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, which are 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.

• Microbiology is essential for understanding disease, biotechnology, ecological balance, and food production.

• Applications include antibiotic development, fermentation, genetic engineering, and environmental remediation.

Major Groups of Microorganisms

• Bacteria: Prokaryotic, unicellular, diverse metabolic types, found in nearly all environments.

• Archaea: Prokaryotic, often extremophiles, distinct from bacteria in genetics and membrane chemistry.

• Fungi: Eukaryotic, includes yeasts (unicellular) and molds (multicellular), decomposers.

• Protozoa: Eukaryotic, unicellular, motile, often aquatic, some are pathogens.

• Algae: Eukaryotic, photosynthetic, aquatic, important oxygen producers.

• Viruses: Acellular, require host cells to replicate, consist of nucleic acid and protein coat.

Prokaryotic vs. Eukaryotic Cells

• Prokaryotes: Lack a nucleus and membrane-bound organelles; DNA is circular and located in the nucleoid.

• Eukaryotes: Have a true nucleus and membrane-bound organelles; DNA is linear and contained within the nucleus.

• Examples: Bacteria and archaea are prokaryotes; fungi, protozoa, algae, and helminths are eukaryotes.

Historical Development of Microbiology

• Robert Hooke: First to observe cells in cork, coined the term "cell."

• Antonie van Leeuwenhoek: First to observe living microorganisms using a simple microscope.

• Louis Pasteur: Disproved spontaneous generation with swan-neck flask experiments; developed pasteurization.

• 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, supporting the theory of biogenesis.

Koch’s Postulates

• A set of criteria to establish a causative relationship between a microbe and a disease:

  1. The microorganism must be found in all cases of the disease.

  2. It must be isolated and grown in pure culture.

  3. The cultured microbe must cause disease when introduced into a healthy host.

  4. The same microbe must be re-isolated from the experimentally infected host.

Microorganisms in Ecological Balance

• Nutrient cycling: Decomposition, nitrogen fixation, and recycling of elements.

• Photosynthesis: Algae and cyanobacteria produce oxygen and organic matter.

• Decomposition: Fungi and bacteria break down dead matter, returning nutrients to the environment.

Normal Microbiota and Human Health

• Normal microbiota are the microorganisms that colonize the human body without causing disease.

• They play roles in digestion, immune system development, and protection against pathogens.

Functional Anatomy of Prokaryotic and Eukaryotic Cells

Prokaryotic vs. Eukaryotic Cell Structure and Function

• Prokaryotic cells are generally smaller, lack a nucleus, and have simpler internal structures.

• Eukaryotic cells are larger, have a nucleus, and possess complex organelles.

Shapes and Arrangements of Bacterial Cells

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

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

• Spirilla: Spiral-shaped.

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

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: Long, whip-like structures for motility.

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

• Pili: Involved in conjugation (DNA transfer) and attachment.

Gram-Positive vs. Gram-Negative Bacterial 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 often more resistant to antibiotics due to the outer membrane.

Plasma Membrane Structure 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.

Bacterial Motility: Flagella and Axial Filaments

• Flagella: Rotate to propel 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: 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.

Prokaryotic vs. Eukaryotic Ribosomes

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

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

Major Eukaryotic Cell Organelles

• Nucleus: Contains genetic material (DNA).

• Endoplasmic reticulum (ER): Rough ER synthesizes proteins; smooth ER synthesizes lipids.

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

• Mitochondria: Site of ATP production (cellular respiration).

• Chloroplasts: Site of photosynthesis in plants and algae.

• Lysosomes: Contain digestive enzymes for breakdown of waste.

The Eukaryotes: Fungi, Algae, Protozoa, and Helminths

Major Groups of Eukaryotic Microorganisms

• Fungi: Yeasts (unicellular), molds (multicellular), reproduce by spores, absorb nutrients.

• Algae: Photosynthetic, aquatic, produce oxygen, can be unicellular or multicellular.

• Protozoa: Unicellular, motile, heterotrophic, reproduce sexually or asexually.

• Helminths: Multicellular parasitic worms (flatworms and roundworms).

General Characteristics of Fungi

• Yeasts: Unicellular, reproduce by budding or fission.

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

• Nutritional adaptations: Absorptive heterotrophs, can grow in acidic, low-moisture environments.

Fungal Cell Walls vs. Bacterial Cell Walls

• Fungal cell walls: Composed mainly of chitin.

• Bacterial cell walls: Composed of peptidoglycan.

Asexual vs. 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).

Medically Important Fungi and Diseases

• Candida albicans: Causes candidiasis (yeast infections).

• Aspergillus: Causes aspergillosis (lung infections).

• Cryptococcus: Causes cryptococcosis (meningitis).

Key Features of Protozoa

• Motility structures: Flagella, cilia, pseudopodia.

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

• Life cycles: Often complex, may include cyst and trophozoite stages.

Major Protozoan Pathogens and Transmission

• 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; flattened bodies.

• Roundworms (Nematoda): Cylindrical, unsegmented bodies.

Helminth Life Cycles and Disease Transmission

• Life cycles often involve multiple hosts and larval stages.

• Transmission can occur via ingestion, skin penetration, or insect vectors.

Arthropods as Disease Vectors

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

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

Significance of Eukaryotic Microbes

• Essential in nutrient cycling, food production, and biotechnology (e.g., antibiotics, enzymes).

• Cause important human, animal, and plant diseases.

Viruses, Viroids, and Prions

General Properties of Viruses

• Viruses are acellular infectious agents that require host cells for replication.

• They lack metabolism and cannot reproduce independently.

Structure and Chemical Composition of Viruses

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

• Capsid: Protein coat surrounding the nucleic acid.

• Envelope: Lipid membrane derived from host cell (not present in all viruses).

• Spikes: Glycoproteins for attachment to host cells.

Classification of Viruses

• Based on host range (animals, plants, bacteria), morphology (shape, presence of envelope), and nucleic acid type.

• Examples: DNA viruses (e.g., Herpesviridae), RNA viruses (e.g., Retroviridae).

Multiplication Cycles of Bacteriophages

• Lytic cycle: Virus replicates and lyses host cell.

• Lysogenic cycle: Viral DNA integrates into host genome as a prophage, replicates with host.

Replication of Animal Viruses vs. Bacteriophages

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

• Uncoating is a step 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: Virus remains in host for long periods, often with ongoing replication (e.g., HIV).

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

Viral Latency and Reactivation

• Some viruses integrate into host DNA or remain as episomes, reactivating under stress or immunosuppression.

• Examples: Herpes simplex virus, HIV.

Viruses and Cancer (Oncogenic Viruses)

• Some viruses can induce cancer by integrating into host DNA and disrupting normal cell regulation.

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

Prions: Structure and Replication

• Prions are infectious proteins that cause neurodegenerative diseases (e.g., Creutzfeldt-Jakob disease).

• They lack nucleic acids and replicate by inducing misfolding of normal proteins.

Prevention and Control of Viral Infections

• Vaccination stimulates immunity against specific viruses.

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

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