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Lecture Six

Microbial GROWTH and Reproduction

The growth of single-celled prokaryote. The growth of an individual bacterial cell is extremely hard to study because they are so small. Therefore, when we talk about growth we are also referring to reproduction - i.e. the increase in the number of individuals. Bacteria basically clone themselves when they reproduce. In many species, one cell splits into two daughter cells of equal size and genetic make up. This process is called:
Binary Fission
Other species can reproduce by budding off of smaller cells:
Some fungi (eukaryotes) like Saccharomyces cerevisiae (brewer's yeast) also bud

Population Growth

Generation time = G = 2 hours / 4 generations = 0.5 hrs.
Generation times for some typical bacteria vary from 5 min to several days
Note pathogens
The great potential for microbial growth can be demonstrated by considering that one E. coli cell (G = 0.333 hr. or 20 minutes) would grow to be a mass 2000 x that of earth if it could grow unchecked for 48 hours!
If we start with one cell and allow it to double every 20 min. in 10 divisions there are 1000 cells ie in 200 min. This 1000 would be 1,000,000 in another 200 min.
1 > 2> 4 > 8 > 16 > 32 > 64 > 128 > 256 > 512 > 1024

Growth curve....

Lag Phase

First part of the growth curve
The lag phase usually occurs when bacteria are introduced into a new environment they usually need to synthesise new enzymes in order to utilise substrates in that environment. Lag phases are even longer if bacteria have to come out of dormancy at the beginning of the growth curve. Many bacteria have shortened lag phases (and faster growth rates) if they are supplied with metabolic intermediates, vitamins, amino acids etc. Bacteria that are have already adapted to their environment will not have to go through a new lag phase if placed in an identical medium. So lag phases can be shortened for commercial fermentations by pre-adapting the bacterial inoculum before use.
Log Phase
Once adapted to its environment bacteria multiply at maximum rate and constant generation time producing a straight line on the growth curve. As nutrients become restricted or waste materials build up the generation time gets less and the curve bend towards the horizontal. Once in the log phase cell will remain like this even if frozen. So once reconstituted a log phase culture will resume directly into its log with out a lag phase. This means that inocula that have been pre-adapted ready for an industrial fermentation can be frozen and stored until needed or sold to other processors.
Stationary Phase
When one or more essential materials, nutrients or oxygen become restricted or waste materials build up to a biostatic level then the rate of growth is zero ie birth = death and a horizontal line is produced.
Decline phase
When an essential nutrient is missing all together or toxic waste become biocidal then the cell start to die of. Death > birth.

The End

Prepared by Barry Brazier