Introduction to Microbiology


General Characteristics of Micro-organisms

Micro-organisms play an important role in the Carbon, Oxygen, Nitrogen and Sulpher cycles. They serve as a source of nutrients at the baseof all food chains. 

General Characteristics:

  • Microscopic (in general)
  • Independent units (many are unicellular)
  • Not simple in structure, but less complex than plants & animals
  • Rapid growth rates
  • Ubiquitous (present, appearing)
  • Diversity in size, complex functions
  • Microbial diversity - structure, function, roles in nature
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Historical Importance of Micro-organisms

Plague - Yersinia pestis, HIV/AIDS


  • Antonie van Leeuwenhoek (1632-1723) - Amateur microscopist - First person to observe and describe micro-organisms accurately.
  • Robert Koch (1843-1910) - German physician - First to produce evidence that bacteria could cause disease (using mice and Bacillus anthracis) -First growth medium of boiled potatoes solidified with gelatine (gelatine is hydrolysed bymany bacteria and melts at 28 degrees) - 1882, isolated Mycobacterium tuberculosis - golden age began...


  • Respective micro-organism must be present in every sick individual (absent in every healthy one)
  • Micro-organism must be isolated and grown in pure culture.
  • Same disease must result when isolated organism is inoculated in healthy host.
  • Same micro-organism must be isolated again from diseased host. 
  • Fannie Hesse - wife of Koch's assistant - Idea of agar (melts at 100 degrees)
  • Richard Petri - Koch's assistant - developed the Petri dish. 
  • Louis Pasteur - discovered that chicken cholera bacterium loses ability to become infectious with age.
  • Edward Jenner (1798) - immuisation against smallpox with cowpox - immunisations with vaccines (pus).
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Importance of Microbes


  • Were largely responsible for creating atmospheric and soil properperties bya. 
  • With fungi, play a large role in decomposition and fixation where C, N, P, S etc released back to environment by decomposers & elements from non-living environ. are returned to living organisms.
  • Organic compounds produced from CO2, serve as building blocks for heterotrophic organisms (animals, fungi, non-photosynthetic protists.)
  • Cyanobacteria - thought to have added O2to atmosphere as a by-product of photosynthesis. 
  • Only certain prokaryotes can reduce free nitrogen gas to ammonia (N2 to NH3) - used to build amino acids, DNA etc. When these orgs die, other prokaryotes (denitrifiers) return N to atmosphere, thus completing the cycle.
  • In aquatic environments, N2 fixation carried out by Cyanobacteria (Anabaena) - contain hetereocysts - specialised cells impermeable to oxygen.
  • In soil, N fixation occurs in plant roots harbouring symbiotic colonies of N-fixing bacteria (eg: Rhizobium with Legumes) - MUTUALISM - Bacteria supply N to plant and plant supplies sugars etc to bacteria.
  • Many bacteria live symbiotically in digestive tracts of animals (eg: cellulase-producing bacteria - cows - digestion of cellulose-rich grasses) 
  • Human intestinal tract - Bacteria aid in digestion and produce vitamins (B12, K) - influence: our maturation, immune system development, metabolism, production of essential biocompounds.


  • Genera, Family, Order, Class, Phylum, Kingdom, Domain.
  • 3 Domains: Archae, Bacteria, Eukarya. (viruses don't feature - not living - considered fragments.
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Domain Eukarya

Domain Eukarya or Eukaryota

Includes: Protista, Fungi, Plantae, Animalia

Protists: Unicellular, colonial or multicellular. Wide variety of reproductive cycles, energy, and carbon derivation.

Fungi: Chemoheterotrophic. Mostly multicellular decomposers. Unique reproductive cycle.

Plantae: Multicellular. Photoautotrophs. Alternation-of-generations life cycle. Rigid cell walls.

Animalia: Mulicellular. Chemoheterotophs. Simple life cycle. No cell walls.

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Domain Eukarya 2

- Believed to have arose from 1st prokaryotic organism bya. by 2 primary processes:

1. Eukaryotic endomembrane system - from membrane in-folding: the plasma membranes of original prokaryotic cells gave rise to first eukaryote ones, folded inward and developed further. (nucleus, ER, Golgi, lysosomes, vacuoles etc)

2. Mitochondria and chloroplasts of eukaryotic cells proposed to have risen from small prokaryotic cells that established residence inside larger ones (endosymbiosis), deriving their outer membranes from plasma membrane of host cell. Mito - from small heterotrophic prokaryotes with effiency in aerobic respiration. Choro - small photosynthetic autotrophs.

- Includes all organisms with Eukaryotic cells (ie: membranous organelles like mitochondria and chloroplasts)


  • Eukaryotic cell structure
  • Unicellular (some protists and yeats), colonial (some protists) or multicellular (most Fungi, Plantae & Animalia)
  • Asexual reproduction via mitosis, sexual reproduction allowing genetic info exchange.
  • Variety of modes of access to energy and carbon, varying with kingdom.
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Domain Bacteria and Domain Archaea


  • Lack membrane-bound organelles (nucleus, ER)
  • All prokaryotes
  • Small size, ability to rapidly reproduce, diverse habitats.
  • Most abundant and diverse group
  • Evidence suggests Archaea & Eukarya more closed related than Bacteria. 


  • Most primitive group
  • Restricted to marginal habitats (hot springs or areas of low O2 conc.)
  • Ie: Archaebacteria
  • Differences in cells walls and biochemistry when compared to bacteria.
  • Gene translation machinery relates more to eukaryotes than eubacteria. 
  • Includes bacteria that grow under extreme conditions - Methanobacteria (Methane as C -source) - Halophiles (high salt conc.) - Thermophiles (grow at temps 80-100)
  • Believed that both bacteria and Archaea descended from common ancestor and that Eukarya later split from Archaea. 
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Prokaryotes vs. Eukaryotes

Shared characteristics:

  • Containing DNA as genetic material
  • Cytoplasm is membrane-bound
  • Use ribosomes for protein synthesis
  • Similar basic metabolic pathways.

Unique to eukaryotes:

  • Compartmentalisation, multicellularity and sexual reproduction.
  • Nucleus (DNA inside) and membrane-bound organelles
  • Higher levels of intracellular division
  • Larger than prokaryotic cells
  • DNA complex and compacted around histones
  • No peptidoglycan cell walls

Unique to prokaryotes:

  • DNA floats freely around cell
  • Lower levels of intracellular division
  • Smaller than eukaryotic cells
  • DNA 'naked'
  • Cell wall of peptidoglycan (single polymer of amino acids and sugar)
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