Question

Microbial Growth Differential, Selective and enriched  Pre Lab Questions:

2. Staphylococcus aureus is a Gram positive cocci. Do you anticipate it growing well on MacConkey agar?

3. A certain species of bacteria will grow at 37 but not at 40 C° another species will not grow at 37 but will at 40 C°. You can control the temperature in your incubator. Is temperature differential or selective?

Answer 1

Staphylococcus aureus being a gram positive cocci is thus unable to grow in MacConkey agar. ManConkey agar is a selective and differential agar medium that contains bile salts, lactose and the dye crystal violet that is known to inhibit the growth of Gram positive bacteria.It helps in the selective isolation of E.coli, a Gram negative lactose fermenting bacteria that grows as red to pink colonies from other Gram negative non-lactose fermenting bacteria that grow as colourless colonies.so that the staphylococcus cannot grow on MacConkey agar.

The primary reason for incubating bacterial cultures at different temperatures is that specific bacteria are adapted to grow best at different temperatures.Bacteria known as mesophiles,which include most pathogens and common ennvironmental bacteria ,grow best in temperature ranging from 20-40 degree celsius.Mesophiles are generally killed by temperatures much hotter or colder. Another reason for incubating at different temperatures is promoting the growth of a target group of bacteria.For example:although both pathogens and environmental bacteria found indoors are mesophiles, pathogen will grow faster than environmental strains at 37 degree celsius,the temperature of the human body.to prevent the pathogen growth ,cultures are often incubated at 25 degree celsius.