Microbiology Study Guide: Prokaryotes, Eukaryotes, and Viruses

General Characteristics of Prokaryotic Organisms

Prokaryotic Cell Structure and Diversity

Prokaryotes are unicellular organisms lacking a nucleus and membrane-bound organelles. They are the most numerous and diverse cellular microbes, thriving in various habitats with sufficient moisture. Only a few prokaryotes cause disease in humans, and archaea are not known to cause disease. The endosymbiotic theory suggests mitochondria are derived from prokaryotes, as evidenced by the similarity of 70S ribosomes in prokaryotes and mitochondria.

• Ribosomes: Sites of protein synthesis; prokaryotes have 70S ribosomes, eukaryotes have 80S.

• Treatments: More available for prokaryotic diseases due to lower toxicity compared to eukaryotic diseases.

• Archaea: Not known to cause disease.

Reproduction of Prokaryotic Cells

All prokaryotes reproduce asexually, using several methods:

• Binary fission: Most common; one cell divides into two.

• Snapping division: Variation of binary fission.

• Reproductive spores: Similar to fungi; source of antibiotics.

• Budding: New cell grows from parent.

• Viviparity: Live offspring emerge from the mother cell (e.g., Epulopiscium).

Arrangements of Prokaryotic Cells

The arrangement of prokaryotic cells results from the planes in which cells divide and whether daughter cells separate. Microscopy is used to group bacteria, aiding diagnosis.

• Bacilli: Most abundant shape; often appear single or in chains (streptobacilli).

• Cocci: Many arrangements based on division planes:

  • One plane: Diplococci (pairs), Streptococci (chains)

  • Two planes: Tetrads (squares)

  • Multiple planes: Sarcinae (cubical packets)

  • Random planes: Staphylococci (clusters)

Modern Prokaryotic Classification

Classification is based on genetic relatedness of rRNA sequences, dividing life into three domains: Archaea, Bacteria, and Eukarya. Bergey’s Manual provides a systematic classification scheme.

Survey of Bacteria and Archaea

Low G + C Gram-Positive Bacteria

These bacteria have less than 50% guanine-cytosine content and are classified in the phylum Firmicutes. They include medically important genera such as Clostridia, Mycoplasmas, Bacillus, Listeria, Lactobacillus, Streptococcus, Enterococcus, and Staphylococcus.

• Clostridia: Spore-forming, obligate anaerobes; cause tetanus, botulism, gas gangrene, and C. diff infections.

• Mycoplasmas: Lack cell walls; cause pneumonia and urinary tract infections; "fried egg" appearance in culture.

• Bacillus: Endospore-forming aerobes; includes Bacillus thuringiensis (insecticide) and Bacillus anthracis (anthrax).

• Listeria: Psychrophile; contaminates refrigerated foods; can cause miscarriage in pregnant women.

• Lactobacillus: Normal flora; used in yogurt and probiotics.

• Streptococcus/Enterococcus: Cause cavities, pneumonia, and opportunistic infections.

• Staphylococcus: Common skin flora; produces toxins and enzymes; causes wound and heart infections.

Phylum/Class	G + C Percentage	Representative Genera	Special Characteristics	Diseases
Firmicutes: Clostridia	Low (<50%)	Clostridium	Obligate anaerobic rod, endospore formers	Tetanus, botulism, gas gangrene, severe diarrhea
Firmicutes: Mollicutes	Low (<50%)	Mycoplasma	Lack cell wall, pleomorphic, smallest free-living cells	Pneumonia, urinary tract infections
Firmicutes: Bacilli	Low (<50%)	Bacillus, Listeria, Lactobacillus, Streptococcus, Staphylococcus	Endospore formers, produce yogurt, butter, pickles, asexual spores, cocci in clusters	Anthrax, food poisoning, wound infections, pneumonia, strep throat, sinus infections
Actinobacteria	High (>50%)	Corynebacterium, Mycobacterium, Actinomyces, Nocardia, Streptomyces	Pleomorphic, filaments, degrade pollutants, produce antibiotics	Diphtheria, tuberculosis, leprosy, rare sinus infections

Gram-Negative Bacteria: Proteobacteria

Proteobacteria are the largest and most diverse group of Gram-negative bacteria, divided into five classes: Alpha-, Beta-, Gamma-, Delta-, and Epsilonproteobacteria.

• Alphaproteobacteria: Includes Rickettsia (typhus), Brucella (brucellosis), and Agrobacterium (plant tumors).

• Betaproteobacteria: Includes Neisseria (meningitis, gonorrhea), Bordetella (whooping cough), Burkholderia (lung infections).

• Gammaproteobacteria: Includes Legionella (Legionnaire’s disease), Coxiella (Q fever), Enterobacteriaceae (GI tract), Vibrioceae (cholera), Pasteurellaceae (Haemophilus).

• Pseudomonads: Soil bacteria; cause nosocomial infections; resistant to antibiotics.

• Epsilonproteobacteria: Includes Campylobacter (blood poisoning), Helicobacter pylori (ulcers, gastric cancer).

Family	Special Characteristics	Representative Genera	Typical Human Diseases
Enterobacteriaceae	Straight rods, oxidase negative, peritrichous flagella or nonmotile	Escherichia, Enterobacter, Serratia, Salmonella, Proteus, Shigella, Yersinia	Rarely pathogenic, gastroenteritis, urinary tract infection, plague, shigellosis
Vibrionaceae	Vibrios, oxidase positive, polar flagella	Vibrio	Cholera
Pasteurellaceae	Cocci or straight rods, oxidase positive, nonmotile	Haemophilus	Meningitis in children, middle ear infections, pneumonia

Characterizing and Classifying Eukaryotes

General Characteristics of Eukaryotic Microorganisms

Eukaryotic microorganisms include protozoa, fungi, algae, and water molds. They are more complex than prokaryotes, with DNA packaged as chromosomes in a nucleus. Eukaryotes reproduce both asexually and sexually, forming gametes and zygotes.

Reproduction of Eukaryotes

• Meiosis: Reduces chromosome number by half.

• Mitosis: Produces identical daughter cells.

• Cytokinesis: Division of cytoplasm; may be delayed in coenocytic cells (multinucleated).

• Schizogony: Multiple mitoses followed by cytokinesis, producing merozoites (seen in protozoa).

Classification of Eukaryotes

Classification has evolved from two kingdoms (Plantae and Animalia) to include Fungi, Protista, and multiple modern groups based on rRNA sequences.

Protozoa

General Characteristics and Morphology

Protozoa are unicellular, eukaryotic organisms lacking a cell wall. They inhabit aquatic environments and are highly diverse in morphology and life cycle stages.

• Contractile vacuoles: Pump water out to prevent cell rupture in hypotonic environments.

• Motility: Move by cilia, flagella, or pseudopods; apicomplexans are non-motile.

• Life cycle stages: Trophozoite (active, feeding), cyst (resting, dormant).

Classification of Protozoa

Protozoa are classified into six groups based on rRNA and distinguishing features.

Category	Distinguishing Features	Representative Genera
Parabasala	Parabasal body, single nucleus, lack mitochondria	Trichomonas
Diplomonadida	Two equal-sized nuclei, lack mitochondria, Golgi bodies, and peroxisomes	Giardia
Euglenozoa	Flagella with internal crystalline rod, disk-shaped mitochondria	Euglena, Trypanosoma, Leishmania
Alveolates	Alveoli, cilia, apical complex of organelles	Balantidium, Plasmodium, Paramecium, Dinoflagellates
Rhizaria	Threadlike pseudopodia, shells of silica	Foraminifera, Radiolaria
Amoebozoa	Lobe-shaped pseudopodia	Naegleria, Acanthamoeba, Entamoeba

Fungi

General Characteristics and Significance

Fungi are chemoheterotrophic eukaryotes with cell walls composed of chitin. They decompose dead organisms, recycle nutrients, and are used in food and drug production. About 30% cause diseases (mycoses) in plants, animals, and humans.

Morphology of Fungi

• Hyphae: Filamentous structures; may be septate (divided) or aseptate (coenocytic).

• Mycelium: Mass of hyphae.

• Dimorphism: Ability to change shape based on temperature; yeast at 37°C, mold at room temperature.

• Pseudohyphae: Yeast cells that fail to separate completely.

Reproduction of Fungi

• Asexual reproduction: Mitosis and cytokinesis; budding in yeasts, fragmentation in molds.

• Spore formation: Asexual spores (sporangiospores, conidia, chlamydospores); sexual spores (zygospores, ascospores, basidiospores).

Classification of Fungi

Division and Type of Sexual Spore	Comments	Representative Genera
Zygomycota (Zygospores)	Coenocytic (aseptate)	Rhizopus
Ascomycota (Ascospores)	Septate; some associated with cyanobacteria or green algae to form lichens	Claviceps, Neurospora, Penicillium, Saccharomyces, Tuber
Basidiomycota (Basidiospores)	Septate	Agaricus, Amanita, Cryptococcus

Other Eukaryotes: Parasitic Helminths and Arthropod Vectors

Arthropod Vectors

Arthropods such as ticks, fleas, lice, flies, mosquitoes, and kissing bugs serve as mechanical or biological vectors for various pathogens, including protozoa and viruses.

Characteristics of Viruses, Viroids, and Prions

Are Viruses Alive?

Viruses are miniscule, acellular infectious agents composed of protein and DNA or RNA. They cannot carry out metabolic pathways, grow, or reproduce independently, and must recruit host cell machinery for replication.

• Virion: Extracellular state; protein coat (capsid) surrounds nucleic acid.

• Capsid: Composed of capsomeres; provides protection and attachment to host cells.

• Envelope: Acquired from host cell membrane; contains viral glycoproteins (spikes).

• Genetic material: DNA or RNA, never both; may be single- or double-stranded, linear or circular.

Classification and Replication of Viruses

• Classification: Based on nucleic acid, envelope presence, shape, and size.

• Lytic cycle: Attachment, entry, synthesis, assembly, release; results in host cell lysis.

• Lysogeny: Viral genome integrates into host DNA; host reproduces normally until lysis.

• Animal viruses: Entry via direct penetration, membrane fusion, or endocytosis; DNA viruses replicate in nucleus, RNA viruses in cytoplasm.

• Latency: Animal viruses may remain dormant in host cells for years.

Viroids and Prions

• Viroids: Infectious RNA molecules lacking capsid; infect plants.

• Prions: Proteinaceous infectious particles; cause spongiform encephalopathies in mammals.

Infection, Infectious Diseases, and Epidemiology

Symbiotic Relationships

• Mutualism: Both organisms benefit (e.g., E. coli in human colon).

• Commensalism: One benefits, other unaffected (e.g., Staphylococcus epidermidis on skin).

• Parasitism: One benefits, other harmed (e.g., Mycobacterium tuberculosis in human lung).

Normal Microbiota

• Resident microbiota: Present throughout life; mostly commensal.

• Transient microbiota: Temporary; outcompeted by resident flora.

• Opportunistic pathogens: Cause disease when introduced to unusual sites or in immunocompromised hosts.

Reservoirs of Infection

• Animal reservoirs: Zoonoses; diseases spread from animals to humans.

• Human carriers: Asymptomatic individuals can transmit disease.

• Nonliving reservoirs: Soil, water, food.

Portals of Entry and Exit

• Entry: Skin, mucous membranes, placenta, parenteral route.

• Exit: Same as entry; pathogens leave in secretions or excretions.

Manifestations and Etiology of Disease

• Symptoms: Subjective (e.g., nausea).

• Signs: Objective (e.g., vomiting).

• Syndrome: Group of symptoms/signs.

• Koch’s Postulates: Criteria for identifying causative agents of disease.

Virulence Factors

• Adhesion: Attachment to host cells via fimbriae, spikes, or biofilms.

• Biofilms: Bacterial communities communicating via quorum sensing.

• Extracellular enzymes: Break down host barriers (e.g., hyaluronidase, collagenase).

• Toxins: Endotoxins (LPS), exotoxins (proteins).

• Antiphagocytic factors: Capsules, chemicals preventing phagocytosis.

Stages of Infectious Disease

• Incubation period

• Prodromal period

• Illness

• Decline

• Convalescence

Modes of Transmission

• Contact transmission: Direct, indirect, droplet.

• Vehicle transmission: Airborne, waterborne, foodborne, bodily fluids.

• Vector transmission: Biological (mosquitoes, ticks), mechanical (flies).

Epidemiology

• Incidence: Number of new cases.

• Prevalence: Total cases (new and existing).

• Endemic: Stable incidence.

• Sporadic: Few cases.

• Epidemic: Higher frequency than usual.

• Pandemic: Disease crosses continents.

Additional info: Academic context and explanations were added to ensure completeness and clarity for exam preparation.