. Transcription: DNA is turned into messenger RNA (mRNA).

  3. Translation: mRNA links with a ribosome (potentially on the Rough ER) to build a protein.

  4. Protein is stored in the ER and sent via transitional vesicles to the Golgi.

  5. The Golgi finalizes the protein and sends it via condensing vesicles to its final destination.

• Vacuoles and Lysosomes: Function as the digestive system.

  • Vacuoles: Membrane-bound storage compartments for things needing breakdown (nutrients or engulfed pathogens like bacteria).

  • Lysosomes: Contain digestive enzymes. They fuse with vacuoles to break down their contents.

• Mitochondria: Known as the powerhouses of the cell where energy is made. Eukaryotes use mitochondria for energy; bacteria use their cell membrane.

• Ribosomes: The site of protein synthesis via translation.

  • Prokaryotic Size: $30S + 50S = 70S$.

  • Eukaryotic Size: $60S + 40S = 80S$.

  • Location: Prokaryotic ribosomes are dispersed in cytoplasm. Eukaryotic ribosomes are in the cytoplasm, on the Rough ER, and inside mitochondria.

The Cytoskeleton

Found in both prokaryotic and eukaryotic cells, providing an internal framework composed of three types of filaments:

1. Actin Filaments: Located just underneath the cell membrane. Responsible for shape changes and a "crawling-like" motion.

2. Microtubules: Long filaments acting as railroad tracks throughout the cytoplasm. They move vesicles (like transitional vesicles) from one location to another.

3. Intermediate Filaments: Responsible for anchoring organelles to the cell membrane so they do not float freely and get jumbled.

Mycology: The Study of Fungi

• Characteristics: Fungi are highly adaptable. Unlike most bacteria, they thrive in harsh conditions: lower (acidic) pH and high salt concentrations (e.g., on human skin).

• Nutrition: Fungi are chemohedrotrophic (other-feeders), obtaining carbon and energy by feeding off other living organisms (plants, animals, bacteria).

• Groups by Size:

  • Macroscopic Fungi: Large enough to see with the naked eye (Mushrooms).

  • Microscopic Fungi: Require a microscope (Yeast and Molds).

• Yeast:

  • Shape: Oval and round.

  • Reproduction: Budding (pushing a new daughter cell out of the membrane).

  • Pseudohyphae: A chain of yeast cells that stay stuck together after budding (similar to streptococcus bacteria).

  • Facultative Anaerobes: Can survive with or without oxygen. In the absence of oxygen, they undergo fermentation, producing alcohol as a byproduct (used for beer and wine).

• Molds and Fleshy Fungi:

  • Hyphae: Long string-like filaments that make up molds.

  • Septate Hyphae: Divided into separate units by cross-walls called septae.

  • Coinciding Hyphae: One long continuous filament with no cross-walls (septa).

  • Mycelium: A large mass of intertwined hyphae.

  • Vegetative Mycelium: Underground; responsible for nutrient absorption.

  • Reproductive Mycelium: Above ground; stores spores in a capsule/head.

• Dimorphic Fungi: Fungi that can change shape between yeast-like (oval) and mold-like (filamentous) based on their environment.

  • Example: Chlamydia alpagam (also referred to as kidney alfanea in transcript) is a mold in the soil but takes a yeast form in the body during infection.

Fungal Reproduction and Mycosis

• Spore Types:

  • Sporangiospores: Spores encapsulated inside a capsule/sac (common in mushrooms).

  • Chandemia (Conidia): Free spores not encased in a sac, freely associated with the end of hyphae. These spread easily by wind.

• Mycosis (Fungal Infection) Categories:

  • Superficial: Targets keratinized tissue (hair, skin, nails, e.g., athlete's foot, toenail fungus).

  • Subcutaneous: Deeper layer of the skin underneath the epidermis (e.g., eomycetoma).

  • Systemic: Targets internal organs (lungs, kidneys, brain). Usually indicates the fungus entered the blood (e.g., histoplasmosis in the lungs).

  • Opportunistic: Targets immunocompromised individuals (e.g., diabetics, AIDS patients, or those suffering from stress/poor diet).

Protozoology and Pathogenic Protozoa

• Characteristics: Eukaryotic, lack a cell wall, found in soil and water, and are other-feeders.

• Life Stages:

  1. Trophozoite: The active, growing, and dividing stage in a good environment with moisture and nutrients.

  2. Cyst: The dormant, encapsulated stage for surviving harsh environments (lack of moisture/nutrients).

• Pathogenic Examples:

  • Giardia lamblia: Causes Giardiosis. Exists as a cyst in soil/fecal matter. When ingested by a host (like a dog), it activates into a trophozoite in the gastrointestinal tract, causing diarrhea. It looks like it is "smiling" under a microscope due to two eyes and a mouth.

  • Trichomonas vaginalis: A sexually transmitted protozoan. It does not form a cyst. Therefore, it must go immediately from one good environment to another via direct contact; it dies within minutes on dry surfaces.

  • Plasmodium: The protozoan that causes Malaria. It is spread by mosquitoes and targets red blood cells, feeding on hemoglobin. This leads to severe anemia and potentially death.

Questions & Discussion

• Question: Does the run-and-tumble motion apply to eukaryotic cells?

• Answer: No, eukaryotic cells move differently; they do not use the specific run-and-tumble circular mechanism of bacteria.

• Question: How do you spell chitin?

• Answer: C-H-I-T-I-N.

• Discussion on Cilia: The instructor explained that cilia are often damaged by smoking or jewelry-like effects of illness, leading to a "smoker's cough" where debris must be mechanically coughed up because the natural elevator is broken.

• Discussion on Mushrooms and Mowing: The instructor noted that mowing over a single mushroom breaks the reproductive mycelium, releasing spores that can result in $50$ new mushrooms within $48$ hours.

• Discussion on Giardia: Giardia is common in dogs. Symptoms include stomach pain and diarrhea. It is treated with antibiotics (like Flagyl) and resolves in a few days. The instructor shared an anecdote about her dog repeatedly getting Giardia at a specific dog park pond.

• Discussion on Probiotics: The instructor advocated for taking probiotics or eating yogurt daily to help with gut issues and immunity, noting they are now found in everything from water to gummies and even cleaning supplies.