Classification of Microorganisms

Introduction to Taxonomy and Phylogeny

Taxonomy is the science of classifying organisms to reflect their evolutionary relationships and similarities. Systematics, or phylogeny, is the study of the evolutionary history of organisms. These disciplines provide a framework for organizing biological diversity and understanding the relationships among different groups of organisms.

• Taxonomy: The science of classification, arranging organisms into groups (taxa) based on similarities.

• Phylogeny: The evolutionary history and relationships among organisms.

• Taxon: A group or level of organization into which organisms are classified.

Historical Perspectives in Classification

The classification of living organisms has evolved over time, reflecting advances in scientific understanding and technology.

• Linnaeus (1735): Introduced the two-kingdom system (Plantae and Animalia).

• Von Nägeli & Haeckel (1800s): Proposed inclusion of bacteria and fungi in Plantae; Haeckel added Protista for microorganisms.

• Murray (1937): Introduced the term "prokaryote" for cells without a nucleus.

• Whittaker (1969): Proposed the five-kingdom system (Monera, Protista, Fungi, Plantae, Animalia).

• Woese (1978): Developed the three-domain system based on rRNA sequencing: Bacteria, Archaea, and Eukarya.

The Three-Domain System

The three-domain system, based on molecular data (especially rRNA sequences), divides all life into Bacteria, Archaea, and Eukarya. This system reflects fundamental differences in cell structure and genetics.

• Bacteria: Prokaryotic, cell walls contain peptidoglycan.

• Archaea: Prokaryotic, cell walls lack peptidoglycan, often live in extreme environments (e.g., methanogens, extreme halophiles, hyperthermophiles).

• Eukarya: Eukaryotic, includes kingdoms Fungi, Plantae, Animalia, and Protists.

Characteristics of the Three Domains

The domains Bacteria, Archaea, and Eukarya differ in several fundamental characteristics, including cell type, membrane lipids, and sensitivity to antibiotics.

Endosymbiont Theory

The endosymbiont theory explains the origin of eukaryotic cells from prokaryotic ancestors. According to this theory, certain organelles (mitochondria and chloroplasts) originated as free-living bacteria that were engulfed by ancestral eukaryotic cells.

• Infoldings of the plasma membrane formed the nuclear envelope.

• Endosymbiotic bacteria evolved into mitochondria and chloroplasts.

Phylogenetic Trees and Molecular Clocks

Phylogenetic trees group organisms based on common properties and evolutionary ancestry. Molecular clocks, such as rRNA and genome sequencing, are used to estimate evolutionary relationships by measuring the accumulation of mutations over time.

• Fossils and genomes provide evidence for evolutionary groupings.

• rRNA sequencing is especially useful for prokaryotes.

Classification and Nomenclature

Scientific Nomenclature

Scientific names are used to avoid confusion caused by regional and linguistic differences. Binomial nomenclature assigns each organism a two-part name: genus and species (e.g., Homo sapiens).

• Genus: Capitalized, italicized or underlined.

• Species (specific epithet): Lowercase, italicized or underlined.

• Names often reflect characteristics, discoverers, or locations.

Major Taxa

Organisms are classified into hierarchical taxa: Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species.

• Culture: Microorganisms grown in laboratory media.

• Clone: Population of cells derived from a single parent cell.

• Strain: Genetically different cells within a clone.

Classification of Eukaryotes

Eukaryotes are classified into four kingdoms: Protista, Fungi, Plantae, and Animalia.

• Protista: Mostly unicellular, nutritionally diverse, grouped by rRNA clades.

• Fungi: Chemoheterotrophic, cell walls of chitin, reproduce by spores or hyphal fragments.

• Plantae: Multicellular, cellulose cell walls, photosynthetic.

• Animalia: Multicellular, no cell walls, ingest organic matter.

Classification of Viruses

Viruses are not classified within the three domains because they are not composed of cells and require a host for replication. Viral species are defined by shared characteristics and ecological niches.

Methods of Classifying and Identifying Microorganisms

Bergey’s Manual

Bergey’s Manual of Systematics of Archaea and Bacteria provides the classification scheme for prokaryotes, while Bergey’s Manual of Determinative Bacteriology offers identification schemes based on morphology, staining, and biochemical tests.

Conventional Identification Methods

• Morphology: Useful for eukaryotes; limited for prokaryotes.

• Differential Staining: Gram and acid-fast stains distinguish cell wall types.

• Biochemical Tests: Identify bacteria based on enzyme activities (e.g., exoenzymes, endoenzymes).

Biochemical Tests

Biochemical tests are widely used to differentiate bacteria based on their metabolic properties. Rapid identification systems can perform multiple tests simultaneously, providing results within 4 to 24 hours.

• Results are often assigned numerical codes for database comparison.

• Automated systems use mass spectrophotometry to analyze protein profiles.

Serological Methods

Serology studies immune responses in serum. Microorganisms are antigenic and stimulate antibody production. Serological tests use known antibodies to identify unknown bacteria.

• Slide Agglutination Test: Bacteria clump when mixed with specific antibodies.

• ELISA (Enzyme-Linked Immunosorbent Assay): Detects antigens (direct ELISA) or antibodies (indirect ELISA) in patient samples.

Phage Typing

Phage typing determines which bacteriophages a bacterium is susceptible to. Clearings (plaques) on a bacterial lawn indicate lysis by specific phages, useful for tracing infection sources.

Flow Cytometry

Flow cytometry separates and analyzes cells based on size and fluorescence. It allows rapid identification without culturing by detecting differences in electrical conductivity or fluorescence.

Molecular Methods

• Whole Genome Sequencing: Compares DNA base composition (G+C content) to assess relatedness.

• Nucleic Acid Hybridization: Measures the ability of DNA from different organisms to hybridize; >70% hybridization indicates same species.

• Nucleic Acid Amplification Tests (NAATs): Use PCR to amplify DNA for identification, including detection of unculturable microbes.

• Southern Blotting: Uses DNA probes to identify microorganisms.

• DNA Chips (Microarrays): Detect pathogens by hybridization with fluorescently labeled probes.

• Ribotyping and FISH: Use rRNA sequencing and fluorescent probes to identify and quantify microorganisms in samples.

Dichotomous Keys and Cladograms

Dichotomous keys are tools for identification based on a series of choices that lead to the correct name of an organism. Cladograms are branching diagrams showing evolutionary relationships and are used for classification rather than identification.

Summary Table: Major Methods for Microbial Classification and Identification