Bacterial growth
Bacterial growth is the asexual
reproduction, or cell division, of a bacterium into two daughter cells,
in a process called binary fission.
Providing no mutational event occurs, the resulting daughter cells are
genetically identical to the original cell. Hence, "local doubling"
of the bacterial population occurs. Both daughter cells from the division do
not necessarily survive. However, if the number surviving exceeds unity on
average, the bacterial population undergoes exponential
growth. The measurement of an exponential bacterial growth curve in
batch culture was traditionally a part of the training of all microbiologists;
the basic means requires bacterial enumeration (cell counting) by direct and
individual (microscopic, flow cytometry[1]), direct and bulk
(biomass), indirect and individual (colony counting), or indirect and bulk
(most probable number, turbidity, nutrient
uptake) methods. Models reconcile theory with the measurements.
Growth is shown as L = log(numbers) where
numbers is the number of colony forming units per ml, versus T (time.)
In autecological studies, the growth of bacteria
(or other microorganisms, as protozoa, microalgae or yeasts) in batch culture can be modeled with
four different phases: lag phase (A), log phase or exponential phase (B), stationary phase (C), and death phase(D).[3]
1.
During lag phase, bacteria adapt themselves to
growth conditions. It is the period where the individual bacteria are maturing and not yet
able to divide. During the lag phase of the bacterial growth cycle, synthesis
of RNA, enzymes and other molecules occurs.
2.
The log phase (sometimes called the
logarithmic phase or the exponential phase) is a period characterized
by cell doubling.[4]The number of new
bacteria appearing per unit time is proportional to the present population. If
growth is not limited, doubling will continue at a constant rate so both the
number of cells and the rate of population increase doubles with each
consecutive time period. For this type of exponential growth, plotting the
natural logarithm of cell number against time produces a straight line. The
slope of this line is the specific growth rate of the organism, which is a
measure of the number of divisions per cell per unit time.[4] The actual rate of this
growth (i.e. the slope of the line in the figure) depends upon the growth
conditions, which affect the frequency of cell division events and the
probability of both daughter cells surviving. Under controlled conditions, cyanobacteria can double their
population four times a day.[5] Exponential growth cannot
continue indefinitely, however, because the medium is soon depleted of
nutrients and enriched with wastes.
3.
The stationary phase is often due to a
growth-limiting factor such as the depletion of an essential nutrient, and/or
the formation of an inhibitory product such as an organic acid. Stationary
phase results from a situation in which growth rate and death rate are equal.
The number of new cells created is limited by the growth factor and as a result
the rate of cell growth matches the rate of cell death. The result is a
“smooth,” horizontal linear part of the curve during the stationary phase.
4.
At death phase (decline phase), bacteria
die. This could be caused by lack of nutrients, environmental temperature above
or below the tolerance band for the species, or other injurious conditions.
This basic batch culture growth model draws
out and emphasizes aspects of bacterial growth which may differ from the growth
of macrofauna. It emphasizes clonality, asexual binary division, the short
development time relative to replication itself, the seemingly low death rate,
the need to move from a dormant state to a reproductive state or to condition
the media, and finally, the tendency of lab adapted strains to exhaust their
nutrients. In reality, even in batch culture, the four phases are not well
defined. The cells do not reproduce in synchrony without explicit and continual
prompting (as in experiments with stalked bacteria [6]) and their exponential
phase growth is often not ever a constant rate, but instead a slowly decaying
rate, a constant stochastic response to pressures both to reproduce and to go
dormant in the face of declining nutrient concentrations and increasing waste
concentrations.
Batch culture is the most common laboratory
growth method in which bacterial growth is studied, but it is only one of many.
It is ideally spatially unstructured and temporally structured. The bacterial
culture is incubated in a closed vessel with a single batch of medium. In some
experimental regimes, some of the bacterial culture is periodically removed and
added to fresh sterile medium. In the extreme case, this leads to the continual
renewal of the nutrients. This is a chemostat, also
known as continuous culture. It is ideally spatially unstructured and
temporally unstructured, in a steady state defined by the rates of nutrient
supply and bacterial growth. In comparison to batch culture, bacteria are
maintained in exponential growth phase, and the growth rate of the bacteria is
known. Related devices include turbidostats and auxostats.
Bacterial growth can be suppressed with bacteriostats,
without necessarily killing the bacteria. In a synecological,
true-to-nature situation in which more than one bacterial species is present,
the growth of microbes is more dynamic and continual.
Liquid is not the only laboratory
environment for bacterial growth. Spatially structured environments such as
biofilms or agar surfaces present
additional complex growth models.
Blood pressure
Blood pressure (BP) is the pressure of circulating blood on the walls of blood vessels. When used without further specification,
"blood pressure" usually refers to the pressure in large arteries of the systemic circulation.
Blood pressure is usually expressed in terms of the systolic (maximum during one heart
beat) pressure over diastolic (minimum in between two
heart beats) pressure and is measured in millimeters of mercury (mmHg), above
the surrounding atmospheric pressure (considered to be zero for convenience).
Systemic arterial pressure
The risk of cardiovascular
disease increases progressively above 115/75 mmHg.[6] In
practice blood pressure is considered too low only if noticeable symptoms are present.[4]
Observational
studies demonstrate that people who maintain arterial pressures at the low end
of these pressure ranges have much better long term cardiovascular health.
There is an ongoing medical debate over what is the optimal level of blood
pressure to target when using drugs to lower blood pressure with hypertension,
particularly in older people.[7]
Mean arterial pressure
The mean arterial
pressure (MAP) is the
average over a cardiac cycle and is determined by the cardiac output (CO), systemic
vascular resistance (SVR),
and central venous
pressure (CVP):[23]
Pulse pressure
Curve of
the arterial pressure during one cardiac cycle. The closing of the aortic valve
causes the notch in the curve.
The pulse pressure is the difference between the measured
systolic and diastolic pressures,[24]
The up
and down fluctuation of the arterial pressure
results from the pulsatile nature of the cardiac output, i.e. the heartbeat. Pulse
pressure is determined by the interaction of the stroke volume of the heart, the compliance (ability
to expand) of the arterial system—largely attributable to the aorta and large elastic arteries—and the resistance to
flow in the arterial tree. By expanding under pressure,
the aorta absorbs some of the force of the blood surge from the heart during a
heartbeat. In this way, the pulse pressure is reduced from what it would be if
the aorta were not compliant.[24] The loss of arterial compliance that occurs
with aging explains the elevated pulse pressures found in elderly patients.
Measurement
Taking another persons blood pressure with a
sphygmomanometer
For each heartbeat, blood
pressure varies between systolic and diastolic pressures. Systolic pressure is
peak pressure in the arteries, which occurs near the end of the cardiac cycle when the ventricles are contracting. Diastolic pressure is minimum pressure in the arteries,
which occurs near the beginning of the cardiac cycle when the ventricles are
filled with blood. An example of normal measured values for a resting, healthy
adult human is 120 mmHg systolic and 80 mmHg diastolic (written as 120/80 mmHg, and spoken as "one-twenty over
eighty").
