Classification of Microorganisms

Classification of Microorganisms

Bacteria and Classification Systems

  • Mitochondria and Chloroplasts

    • Origin of Mitochondria:

    • Endosymbiotic origin is a concept which explains the presence of mitochondria in eukaryotic cells.

    • Origin of Chloroplasts:

    • Similar to mitochondria, chloroplasts originated from endosymbiotic relationships.

  • Figure 10.1 Three-Domain System:

    • Domains of life: Bacteria, Archaea, Eukarya.

    • Communities of early cells underwent horizontal gene transfer.

    • All organisms have evolved from cells formed over 3 billion years ago.

    • Conserved DNA: Ancestors’ DNA is preserved across generations.

  • Domain Eukarya:

    • Comprises Kingdoms Fungi, Plantae, and Animalia, alongside various protists.

    • Bacteria and Archaea are classified as prokaryotes.

Characteristics of Cellular Domains

  • Table 10.1: Characteristics of Archaea, Bacteria, and Eukarya

    • Archaea: Prokaryotic cells, no peptidoglycan in cell walls, branched carbon chains.

    • Bacteria: Prokaryotic cells, peptidoglycan present, branched carbon chains.

    • Eukarya: Has cell walls composed of carbohydrates; contains mitochondria and chloroplasts.

    • Protein Synthesis:

    • Archaea: First amino acid is Methionine.

    • Bacteria: First amino acid is Formylmethionine.

    • Eukarya: First amino acid is Methionine.

  • Antibiotic Sensitivity:

    • Archaea: No sensitivity; Bacteria: Yes; Eukarya: Varies.

Evolution and Historical Perspective

  • Taxonomy: The classification of organisms into groups based on similarities and differences.

  • Systematics (Phylogeny): The study of the evolutionary history of organisms.

  • History of Phylogenetic Relationships:

    • 1735: Linnaeus proposed kingdoms Plantae and Animalia.

    • 1800s: Bacteria, fungi classified under Plantae; Haeckel proposed Protista.

    • 1937: Concept of prokaryotes defined.

    • 1968: Murray introduced the Prokaryotae kingdom.

    • 1969: Whittaker proposed the five-kingdom system.

The Three Domains of Life

  • Woese’s Three Domains System:

    • Bacteria (Prokaryotes)

    • Archaea (Prokaryotes)

    • Eukarya (Eukaryotes)

  • Evidence of Common Ancestors:

    • Ancestor cells arose 3.5 to 4.5 billion years ago; led to Bacteria, Archaea, and later the nucleoplasm of eukaryotes.

  • Horizontal Gene Transfer across domains contributes to genetic diversity.

Origin of Eukaryotic Cells

  • Eukaryotic Development: Eukaryotic cells originated from prokaryotic cells, possibly through symbiogenesis.

    • The original nucleoplasmic cells had rudimentary nuclei that may have given rise to complex organelles.

  • Membrane Foldings:

    • Formation of nuclear membranes and organelles (ex: Endoplasmic Reticulum) through membrane folding.

    • Gemmata obscuriglobus: A bacterium with a true nucleus representing evolutionary significance.

Evidence Supporting Endosymbiotic Theory

  • Endosymbiotic Theory:

    • Proposes that eukaryotic cells evolved from interactions with prokaryotic cells.

    • Prokaryotic cells served as hosts for symbiotic bacteria forming organelles like chloroplasts and mitochondria.

    • **Key evidence: ** Mitochondria and chloroplasts have 70S ribosomes and possess circular DNA characteristic of prokaryotes.

Phylogenetic Trees and Relationships

  • Phylogenetic Tree Definition: A graphical representation of shared characteristics among organisms, suggesting common ancestry.

    • Higher organisms' relationships can be inferred based on fossil evidence, although microorganisms are less likely to be fossilized.

  • Molecular Clock Concept:

    • Useful for estimating the evolutionary time scale based on DNA mutation rates. Highly conserved genes mutate slower and can reveal divergence times.

Cladograms and Evolutionary Relationships

  • Cladograms:

    • Represent evolutionary relationships visually, where branch points indicate common ancestors.

  • Construction Steps:

    1. Alignment of small rRNA sequences among species.

    2. Calculation of percentage similarity between organisms.

    3. Drawing horizontal branches proportional to the similarity values.

Classification of Organisms

  • Scientific Nomenclature:

    • Binomial Nomenclature consists of the genus name and specific epithet (species).

    • Example: Escherichia coli, where Escherichia is the genus and coli is the species.

  • Taxonomic Hierarchy:

    • Organisms are categorized into the following hierarchy: Domain > Kingdom > Phylum > Class > Order > Family > Genus > Species.

Prokaryotic Classification

  • Prokaryotic Species: Defined as populations of cells with similar characteristics.

    • Pure cultures are often clones derived from singular cells, which theoretically should be identical.

    • Strains in a pure culture may exhibit non-identical characteristics.

Eukaryotic Classifications

  • Eukaryotic Kingdoms:

    • Protista: Autotrophic and heterotrophic, originally a large and diverse grouping of organisms.

    • Fungi: Chemoheterotrophic, with unicellular and multicellular forms, having cell walls made of chitin.

    • Plantae: Multicellular with cellulose in cell walls; primarily photosynthetic.

    • Animals: Multicellular, lacking cell walls, and chemoheterotrophic.

Classification of Viruses

  • Viruses: Defined as acellular entities that require a host for replication and are not classified within the domain framework.

    • Viral species refers to a population with similar characteristics occupying a specific niche.

Microorganism Classification and Identification Methods

  • Classification: Organizing organisms by shared traits and known specific characteristics in databases such as Bergey’s Manual.

  • Identifying Unknowns: Involves matching unknown organisms against defined characteristics from known databases.

  • Clinical Microbiology Requisition Forms: Used to obtain specimens and outline required tests.

Morphological and Biochemical Identification

  • Morphological Characteristics: Useful but don't necessarily provide insight into phylogenetic relationships.

  • Differential Staining: Methods such as Gram staining help identify different cell wall structures (Gram-positive vs. Gram-negative).

    • Fast Acid Staining: Specific for identifying acid-fast bacteria.

  • Biochemical Tests: Identify bacteria via enzymatic activity that reveals the presence of specific enzymes.

Advanced Microbial Identification Techniques

  • Automated Rapid Identification Systems: Utilize specific tests to yield results in hours, employing mass spectrometry (MALDI-TOF) for identifying bacterial pathogens incorporating genome analysis.

    • MALDI-TOF Explained: Utilizes mass spectrometry to detect organism identity based on molecular weight and characteristics of ribosomal proteins.

Serological Identification Methods

  • Serology Definition: The study of serum in relation to immune responses,

    • Agglutination Testing: Uses antisera to identify bacteria based on clumping which indicates a positive reaction.

  • ELISA: A sensitive technique to detect antibodies or antigens in a sample, utilizing enzyme-linked antibodies for signal detection.

Further Identification Techniques

  • Western Blotting: Used for detecting specific proteins via immunological methods.

  • Phage Typing: Involves using bacteriophages to identify bacterial strains based on susceptibility assays.

  • Fatty Acid Profiles: Characteristic profiles of fatty acids can help uniquely identify species based on their constant composition.

Nucleic Acid Techniques

  • DNA Base Composition: Used to compare species based on GC content, assisting in taxonomy.

  • DNA Fingerprinting: Involves restriction enzyme analysis of DNA fragments to reveal unique patterns for identification.

  • Nucleic Acid Amplification Tests (NAATs): Include PCR and its modifications, crucial for amplifying and detecting specific genetic sequences, even in low quantities.

Hybridization and Sequencing Techniques

  • DNA Hybridization: Techniques such as Southern Blotting can reveal genetic relatedness among different species.

  • DNA Microarrays: Useful for identifying multiple species or genes simultaneously.

  • Next-Generation Sequencing (NextGen): Rapid sequencing technology that can provide entire genome sequences quickly.

  • Fluorescent in Situ Hybridization (FISH): Allows for the detection of microorganisms directly without cultivation.

Ribotyping and Adding to Classifications

  • Ribotyping: Involves sequencing ribosomal RNA, providing a highly conserved marker useful for distinguishing species.