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Module: BIOM - 1009

Lecturer: Dr Martin

Date: 15/11/16

Microbial Growth
o

Microbes can be present in a number of samples depending on what they prefer in terms of their
environment, some environments include:
 Patients
 Food
 Water
 Agriculture

o

Knowing the key to certain microbial organisms’ growth can help us identify what will kill or prevent the
growth of them
 This can be physical or chemical

o

To grow a sample, you must first sterilize the area and remove all pre-existing bacteria, this can be done in
the following ways:
 Heat
 This is the most widely used form
 It may kill endospores
 Examples include: boiling or incineration
 Irradiation
 This will destroy the bacteria’s nucleic acids
 Examples include: microwaves and gamma waves
 Filtration
 This is important for areas that can’t be physically or chemically tampered with, such as
antibiotics, as it will physically filter the bacteria out
 Chemicals
 These are toxic chemicals that kill bacteria
 Such as: formaldehyde

o

There are 4 phases of bacterial growth (see right):
 Lag
 This is when the bacteria are first introduced to their new media/environment
 They are examining their surroundings
 They are still metabolically active
 This can last an hour, to several days
 Log
 This is when rapid cell growth occurs
 The population will double every
generation
 The microbes are most sensitive to
adverse conditions during this phase
 Such as antibiotics and
antimicrobial agents
 Stationary
 This stage occurs when the death rate =
the reproductive rate
 They begin to encounter stresses such as:
 Lack of nutrients, water, space, oxygen etc
...
coli
o They grow at 37°C
o They contaminate food and immunocompromised individuals
o They cause fever, muscle aches etc
...
5 and 7
...
0
 Many foods are preserved in acids
 Such as pickles
 Osmotic Pressure
 Microbes obtain almost all their nutrients from the surrounding water solution
 Therefore, the water availability is very important
 Environments are available with different osmotic pressures (tonicity):
 Isotonic
o These environments have an equal amount of solute as the bacteria have
inside their cells
 Hypertonic
o This is when the solute concentration outside the bacteria is higher than
inside, so all the water diffuses out
 Hypotonic
o This is when the solute concentration is less than inside the bacteria and so
all the water diffuses inside the bacteria
 Chemicals
 Microbes require both macro and micro elements in their surrounding solution:
 Macro
o Nitrogen for amino acids
o Phosphorus for nucleic acids and phospholipids
 Micro
o Trace elements of Ca, Cu, Fe, Mg, Zn etc
...
available in the sample will give an
indication to the number of cells
 Bacterial inoculum size
 This is important for antibiotic testing

o

Bacteria counting comes with many errors however:
 Suitability of the culture
 Length of incubation
 Pipetting errors
 Human errors and cell clumping



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