Eukaryotes of Microbiology: Structure, Function, and Pathogenic Protozoa

Introduction to Infectious Diseases

Overview

This section introduces the eukaryotes of microbiology, focusing on their cellular structures, distinguishing features, and their roles as pathogens. It covers the comparison between eukaryotic cells and prokaryotes, and details the major groups of pathogenic protozoa and their associated diseases.

Distinguishing Characteristics of Eukaryotic Cells

Comparison with Bacteria and Archaea

• Eukaryotic cells possess a true nucleus enclosed by a nuclear membrane, whereas bacteria and archaea are prokaryotic and lack a membrane-bound nucleus.

• Eukaryotes have membrane-bound organelles (e.g., mitochondria, endoplasmic reticulum, Golgi apparatus), while prokaryotes do not.

• Cell size: Eukaryotic cells are generally larger (10–100 μm) than prokaryotic cells (0.1–5 μm).

• Ribosomes: Eukaryotes have 80S ribosomes in the cytoplasm and rough ER, and 70S ribosomes in mitochondria and chloroplasts; prokaryotes have only 70S ribosomes.

Unique Characteristics of Eukaryotes

Cell Shapes and Structures

• Eukaryotic cells exhibit diverse shapes: spheroid (Chromulina), fusiform (Trypanosoma), bell-shaped (Vorticella), ovoid (Paramecium), and ring-shaped (Plasmodium ovale).

Nucleus and Nucleolus

• Nucleus: Contains the cell’s genetic material, surrounded by a nuclear envelope. The nucleolus is the site of rRNA synthesis and ribosome assembly.

• Nucleolus: Dense area within the nucleus, visible in electron micrographs.

Ribosomes

• Eukaryotic cells contain 80S ribosomes (membrane-bound and free) and 70S ribosomes in mitochondria and chloroplasts.

Endomembrane System

• Smooth endoplasmic reticulum (ER): Lipid biosynthesis, carbohydrate metabolism, detoxification.

• Rough ER: Protein synthesis via membrane-bound 80S ribosomes.

• Golgi apparatus: Processes and packages proteins and lipids, adds sugar molecules, produces glycoproteins/glycolipids.

• Lysosomes: Contain digestive enzymes for breaking down particles via endocytosis and phagocytosis.

Cytoskeleton

• Composed of microfilaments, intermediate filaments, and microtubules.

• Provides structural support and a network for intracellular transport.

Centrosomes

• Composed of two centrioles at right angles, each made of nine triplets of microtubules.

• Serve as microtubule-organizing centers during mitosis.

Mitochondria

• Double-membraned organelles; inner membrane forms cristae for increased surface area.

• Site of ATP production and metabolic activities.

Chloroplasts

• Site of photosynthesis in plants and algae.

• Contain stacks of thylakoids (grana) forming a third membrane layer.

Cell Membrane

• Composed of a lipid bilayer with embedded proteins; contains cholesterol for membrane stability.

• Glycoproteins and glycolipids are important for cell recognition and pathogen interaction.

• Transport mechanisms include endocytosis (phagocytosis, pinocytosis, receptor-mediated) and exocytosis.

• Some eukaryotes have a cell wall (fungi, algae, plants); animal cells have an extracellular matrix.

Flagella and Cilia

• Flagella: Structurally distinct from prokaryotic flagella; used for locomotion.

• Cilia: Shorter, used for locomotion, feeding, or movement of extracellular particles.

• Both are composed of a 9+2 array of microtubules.

Protists

General Characteristics

• Diverse group of eukaryotic organisms; may be unicellular or multicellular.

• Vary in nutrition, morphology, locomotion, and reproduction.

• Key structures: contractile vacuoles, cilia, flagella, pellicles, pseudopodia.

Examples

• Paramecium: Uses cilia for locomotion.

• Amoeba: Uses lobelike pseudopodia for movement and feeding.

• Euglena: Uses a flagellum for propulsion.

Protozoal Infections

Major Human Diseases

• Malaria (Plasmodium spp.)

• Leishmaniasis (Leishmania donovani)

• Amoebiasis (Entamoeba histolytica)

• Giardiasis (Giardia lamblia)

• Trichomoniasis (Trichomonas vaginalis)

• Trypanosomiasis (Trypanosoma brucei)

• Toxoplasmosis (Toxoplasma gondii)

Transmission Routes

• Person-to-person contact

• Ingestion of contaminated water or food

• Direct contact with the parasite

• Insect (mosquito) or tick bite

Risk Factors

• Immunocompromised patients (transplant recipients, leukemia, AIDS)

• Protozoal infections can be fatal in these populations.

Malaria

Significance and Epidemiology

• Most significant protozoal disease globally (350–500 million infected; 1–2 million deaths annually).

• Highest prevalence in sub-Saharan Africa, Southeast Asia, Latin America.

Pathogenesis and Transmission

• Caused by Plasmodium protozoa (four species).

• Transmitted by bite of infected female Anopheles mosquito.

• Can also be transmitted congenitally, via blood transfusion, or contaminated needles.

Life Cycle

• Sexual cycle occurs in the mosquito; asexual cycle occurs in humans.

• Medications are effective only during the asexual phase in humans.

Clinical Features

• Classic paroxysm: chills, rigors, fever (up to 40°C), diaphoresis, fatigue, sleep.

• Symptoms repeat every 48–72 hours; other symptoms include headache, nausea, joint pain.

Treatment

• Diagnosis must be confirmed before treatment.

• Treatment depends on Plasmodium species, patient status, and drug susceptibility.

Antimalarial Drugs

• Target the parasite during the asexual phase.

• Erythrocytic phase drugs: chloroquine diphosphate, hydroxychloroquine, quinine sulphate, quinidine sulphate, mefloquine.

• Primaquine phosphate: kills parasite in both phases.

• Drugs may be combined for synergistic effects.

Other Pathogenic Protozoa

Giardiasis

• Caused by Giardia lamblia; transmitted via contaminated water.

• Most common human intestinal parasite in the US.

African Trypanosomiasis

• Caused by Trypanosoma brucei; transmitted by tsetse fly.

• Life cycle involves both human and insect hosts.

Other Protozoal Diseases

• Leishmania donovani: Leishmaniasis

• Entamoeba histolytica: Amoebiasis

• Trichomonas vaginalis: Trichomoniasis

• Toxoplasma gondii: Toxoplasmosis

Pathogenic Protozoans Table

System Most Often Affected	Pathogenic Protozoan	Infection	Key Symptoms
Skin and eyes	Acanthamoeba spp., Leishmania spp.	Amoebic keratitis, Cutaneous leishmaniasis	Corneal ulceration, blindness, skin ulceration
Nervous system	Naegleria fowleri, Trypanosoma brucei	Amoebic meningoencephalitis, African sleeping sickness	Fatal destruction of brain tissue, extreme fatigue
Cardiovascular and lymphatic systems	Trypanosoma cruzi, Babesia microti, Toxoplasma gondii	Chagas disease, Babesiosis, Toxoplasmosis	Congestive heart failure, fever/chills/sweats, asymptomatic or flu-like symptoms
Gastrointestinal system	Plasmodium spp., Entamoeba, Balantidium coli, Giardia lamblia, Leishmania spp., Cryptosporidium spp.	Malaria, Amoebic dysentery, Balantidiasis, Giardiasis, Leishmaniasis, Cryptosporidiosis	Significant fetal damage, cycles of fever/chills, ulceration, colon perforation, persistent diarrhea, liver/spleen swelling, abdominal cramping
Urogenital system	Trichomonas vaginalis	Trichomoniasis	Profuse, yellow-green vaginal discharge, foul odor

Summary

This guide covers the essential features of eukaryotic cells, their organelles, and the major pathogenic protozoa relevant to microbiology. It provides a foundation for understanding infectious diseases caused by eukaryotic microorganisms and their clinical significance.