Microbiology Lecture 2

Growth of micro-organisms

In order to grow all organisms require :-

• Chemical elements for synthesis of cell constituents
• Energy-generating system

-->Autotrophs – light or oxidation of inorganic compounds

-->Heterotrophs – organic compounds

Carbon Sources

• Glucose utilised by virtually all micro-organisms
• Maltose and sucrose (disaccharides) good sources of carbon for most micro-organisms
• Starch (polysaccharide) good source of carbon for most micro-organisms
• Cellulose slowly hydrolysed by some bacteria and many ‘higher fungi’

Nitrogen Sources

• Micro-organisms utilise inorganic and organic sources of nitrogen
• inorganic – atmospheric N2,, NH4+ , NO3_ , NO2_
• organic – amino acids

Nitrogen Sources

• Nitrates may be a good source of nitrogen for many micro-organisms but an inability to utilise nitrates is common and may be ecologically important
• Few micro-organisms are unable to use NH4+ 
• Those that are unable to use NH4+ depend upon amino acids – wide range utilised but asparagine is most often employed in synthetic culture media
• Atmospheric N2 is reduced to NH3 by nitrogen-fixing bacteria.
• The ability to carry out N-fixation restricted to prokaryotes i.e. bacteria.  

Culture media

Defined media

Precise chemical composition,  e.g. Glucose, (NH4)2SO4

Complex media

Digests of chemically undefined substances, e.g. plant, animal or yeast products  …