Unit 1: History of Microbiology
History of Microbiology
Microbiology is the study of living organisms that are too small to be seen with the naked eye, including bacteria, archaea, viruses, fungi, and protozoa.
Major Discoveries and Contributors
- Antonie van Leeuwenhoek (1632-1723): The "Father of Microbiology." He was the first to observe and describe single-celled organisms, which he called "animalcules," using his high-powered single-lens microscopes.
- Louis Pasteur (1822-1895):
- Disproved spontaneous generation with his famous swan-neck flask experiments, proving "life comes from life."
- Developed pasteurization, a process of mild heating to kill pathogens and spoilage microbes in milk and wine.
- Developed vaccines for anthrax and rabies.
- Helped establish the Germ Theory of Disease.
- Robert Koch (1843-1910):
- Provided the first definitive proof for the Germ Theory, linking a specific microbe (Bacillus anthracis) to a specific disease (anthrax).
- Developed Koch's Postulates, a set of criteria to establish the cause of an infectious disease.
- Pioneered solid culture media (using agar) to isolate pure colonies.
Koch's Postulates:
- The microorganism must be found in all organisms suffering from the disease, but should not be found in healthy organisms.
- The microorganism must be isolated from a diseased organism and grown in pure culture.
- The cultured microorganism should cause the same disease when introduced into a healthy organism.
- The microorganism must be re-isolated from the inoculated, diseased experimental host.
Microbial Classification
Classification systems have evolved as our understanding of life has improved.
Haeckel's 3-Kingdom System (1866)
Ernst Haeckel proposed a three-kingdom system, which was one of the first to formally recognize microbes.
- Plantae (Plants)
- Animalia (Animals)
- Protista (All other "simple" organisms, including bacteria, fungi, algae, and protozoa)
Whittaker's 5-Kingdom System (1969)
Robert Whittaker's system became the standard for decades. It was based on cell type (Prokaryotic vs. Eukaryotic) and nutritional mode (Autotroph vs. Heterotroph).
- Monera: Prokaryotic, unicellular (e.g., Bacteria).
- Protista: Eukaryotic, unicellular (e.g., Amoeba, Algae).
- Fungi: Eukaryotic, multicellular (mostly), heterotrophic (saprophytic).
- Plantae: Eukaryotic, multicellular, autotrophic (photosynthetic).
- Animalia: Eukaryotic, multicellular, heterotrophic (ingestive).
Woese's 3-Domain System (1977)
Carl Woese revolutionized classification using 16S ribosomal RNA (rRNA) sequencing. This molecular approach revealed a new, deep split in the prokaryotic world. This is the current, most accepted system.
- Domain Bacteria: True bacteria (prokaryotic).
- Domain Archaea: Prokaryotic microbes, often "extremophiles" (live in salt, heat, etc.). Genetically distinct from bacteria.
- Domain Eukarya: All eukaryotes (plants, animals, fungi, protists).
Exam Tip: Know the basis for each system.
- Whittaker: Based on morphology and nutrition (Prokaryote vs. Eukaryote, Autotroph vs. Heterotroph).
- Woese: Based on genetics (16S rRNA sequencing).
Overview of Major Groups of Microorganisms
Bacteria (Domain Bacteria)
- Cell Type: Prokaryotic.
- Cell Wall: Contains peptidoglycan.
- Reproduction: Binary fission.
- Nutrition: Diverse; can be photoautotrophs, chemoautotrophs, or heterotrophs.
Algae (Domain Eukarya)
- Cell Type: Eukaryotic.
- Cell Wall: Contains cellulose.
- Reproduction: Asexual and sexual.
- Nutrition: Photoautotrophic (photosynthetic).
- Note: Includes unicellular (e.g., diatoms) and multicellular (e.g., seaweed) forms.
Fungi (Domain Eukarya)
- Cell Type: Eukaryotic.
- Cell Wall: Contains chitin.
- Reproduction: Asexual (spores) and sexual.
- Nutrition: Heterotrophic (saprophytic - decomposers, or parasitic).
- Note: Includes unicellular (yeast) and multicellular (molds) forms.
Protozoa (Domain Eukarya)
- Cell Type: Eukaryotic.
- Cell Wall: Absent.
- Reproduction: Asexual and sexual.
- Nutrition: Heterotrophic (ingestive). Often motile (e.g., *Amoeba*, *Paramecium*).
Viruses
- Cell Type: Acellular (not cells). Not in any Domain or Kingdom.
- Structure: A nucleic acid (DNA or RNA, never both) enclosed in a protein coat (capsid).
- Reproduction: Obligate intracellular parasites (must hijack a host cell).
Comparison Table
Unique Features of Viruses
Viruses are unique from all cellular life (Bacteria, Archaea, Eukarya).
- Acellular: They are not cells. They are infectious particles.
- Obligate Intracellular Parasites: They cannot replicate, metabolize, or perform any life functions on their own. They must invade a living host cell and use its machinery (e.g., ribosomes) to make new viruses.
- Genetic Material: Their genome can be DNA or RNA, and it can be single-stranded or double-stranded. Cellular life always has double-stranded DNA. A virus never has both DNA and RNA.
- No Metabolism: They have no ribosomes, no mitochondria, and no metabolic pathways. They are metabolically inert outside a host cell.