The majority of bacterial cells multiply in number by a process of binary fission. That is, each individual will increase in size until it is large enough to divide into two identical daughter cells.
BACTERIAL REPRODUCTION AND GROWTH KINETICS
The majority of
bacterial cells multiply in number by a process of binary fission. That is,
each individual will increase in size until it is large enough to divide into
two identical daughter cells. At the point of separation each daughter cell
must be capable of growth and reproduction. While each daughter cell will
automatically contain those materials that are dispersed throughout the mother
cell (mRNA, rRNA, ribosomes, enzymes, cytochromes, etc.), each must also carry
at least one copy of the chromosome. The bacterial chromosome is circular and
attached to the cytoplasmic membrane where it is able to uncoil during DNA
replication. The process of DNA replication proceeds at a fixed rate dependent
on temperature, therefore the time taken to copy an entire chromosome depends
on the number of base pairs within it and the growth temperature. For Escherichia coli growing at 37 °C,
replication of the chromosome will take approximately 45 minutes. These copies
of the chromosome must then segregate to opposite sides of the cell before cell
division can proceed. Division occurs in different ways for Gram-positive and
Gram-negative bacteria. Gram-negative cells do not have a rigid cell wall and
divide by a process of constriction followed by membrane fusion. Gram positive
cells, on the other hand, having a rigid cell wall, must develop a cross-wall that divides the cell into two equal halves. Constriction and
cross-wall formation takes approximately 15 minutes to complete. DNA
replication, chromosome segregation (C-phase) and cell division (D-phase) occur
sequentially in slow growing cells with generation times of greater than 1 hour
and are the final events of the bacterial cell cycle. Cells are able to
replicate faster than once every hour by initiating several rounds of DNA
replication at a time. Thus partially replicated chromosomes become segregated
into the newly formed daughter cells. In this fashion it is possible for some
organisms growing under their optimal conditions to divide every 15–20 minutes.
Rod shaped organisms maintain their diameter during the cell cycle and increase
their mass and volume by a process of elongation. When the length of the cell
has approximately doubled then the division/constriction occurs centrally.
Coccal forms increase in size by radial expansion, with the division plane
going towards the geometric centre. In some genera the successive division
planes are always parallel. Under such circumstances the cells appear to form
chains (i.e. streptococci). In staphylococci successive division planes are
randomized, giving dividing clusters of cells the appearance of a bunch of
grapes. Certain genera, e.g. Sarcina,
rotate successive division planes by 90 ° to form tetrads and cubical octets. The appearance of dividing cells under
the microscope can therefore be a useful initial guide to identification.
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