Pathogenic bacteria might hide in our daily environment and can cause infectious diseases
and outbreak. The trays used by different canteen are often not well cleaned due to the
large size of students in the cateen. Bacteria might thrive on the surface on the trays of
canteen due to the worm climate in Singapore.
Learning how to isolate and cultivate bacteria from everyday environment.
3. THEORETICAL BACKGROUND
In this experiment, students will collect samples from surface of food trays from different
NTU canteens and cultivate them in liquid nutrient medium and then transfer to petri dish. A
culture medium is defined as a solid or liquid preparation used for the growth, transport,
and storage of microorganisms. The effective culture medium must contain all the nutrients
required for the growth of the microorganism.
Specialized media are widely employed for the isolation and identification of
microorganisms. Selective media allows the growth of certain type of organisms, while
inhibiting the growth of other organisms. This selectivity is achieved in several ways. For
example, organisms that have the ability to utilise a given sugar are screened easily by
making that particular sugar the only carbon source in the medium for the growth of the
microorganism. Like-wise, the selective inhibition of some types of microorganisms can be
studied by adding certain dyes, antibiotics, salts or specific inhibitors that will affect the
metabolism or enzymatic systems of the organisms. For example, media containing
potassium tellurite, sodium azide or thallium acetate at different concentrations of 0.1 - 0.5
g/l will inhibit the growth of all Gram-negative bacteria. Media supplemented with the
antibiotic penicillin concentration 5-50 units/ml or crystal violet 2 mg/l inhibit the growth of
Gram-positive bacteria. Tellurite agar, is used to select for Gram-positive organisms, and
nutrient agar supplemented with the antibiotic penicillin can be used to select for the
growth of Gram negative organisms.
Eg., Mannitol salt agar, Hektoen enteric agar (HE), Phenylethyl alcohol agar
Differential media are widely used for differentiating closely related organisms or groups of
organisms. Because of the presence of certain dyes or chemicals in the media, the
organisms will produce certain characteristic changes or growth patterns that are used for
identification or differentiation of microorganism.
Eg., Mac Conkey (MCK) agar, Eosin Methylene Blue (EMB) agar