3.1.2 + 3.1.5 Lab Procedures when Handling Micro-organisms
Micro-organisms
• Can only be seen with the help of a microscope.
• Include bacteria, some fungi (yeast) and some
protists (plankton).
• Occupy a wide range of habitats, e.g. salt water,
fresh water, soil, dust, air, hot springs, etc.
• Fungi are mostly terrestrial.
• Bacteria can be found in extreme environments from
the inside of volcanoes to inside the human gut.
Procedures when handling
micro-organisms
Asepsis means that measures are taken to exclude
unwanted organisms.
Sterile means that all micro-organisms are destroyed,
i.e. there is nothing living.
Inoculation is the addition of cells to the nutrient
medium.
Incubation is the growing of the microbes in a warm
environment.
Growth of micro-organisms
• Under suitable conditions one micro-organism can
be grown into a colony of micro-organisms which is
visible to the naked eye.
• Micro-organisms are grown on a special medium
containing a food supply – usually nutrient agar.
• The procedures involved require caution as the
micro-organism to be grown (or any contaminating
micro-organism, if present) may be a disease
causing microbe.
• As a result certain precautions are taken when
handling micro-organisms to reduce the possibility of
contamination and to prevent the growth of
undesirable micro-organisms.
Precautions when handling
micro-organisms
• Wash hands before and after the experiment.
• Wash the bench with disinfectant before and after
the experiment.
• Sterilise all equipment before and after use. This can
be done by placing all equipment:
• In an autoclave (or pressure cooker) at 120 °C
for 15 minutes or
• in Dettol for 24 hours
• After an experiment material can be placed in a
dustbin only after sterilisation. Sterilised equipment
can be reused
• Flame all needles, loops and necks of test-tubes by
heating them in the flame of a Bunsen burner.
• Turn off the Bunsen burner when not in use or make
sure that the flame is visible.
• Open all containers for the shortest possible time.
Open lids the shortest possible distance.
• Seal all plates once inoculated.
• Label all plates once prepared.
How to grow a culture.
• Take a sterile agar plate.
• Flame an inoculating loop to
sterilise it.
• Dip the loop into sterile water or
alcohol to cool it.
• Flame the neck of the container
from which the sample is to be
taken.
• Inoculate the loop by placing it
briefly into the sample to be grown.
• Re-flame the neck of the container.
• While only slightly opening the
agar plate streak the inoculating
loop across the surface of the agar.
• Sterilise the loop again by flaming it.
• Label and seal the petri-dish and
incubate it upside down for
2 – 3 days.
• Examine the plate and record the results.
Growth conditions
• Once agar plates are sealed and
labelled turn them upside down and
place them in an incubator to allow
any bacteria and fungi to grow.
• Placing them upside down reduces
problems due to condensation.
• Yeasts and moulds grow best at 25 – 30 °C.
• Bacteria generally require 35 °C.
Results
• Bacterial colonies are usually
seen as white, cream or yellow
shiny dots.
• Fungal colonies appear as a
powder or as a fuzzy growth,
similar to cotton wool in appearance
Disposal
• All micro-organisms should be destroyed after use
by sterilising all petri-dishes and equipment:
• by placing them in an autoclave (or pressure
cooker) at 120°C for 15 minutes or
• by placing them in Dettol for 24 hours.