Bacterial Growth & Requirements
Bacteria are all around us. Given good growing conditions, a
bacterium grows slightly in size or length, a new cell wall grows
through the center forming two daughter cells, each with the same
genetic material as the parent cell. If the environment is optimum,
the two daughter cells may divide into four in 20 minutes, and
four into eight. The time required for cell to divide for population
to double is called generation time.
Bacterial growth can be modeled with four different phases: lag
phase (A), exponential or log phase (B), stationary phase (C), and
death phase (D).
(C)
(D)
(B)
(A)
Figure 1:
Lag phase: in this phase bacteria adapt themselves to growth
conditions. It is the period where the individual bacteria are
maturing and not yet able to divide.
Exponential phase: the number of new bacteria appearing per unit
time is proportional to the present population. This gives rise to the
classic exponential growth curve. Exponential growth cannot
continue indefinitely, however, because the medium is soon depleted
of nutrients.
Stationary phase: the growth rate slows as a result of nutrient
depletion. This phase is reached as the bacteria begin to exhaust the
resources that are available to them.
Death phase: bacteria run out of nutrients and die.
In reality, these phases are not so well defined, and the curve is
much more continuous.
Requirements for microbial growth are divided into two categories,
physical and chemical. Physical aspects include temperature, pH, and
osmotic pressure. Chemical requirements include water, sources of
carbon and nitrogen, minerals, oxygen, and organic growth factors.
A. Temperature: most microbes live within restricted ranges of
temperature with a Range of Tolerance (minimum maximum) which
includes an optimum temperature (most rapid growth in the shortest
period of time); while the optimum is usually closer to the maximum
temperature, this temperature may not be best for all cellular
activities psychrophiles (0-200 C), mesophiles (25-450 C),
thermophiles (50-700 C). A temperature of 750 C for 20 minutes will
kill vegetative cells but not endospores.
B. pH: most bacteria grow in the range of pH near neutrality;
usually between (6.5 -75), they are called neutrophiles. As bacteria
grow, their metabolic activities change the pH of their living
environment. Some bacteria are capable of living in what would be
considered extreme pH ranges, e.g. acidophiles acid lovers can be
found in acid runoff with pH as low as 1.6; Thiobacillus thiooxidans;
alkalophiles live in basic environments such as the ocean, pH about
8.2 .
C. Oxygen respiratory requirements: microbes that use oxygen are
called AEROBES; organisms that require oxygen in order to live are
called OBLIGATE AEROBES; some representative examples are:
Bacillus, Pseudomonas, Mycobacterium; microbes that cannot use
oxygen or for whom oxygen is actively toxic are called OBLIGATE
ANAEROBES; some representative examples are found in the genus
Clostridium.
in both of the foregoing situations, regulating the amount of oxygen
present represents a means of controlling growth or the rate of
growth of the microbes. Organisms that can grow in the presence of
oxygen or in its absence are called FACULTATIVE ANAEROBES;
representative examples include both Escherichia coli and
Staphylococcus aureus. Additionally, some organisms can tolerate
the presence of oxygen but not use it for growth; these are termed
AEROTOLERANT ANAEROBES; other microbes can use oxygen,
but only if the concentration is less than that found in the ambient air
(20%); these microbes are called MICROAEROPHILIC.
Figure 2: Oxygen required for bacterial growth.
Microorganisms are cultured in water to which appropriate dissolved
nutrients are added. These nutrients fall into three categories:
1. Energy sources
2. Cell structural components (Elemental Requirements)
3. Miscellaneous Growth factors
1. Energy Sources
a. Organic energy sources: sugars, starches, fats, protein; glucose,
acetic, glutamic, lactic acid; used by most bacteria, all fungi and
protozoa.
b. Inorganic energy sources: NH4+, nitrite, iron, H2S; only bacteria
use these sources.
c. Light photoautotrophs: (cyanobacterium)
2. Elemental requirements
Macro and micro or trace elements or nutrients; including but not
limited to: C, H, O, P, K, I, N, S, Ca, Fe, Mg
3. Miscellaneous growth factors
a. Vitamins: B1, biotin, pyroxidine (B6), B12, others may be needed
b. Amino Acids: get from protein digests; e.g. casein (milk protein),
peptone (meat protein).
c. Purines and Pyrimidines.
d. Heme.
Growth is an orderly increase in the quantity of cellular
constituents. It depends upon the ability of the cell to form new
protoplasm from nutrients available in the environment. In most
bacteria, growth involves increase in cell mass and number of
ribosomes, duplication of the bacterial chromosome, synthesis of new
cell wall and plasma membrane, partitioning of the two
chromosomes, septum formation, and cell division. This asexual
process of reproduction is called BINARY FISSION.
Figure 3: Binary Fission.
For unicellular organisms such as the bacteria, growth can be
measured in terms of two different parameters: changes in cell mass
and changes in cell numbers.
Methods for measurement of the cell mass involve both direct and
indirect techniques:
1. Direct physical measurement of dry weight, wet weight, or volume
of cells after centrifugation.
2. Direct chemical measurement of some chemical component of the
cells such as total N, total protein, or total DNA content.
3. Indirect measurement of chemical activity such as rate of O2
production or consumption, CO2 production or consumption, etc.
4. Turbidity measurements employ a variety of instruments to
determine the amount of light scattered by a suspension of cells.
Measuring techniques involve direct counts, visually or
instrumentally, and indirect viable cell counts.
1. Direct microscopic counts are possible using special slides known
as counting chambers. Dead cells cannot be distinguished from living
ones. Only dense suspensions can be counted (>107 cells per ml), but
samples can be concentrated by centrifugation or filtration to
increase sensitivity.
2. Electronic counting chambers count numbers and measure size
distribution of cells. For cells the size of bacteria the suspening
medium must be very clean. Such electronic devices are more often
used to count eukaryotic cells such as blood cells.
3. Indirect viable cell counts, also called plate counts, involve plating
out (spreading) a sample of a culture on a nutrient agar surface. The
sample or cell suspension can be diluted in nontoxic diluents (e.g.
water or saline) before plating. If plated on a suitable medium, each
viable unit grows and forms a colony. Each colony that can be
counted is called a colony forming unit (cfu) and the number of cfu's
is related to the viable number of bacteria in the sample.
1- http://en.wikipedia.org/wiki/bacterial-growth
2- http://www.cellsalive.com/ecoli.htm
3- http://textbookofbacteriology.net/growth.html
4- http://www.angelfire.com/de/nestsite/micro7.html
5- http://microvet.arizona.edu