1. Microbial Evolution
Evolution in microbiology refers to how microorganisms originated, diversified, and adapted to various environments.
Thank you for reading. Don't forget to subscribe & share!
a) Origin of Microorganisms
- Life is believed to have originated ~3.5–4.0 billion years ago.
- RNA world hypothesis: RNA was the first genetic material; microbes evolved from self-replicating RNA molecules.
- Fossil evidence: Stromatolites (cyanobacteria-like structures) are among the earliest microbial fossils.
b) Major Events in Microbial Evolution
- Prokaryotic cells (Archaea & Bacteria) were the first life forms.
- Photosynthetic bacteria (cyanobacteria) evolved oxygenic photosynthesis → led to the “oxygen revolution” (~2.4 billion years ago).
- Endosymbiotic theory: Eukaryotic cells evolved when ancestral eukaryotes engulfed prokaryotic cells, which became mitochondria and chloroplasts.
- Continuous genetic change occurs through mutation, horizontal gene transfer (HGT), and natural selection.
c) Evolutionary Forces in Microbes
- Mutation: Random changes in DNA.
- Horizontal gene transfer: Transformation, transduction, conjugation.
- Selection pressure: Antibiotic resistance, environmental adaptation.
- Speciation: Formation of new microbial species under isolation and adaptation.
2. Systematics in Microbiology
Systematics = Study of the diversity of organisms and their evolutionary relationships.
Components of Microbial Systematics
- Classification → arranging organisms into groups (taxa) based on similarities.
- Nomenclature → assigning names according to internationally accepted rules.
- Identification → determining whether a particular isolate belongs to a known taxon.
Approaches in Microbial Systematics
- Phenetic approach (classical): Based on morphology, physiology, biochemical traits.
- Genotypic approach: Based on DNA-DNA hybridization, GC content, sequence similarity.
- Phylogenetic approach: Based on evolutionary relatedness using rRNA sequencing (16S rRNA for prokaryotes, 18S rRNA for eukaryotes).
3. Microbial Taxonomy
Taxonomy = The science of classification, nomenclature, and identification of organisms.
a) Levels of Taxonomy (Hierarchical System)
- Domain → Kingdom → Phylum → Class → Order → Family → Genus → Species → Subspecies/Strain.
b) The Three-Domain System (Carl Woese, 1977)
Based on 16S rRNA sequencing:
- Bacteria → True prokaryotes (e.g., E. coli).
- Archaea → Methanogens, halophiles, thermophiles.
- Eukarya → Protists, fungi, plants, animals.
c) Nomenclature Rules
- Governed by the International Code of Nomenclature of Bacteria (ICNB).
- Names usually derived from Latin/Greek.
- Binomial system (Genus + species, italicized, e.g., Escherichia coli).
d) Modern Taxonomic Tools
- Molecular techniques: rRNA sequencing, whole-genome sequencing, multilocus sequence typing (MLST).
- Chemotaxonomy: Based on cell wall components, fatty acids, quinones.
- Numerical taxonomy: Use of computers to compare large sets of phenotypic traits.
4. Significance of Microbial Evolution, Systematics, and Taxonomy
- Evolution: Explains origin and adaptation of microbial diversity.
- Systematics: Provides framework for studying microbial ecology, biotechnology, and medicine.
- Taxonomy: Ensures accurate communication, identification of pathogens, and discovery of new species.
