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Summary of Microbiology
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Microbiology RESITLecture I – Introduction and Microbial growth Poolman
Chapter 5
Generation time = the time required for the separation of one cell into two cells.
It depends on nutritional and genetic factors, and on temperature.
Clostridium perfringens and Vibrio natriegens have optimal generation times of 10 min the fastest
growing organisms.
It takes half a minute to synthesize a protein (E.coli).
The fast growing cells are often the smaller cells (micrometer size).
They do not synthesize everything themselves, building blocks can be imported.
High surface to volume ratio
Processes such as transcription, translation and insertion occur simulataneously while
transcription takes place, ribosomes can already attach to the template.
SecTranslocon
Inserts membrane proteins into the membrane = insertion
Translocates proteins across the membrane = translocation
Transertion: all processes at the same time (transcription, translation, insertion and translocation).
Binary fission: cells elongate to approximately twice their original length and then form a partition
that constricts the cell into two daughter cells. The partition that forms between dividing cell =
septum.
Most bacteria use this form of division including E.coli
Allows rapid growth
Budding: division as a result of unequal cell growth, forms a totally new daughter cell with the
mother cell retaining its original identity. Formation of a new cell wall occurs from a single point
(polar growth) rather than throughout the whole cell (intercalary growth) as in the binary fission.
Some budding bacteria form cytoplasmic extensions such as stalks (Caulobacter), hyphae
(Hyphomicrobium), and appendages (Ancalomicrobium)
Simple budding: yeast
,During division the cells can grow either in suspension or attached to surfaces. Planktonic growth =
the suspended life style, used by many bacteria that live in nature.
Sessile growth = growing attached to a surface. These cells can form biofilms = an attached
polysaccharide matrix containing embedded bacterial cells. Cells need EPS production to stick on the
surface.
Microbial mats = biofilms that form multi-layered sheets with different organisms present in the
individual layers. Mats composed of various phototropic and chemotropic bacteria are common in
the outflows of hot springs and in marine intertidal regions.
Implants in human body but also teeth are susceptible for biofilm formation infections.
The doubling time of E.coli is 20 minutes.
Exponential growth = a repetitive pattern where the number of cells doubles in a constant time
interval. Semilogarithmic graphs are useful for estimating the generation time of a culture from
actual growth data.
N = N02n
N is the final cell number
N0 is the initial cell number
n is the number of generations during the period of exponential growth
Generation time (g) of the exponentially growing population is: g = t/n (=doubling time, td)
t is the duration of exponential growth (days/hours/minutes)
n is the number of generations during the period of exponential growth
Doubling of number of cells: lnNt /N0 = ln2
Nt is the number of cells at a given time
= ln2 / td
is the specific growth rate
Batch culture = a closed-system microbial culture of fixed volume. An organism in such culture cannot
grow exponentially indefinitely.
,Lag phase: growth begins only after a period of time.
This depends on the history of the cells used as inocula and the composition of the growth medium
and growth conditions.
If the inoculum is taken from an old culture, there is usually a lag because the cells are depleted of
various essential constituents and time is required for their biosynthesis.
There is also a lag when a microbial culture is transferred from a nutrient-rich medium to a nutrient-
poor medium because new enzymes have to be synthesized.
Exponential phase: growing cell population doubles at regular intervals.
Cells are in their healthiest state.
Stationary phase: exponential phase ceases when essential nutrient is depleted or the organism’s
waste products accumulate.
There is no net increase or decrease in cell number growth rate is zero.
There might be division of cells but others die, so they balance each other out (cryptic growth).
Death phase
Occurs exponentially, but the rate is slower than the rate of exponential growth.
Culture media:
Nutrient solutions used to grow microbes in the laboratory
Typically sterilized in an autoclave
Two broad classes:
defined media: exact chemical composition known.
complex media: composed of digests of microbial, animal, or plant products (e.g., yeast and
meat extracts)
Defined media:
Prepared by adding precise amounts of pure inorganic or organic chemicals to distilled water the
exact composition of a defined medium is known.
Complex media:
Are made from digests of microbial, animal, or plant products.
Its exact nutritional condition is unknown not essential when culturing many microorganisms.
Enriched medium: used for the culture of nutritionally demanding (fastidious) microbes. It is
a complex medium to which additional highly nutritious substances are added
(serum/blood).
Selective medium: contains compounds that inhibit/stimulate the growth of some
microorganisms but not others.
Differential medium: an indicator is added which reveals by a colour change whether a
particular metabolic reaction has occurred during growth.
E.coli needs yeast extract as nutrition.
High K+ is needed for the osmotic pressure.
For successful cultivation of a microbe, it is important to know the nutritional requirements and
supply them in proper form and proportions in a culture medium.
Cells can be grown in liquid or solid culture media.
Solid media are prepared by addition of the gelling agent agar to liquid media. When grown
on solid media, cells form isolated masses (colonies)
, Why do some cells grow much faster than others?
Synthesis of nutrients takes time
Availability of nutrients
Certain cells have more of the same enzymes than others
Total cell count:
Microscopic cell count: observing and enumerating cells present
Dried on slides or on liquid samples
counting chambers with squares etched on a slide for liquid samples
Quick and easy way to of estimating microbial cell numbers
Limitation:
o Dead cells cannot be distinguished from live cells
o Small cells are difficult to see
o Motile cells must be killed/immobilized
o Precision is difficult to achieve
o Phase-contrast microscope required if a stain is not used
o Cell suspensions of low density (< 106 cells/ml) hard to count
o Debris in sample can be mistaken for cells
Microscopic cell counts in microbial ecology
Use stains to visualize and provide phylogenetic information or metabolic properties
The dye DAPI reacts with DNA
Often used on natural samples
Other fluorescent stains differentiate live and dead cells
Phylogenetic stains can determine proportion of Bacteria or Archaea (usually probes for
rRNA)
Viable cell = a cell that is able to divide and form offspring.
Viable count:
Spread-plate method: a volume of an appropriate diluted culture is spread over the surface
of an agar plate using a sterile glass spreader.
Pour-plate method: a known volume of culture is pipetted into a sterile Petri plate. Then
molten agar medium is added and mixed.
Sources of error:
Depends on inoculum size, viability, culture medium, incubation conditions
Mixed cultures grow at different rates
Plating inconsistencies
Reporting in colony-forming units instead of number of viable cells accounts for clumps
Applications:
Quick and easy
Used in food, dairy, medical, and aquatic microbiology, and water analyses
High sensitivity
Can target particular species in mixed samples
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