Bacterial growth and division WORLD OF MICROBIOLOGY AND IMMUNOLOGY50•
dependent on the rate of addition of the fresh medium.
Bacteria can be tailored to grow relatively slow or fast and,
if the set-up is carefully maintained, can be maintained for a
long time.
Bacterial growth requires the presence of environmental
factors. For example, if a bacterium uses organic carbon for
energy and structure (chemoheterotrophic bacteria) then
sources of carbon are needed. Such sources include simple
sugars (glucose and fructose are two examples). Nitrogen is
needed to make amino acids, proteins, lipids and other com-
ponents. Sulphur and phosphorus are also needed for the man-
ufacture of bacterial components. Other elements, such as
potassium, calcium, magnesium, iron, manganese, cobalt and
zinc are necessary for the functioning of enzymesand other
processes.
Growth is also often sensitive to temperature.
Depending on the species, bacteria exhibit a usually limited
range in temperatures in which they can growth and repro-
duce. For example, bacteria known as mesophiles prefer tem-
peratures from 20°–50° C (68°–122° F). Outside this range
growth and even survival is limited.
Other factors, which vary depending on species,
required for growth include oxygen level, pH, osmotic pres-
sure, light and moisture.The obvious events of growth and division that are
apparent from measurement of the numbers of living bacteria
are the manifestation of a number of molecular events. At the
level of the individual bacteria, the process of growth and
replication is known as binary division. Binary division occurs
in stages. First, the parent bacterium grows and becomes larger.
Next, the genetic material inside the bacterium uncoils from the
normal helical configuration and replicates. The two copies of
the genetic material migrate to either end of the bacterium.
Then a cross-wall known as a septum is initiated almost pre-
cisely at the middle of the bacterium. The septum grows inward
as a ring from the inner surface of the membrane. When the
septum is complete, an inner wall has been formed, which
divides the parent bacterium into two so-called daughter bacte-
ria. This whole process represents the generation time.
Bacterial division is initiated by as-yet unidentified sen-
sors of either the volume or the length of the bacterium. The
sensors trigger a series of events, including the formation of
the septum. In septum formation are number of proteins are
recruited to the site where septal formation will begin. They
may be guided to the site by the concentration of a trio of pro-
teins that either inhibit or promote the formation of a so-called
Z-ring. The Z-ring is analogous to a drawstring, and is likely
an integral part of the inwardly growing septum wall.Freeze fracture electron micrograph showing dividing Streptococcus bacteria. The division plane between the daughter cells is evident in some
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