Exam 3: Microbiology Study Guide: Classification, Pathogenicity, and Disease

Classification and Identification of Microorganisms

The Three Domains of Life

The classification of all living organisms is based on three fundamental domains: Bacteria, Archaea, and Eukarya. Each domain possesses unique characteristics that distinguish them from one another.

• Bacteria: Prokaryotic cells, peptidoglycan cell walls, reproduce by binary fission, diverse metabolic pathways.

• Archaea: Prokaryotic cells, lack peptidoglycan, often inhabit extreme environments, unique membrane lipids.

• Eukarya: Eukaryotic cells, include kingdoms Plantae, Animalia, Fungi, and Protista.

Kingdom Classification for Eukaryotes:

• Plantae: Multicellular, photosynthetic, cell walls of cellulose.

• Animalia: Multicellular, heterotrophic, lack cell walls.

• Fungi: Unicellular or multicellular, chitin cell walls, absorb nutrients.

• Protista: Mostly unicellular, diverse modes of nutrition and reproduction.

Species Definitions

• Prokaryotic species: A population of cells with similar characteristics.

• Eukaryotic species: A group of organisms that can interbreed and produce fertile offspring.

• Viral species: A group of viruses sharing the same genetic information and ecological niche.

Binomial Nomenclature

The binomial naming system assigns each organism a two-part Latin name: genus and species (e.g., Escherichia coli). This system provides universal identification and classification.

Methods of Identifying Bacteria

• Morphological characteristics: Shape, arrangement, and structure.

• Staining: Gram stain, acid-fast stain, etc.

• Biochemical tests: Enzyme activity, metabolic capabilities.

• Other mechanisms: Serology, phage typing, fatty acid profiles, flow cytometry, DNA base composition, DNA fingerprinting, NAATs/PCR, nucleic acid hybridization, DNA chips.

Phylogenetic System and Relationships

Bergey’s Manual and Bacterial Classification

Bergey’s Manual of Systematic Bacteriology provides a phylogenetic framework for classifying bacteria based on genetic and phenotypic characteristics. The new system emphasizes evolutionary relationships.

• Bacteria are divided into subgroups based on genetic, metabolic, and structural features.

• Relationships among domains are determined by molecular data, such as ribosomal RNA sequences.

Eukaryotic Microorganisms

Defining Characteristics

• Fungi: Eukaryotic, chitin cell walls, reproduce by spores, absorb nutrients.

• Algae: Eukaryotic, photosynthetic, cellulose cell walls, aquatic habitats.

• Protozoa: Eukaryotic, mostly unicellular, motile, lack cell walls.

• Helminths: Eukaryotic, multicellular parasitic worms, complex life cycles.

Viruses, Viroids, and Prions

Basic Characteristics of Viruses

• Nonliving infectious agents, consist of nucleic acid (DNA or RNA) and protein coat (capsid).

• Obligate intracellular parasites; require host cells for replication.

Viral Species Definition

Viral species are defined by genetic similarity and ecological niche, differing from traditional species concepts based on reproduction.

Lytic vs. Lysogenic Cycles

• Lytic cycle: Virus replicates and lyses host cell, releasing new virions.

• Lysogenic cycle: Viral genome integrates into host DNA, replicates with host, may later enter lytic cycle.

Prions and Viroids

• Prions: Infectious proteins causing neurodegenerative diseases (e.g., Creutzfeldt-Jakob disease).

• Viroids: Infectious RNA molecules, cause plant diseases, lack protein coat.

Principles of Disease and Epidemiology

Key Terminology

• Pathology: Study of disease.

• Etiology: Cause of disease.

• Infection: Invasion and colonization by pathogens.

• Disease: Abnormal state in which body function is impaired.

Microbial Relationships

• Normal microbiota: Permanent residents, usually harmless.

• Transient microbiota: Temporary, may cause disease under certain conditions.

• Opportunistic microbes: Cause disease when host defenses are compromised.

Koch’s Postulates

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

• Significance: Foundation of medical microbiology.

Disease Classification

• Frequency: Sporadic, endemic, epidemic, pandemic.

• Severity/duration: Acute, chronic, latent.

• Extent: Local, systemic, focal.

Herd Immunity

Herd immunity occurs when a large portion of a population is immune, reducing disease spread. Important for controlling epidemics.

Sequence of Disease Development

• Incubation period

• Prodromal period

• Period of illness

• Period of decline

• Period of convalescence

Reservoirs of Infection

• Human: Carriers, symptomatic individuals.

• Animal: Zoonoses.

• Nonliving: Soil, water, fomites.

Methods of Disease Transmission

• Contact: Direct, indirect, droplet.

• Vehicle: Water, food, air.

• Vector: Arthropods (e.g., mosquitoes).

Hospital Acquired Infections (HAIs)

• Caused by opportunistic pathogens, invasive procedures, antibiotic resistance.

• Prevention: Hand hygiene, sterilization, isolation protocols.

Re-emerging Infectious Diseases

• Antibiotic resistance, changes in human behavior, environmental changes.

Microbial Mechanisms of Pathogenicity

Portals of Entry and Exit

• Entry: Skin, mucous membranes, parenteral route.

• Exit: Respiratory tract, gastrointestinal tract, genitourinary tract.

Adherence to Host Cells

• Microbes use adhesins (e.g., fimbriae, pili, surface proteins) to attach to host cells.

• Example: Streptococcus pyogenes uses M protein for adherence.

Role of Capsules and Cell Wall Components

• Capsules prevent phagocytosis.

• Cell wall components (e.g., M protein, mycolic acid) enhance pathogenicity.

Enzymes Contributing to Pathogenicity

Enzyme	Function
Coagulases	Clot fibrin, protect bacteria from immune system
Kinases	Break down clots, spread infection
Hyaluronidase	Breaks down connective tissue
Collagenase	Breaks down collagen
IgA protease	Destroys IgA antibodies

Siderophores

Siderophores are molecules secreted by bacteria to scavenge iron from the host, essential for bacterial growth.

Direct Damage vs. Toxin Production

• Direct damage: Pathogen invades and destroys host cells.

• Toxin production: Pathogen releases toxins that disrupt host function.

Exotoxins vs. Endotoxins

Type	Source	Properties
Exotoxins	Secreted by Gram-positive and Gram-negative bacteria	Specific, potent, protein-based
Endotoxins	Part of Gram-negative cell wall (lipid A of LPS)	General effects, less potent, heat stable

Mechanisms of Toxin Action

• A-B toxins: Two-part toxins; A (active) and B (binding) components.

• Membrane-disrupting toxins: Cause cell lysis by disrupting membranes.

• Superantigens: Trigger excessive immune response.