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:
Other species can reproduce by budding
off of smaller cells:
Some fungi (eukaryotes) like Saccharomyces
cerevisiae (brewer's yeast) also bud
Generation time = G = 2 hours / 4 generations = 0.5
Generation times for some typical bacteria vary from 5 min
to several days
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
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.
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.
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.
When an essential nutrient is missing all together or toxic
waste become biocidal then the cell start to die of. Death > birth.