Microbiology Study Guide: Foundations, Cell Structure, Eukaryotic Microbes, and Viruses

The Microbial World and You

Definition, Scope, and Importance of Microbiology

Microbiology is the study of microorganisms—organisms too small to be seen with the naked eye. This field encompasses a wide range of life forms and has profound impacts on science, medicine, industry, and the environment.

• Microorganisms include bacteria, archaea, fungi, protozoa, algae, and viruses.

• They play essential roles in nutrient cycling, decomposition, and as part of the normal microbiota in humans.

• Microbiology is crucial for understanding infectious diseases, biotechnology, and ecological balance.

Major Groups of Microorganisms

• Bacteria: Single-celled prokaryotes with diverse metabolic capabilities.

• Archaea: Prokaryotes distinct from bacteria, often found in extreme environments.

• Fungi: Eukaryotic organisms including yeasts and molds; important decomposers.

• Protozoa: Unicellular eukaryotes, often motile, with complex life cycles.

• Algae: Photosynthetic eukaryotes, important for oxygen production and as aquatic primary producers.

• Viruses: Acellular infectious agents requiring host cells for replication.

Prokaryotic vs. Eukaryotic Cells

• Prokaryotic cells lack a nucleus and membrane-bound organelles; DNA is in a nucleoid region.

• Eukaryotic cells have a true nucleus and various organelles (e.g., mitochondria, ER, Golgi apparatus).

• Prokaryotes include Bacteria and Archaea; eukaryotes include Fungi, Protozoa, Algae, and all multicellular organisms.

Historical Development of Microbiology

• Robert Hooke: First to describe cells (1665).

• Antonie van Leeuwenhoek: Observed living microorganisms using a simple microscope.

• Louis Pasteur: Disproved spontaneous generation, developed pasteurization, and supported the theory of biogenesis.

• Robert Koch: Established Koch’s postulates, linking specific microbes to specific diseases.

Pasteur’s Experiments and Biogenesis

• Pasteur’s swan-neck flask experiments showed that microorganisms do not arise spontaneously but come from other microbes (biogenesis).

• This refuted the theory of spontaneous generation.

Koch’s Postulates

• Set of criteria to establish a causative relationship between a microbe and a disease.

• Four main postulates, including isolation and re-infection experiments.

Microorganisms in Ecological Balance

• Microbes recycle nutrients (e.g., carbon, nitrogen cycles), perform photosynthesis, and decompose organic matter.

Normal Microbiota

• Normal microbiota are the collection of microbes living on and in the human body.

• They protect against pathogens, aid digestion, and contribute to immune system development.

Functional Anatomy of Prokaryotic and Eukaryotic Cells

Comparing Prokaryotic and Eukaryotic Cells

• Prokaryotes: No nucleus, single circular chromosome, 70S ribosomes, cell wall (peptidoglycan in bacteria), simple structure.

• Eukaryotes: Nucleus, multiple linear chromosomes, 80S ribosomes, membrane-bound organelles, complex structure.

Bacterial Shapes and Arrangements

• Cocci: Spherical (e.g., Staphylococcus).

• Bacilli: Rod-shaped (e.g., Escherichia coli).

• Spirilla: Spiral-shaped.

• Arrangements: chains (strepto-), clusters (staphylo-), pairs (diplo-).

Key Prokaryotic Cell Structures

• Cell wall: Provides shape and protection; composed of peptidoglycan in bacteria.

• Plasma membrane: Selectively permeable barrier; site of metabolic processes.

• Ribosomes: Sites of protein synthesis (70S in prokaryotes).

• Flagella: Motility structures; rotate to propel the cell.

• Fimbriae: Short, hair-like structures for attachment.

• Pili: Longer structures involved in conjugation (DNA transfer).

Gram-Positive vs. Gram-Negative Cell Walls

• Gram-positive: Thick peptidoglycan layer, teichoic acids, stains purple.

• Gram-negative: Thin peptidoglycan, outer membrane with lipopolysaccharide (LPS), stains pink/red.

• Implications: Gram-negative bacteria are generally more resistant to antibiotics due to the outer membrane.

Plasma Membrane and Transport

• Structure: Phospholipid bilayer with embedded proteins.

• Passive transport: Movement down concentration gradient (diffusion, osmosis, facilitated diffusion).

• Active transport: Requires energy (ATP) to move substances against gradient.

Motility: Flagella and Axial Filaments

• Flagella: Rotate to move the cell; arrangement varies (monotrichous, lophotrichous, peritrichous).

• Axial filaments: Found in spirochetes; enable corkscrew motion.

• Chemotaxis: Movement toward or away from chemical stimuli.

Endospores: Formation and Function

• Endospores are highly resistant, dormant structures formed by some bacteria (e.g., Bacillus, Clostridium).

• Sporulation: Process of endospore formation under stress.

• Germination: Return to vegetative state when conditions improve.

Ribosomes: Prokaryotic vs. Eukaryotic

• Prokaryotic ribosomes: 70S (50S + 30S subunits).

• Eukaryotic ribosomes: 80S (60S + 40S subunits); mitochondria and chloroplasts have 70S ribosomes.

Major Eukaryotic Cell Organelles

• Nucleus: Contains DNA, site of transcription.

• Endoplasmic reticulum (ER): Rough ER (protein synthesis), smooth ER (lipid synthesis).

• Golgi complex: Modifies, sorts, and packages proteins and lipids.

• Mitochondria: Site of ATP production via cellular respiration.

• Chloroplasts: Site of photosynthesis in plants and algae.

• Lysosomes: Contain digestive enzymes for breakdown of macromolecules.

The Eukaryotes: Fungi, Algae, Protozoa, and Helminths

Major Groups of Eukaryotic Microorganisms

• Fungi: Includes yeasts (unicellular) and molds (multicellular); cell walls contain chitin.

• Algae: Photosynthetic, aquatic organisms; cell walls often contain cellulose.

• Protozoa: Unicellular, motile, lack cell walls; diverse life cycles.

• Helminths: Parasitic worms, including flatworms (Platyhelminthes) and roundworms (Nematoda).

General Characteristics of Fungi

• Yeasts: Unicellular, reproduce by budding or fission.

• Molds: Multicellular, composed of hyphae forming a mycelium.

• Reproduction: Asexual (spores, budding) and sexual (spores).

• Nutritional adaptations: Absorptive heterotrophs, often decomposers.

Fungal Cell Walls vs. Bacterial Cell Walls

• Fungal cell walls contain chitin; bacterial cell walls contain peptidoglycan.

• Chitin provides rigidity and resistance to environmental stress.

Asexual and Sexual Spore Formation in Fungi

• Asexual spores: Formed by mitosis (e.g., conidiospores, sporangiospores).

• Sexual spores: Formed by fusion of nuclei from two mating types (e.g., zygospores, ascospores, basidiospores).

• Example: Aspergillus produces conidiospores asexually.

Medically Important Fungi

• Candida albicans: Causes candidiasis (yeast infections).

• Aspergillus: Causes aspergillosis, especially in immunocompromised individuals.

• Cryptococcus: Causes cryptococcal meningitis.

Key Features of Protozoa

• Motility structures: Flagella, cilia, pseudopodia.

• Reproduction: Asexual (binary fission, budding) and sexual (conjugation).

• Complex life cycles, often involving multiple hosts.

Major Protozoan Pathogens

• Giardia: Causes giardiasis; transmitted via contaminated water.

• Plasmodium: Causes malaria; transmitted by Anopheles mosquitoes.

• Trypanosoma: Causes sleeping sickness; transmitted by tsetse flies.

Parasitic Helminths: Flatworms vs. Roundworms

• Flatworms (Platyhelminthes): Includes flukes and tapeworms; often have complex life cycles with multiple hosts.

• Roundworms (Nematoda): Cylindrical, unsegmented worms; many are intestinal parasites.

Helminth Life Cycles and Disease Transmission

• Life cycles often involve eggs, larvae, and adult stages.

• Transmission can occur via ingestion, skin penetration, or vector-borne routes.

Arthropods as Disease Vectors

• Arthropods (e.g., mosquitoes, ticks, fleas) transmit many eukaryotic and prokaryotic diseases.

• Examples: Malaria (mosquito), Lyme disease (tick).

Significance of Eukaryotic Microbes

• Essential in ecosystems (decomposition, photosynthesis).

• Medical importance (pathogens, antibiotics).

• Biotechnological applications (fermentation, enzyme production).

Viruses, Viroids, and Prions

General Properties of Viruses

• Viruses are acellular, obligate intracellular parasites.

• They lack metabolism and cannot reproduce independently.

• Composed of nucleic acid (DNA or RNA) and a protein coat (capsid); some have envelopes.

Structure and Chemical Composition of Viruses

• Nucleic acids: DNA or RNA, single- or double-stranded.

• Capsid: Protein shell surrounding the nucleic acid.

• Envelope: Lipid membrane derived from host cell (in some viruses).

• Spikes: Glycoproteins for attachment to host cells.

Classification of Viruses

• Based on host range (bacteria, plants, animals), morphology (helical, icosahedral, complex), and nucleic acid type.

• Examples: Bacteriophages infect bacteria; Influenza virus infects animals.

Multiplication Cycles of Bacteriophages

• Lytic cycle: Virus replicates and lyses host cell.

• Lysogenic cycle: Viral DNA integrates into host genome (prophage), can later enter lytic cycle.

Replication of Animal Viruses vs. Bacteriophages

• Animal viruses often enter cells by endocytosis or membrane fusion; bacteriophages inject DNA.

• Uncoating step is unique to animal viruses.

Growth, Identification, and Quantification of Viruses

• Viruses are grown in cell cultures, embryonated eggs, or living animals.

• Identified by cytopathic effects, serology, or molecular methods.

• Quantified by plaque assays or counting infectious units.

Types of Viral Infections

• Acute: Rapid onset, short duration (e.g., influenza).

• Latent: Virus remains dormant, can reactivate (e.g., herpesviruses).

• Persistent: Continuous low-level production (e.g., hepatitis B).

• Slow: Gradual increase in damage over time (e.g., HIV).

Viral Latency and Reactivation

• Some viruses integrate into host DNA or persist as episomes.

• Reactivation can occur due to stress or immunosuppression (e.g., herpes simplex, HIV).

Viruses and Cancer (Oncogenic Viruses)

• Some viruses can induce cancer by integrating oncogenes or disrupting host genes.

• Examples: Human papillomavirus (HPV), Epstein-Barr virus (EBV).

Prions: Structure and Replication

• Prions are infectious proteins lacking nucleic acids.

• Cause neurodegenerative diseases (e.g., Creutzfeldt-Jakob disease).

• Replicate by inducing misfolding of normal proteins.

Prevention and Control of Viral Infections

• Vaccination stimulates adaptive immunity.

• Antiviral drugs inhibit viral replication (e.g., reverse transcriptase inhibitors for HIV).

• Public health measures include sanitation, vector control, and surveillance.