Tuesday, 3 June 2014

Introduction, Objectives and The Theoretical Background


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.

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

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