Question

- Describe a circumstance (e.g. in its’ natural environment) where Staphylococcus species bacteria might benefit from being salt tolerant.

- How might the ability to perform hemolysis benefit a hemolytic organism in its’ natural environment?

- Imagine that you have a culture containing a single bacterial species. This culture may be one of several possible species. Describe a scenario where you use Mannitol Salt agar, Blood Agar, and EMB plates to identify the unknown bacterial species. That is, how would you use these plates to help you identify it?

- In the table below are a variety of agar plate media available to microbiologists to cultivate bacteria. Denote whether each medium is non-selective (general growth media), selective, and/or differential by placing an “X” in the appropriate column or columns. (2 points for correctly filling out the entire table).

Type of Agar Plate	Non-selective Media	Selective Media	Differential Media
Nutrient Agar (NA)
Tryptic Soy Agar (TSA)
TSA with 5% Sheep Blood (BAP)
Chocolate Agar Plate (CAP)
MacConkey (MAC)			X
Eosin Methylene Blue (EMB)			X
Hektoen Agar (HE)
Phenyl Ethyl Alcohol (PEA)
Mannitol Salt Agar (MSA)		X

Answer 1

Ans. TSA (Trypticase Soy Agar) and NA (Nutrient Agar) are not selective or differential media but are a general-purpose media whose goal is simply to grow many bacterial types cheaply.

TSA with 5% Sheep Blood (BAP) is an enriched medium for general purposes used to cultivate fastidious organism and differentiate bacteria by their hemolytic properties. This medium is a highly nutritious medium commonly used for the preparation of blood agar by blood supplement. Usually, it has a 5 percent Sheep blood tryptic soy agar base. But can be made selective by the addition of antibiotics.

The lysed blood agar is Chocolate Agar (CAP). This name derives from the fact that the medium is color chocolate-brown due to the lysis of red blood cells ( RBC). Incubated at 35-37 ° C and 5 percent CO₂ atmosphere, chocolate agar is used for the isolation of fastidious species such as Haemophilus influenza. It is a differential medium for gram-positive cocci and can be made selective by the addition of antibiotics.

The MacConkey Agar (or MAC) is a medium of differential and selective media. The media contain crystal violet, bile salts, lactose, and pH-indicator Neutral Red are responsible for the selective and differential aspects. MAC is a selective medium since certain bacteria flourish while other bacteria are inhibited. MAC is a differential medium because it includes sugar lactose and a pH-indicator that shows species differences.

A selective differential medium used to isolate fictional coliforms is Eosin methylene blue agar (EMB). Eosin Y and methylene blue are pH indicator colors, which combine into dark purple precipitation with a low pH and inhibit the development of the majority of Gram-positive species. Lactose and sucrose are fermentable sources of carbon dioxide that promote the growth and differentiation of fecal coliforms.

Hektoen agar (HE) is a selective and differential agar medium commonly used in patient specimens to recover Salmonella and Shigella. HE includes lactose fermentation indicators and hydrogen sulfide generation indicators and inhibitors that prevent gram-positive bacteria from developing. It was named after the Chicago Hektoen Institute, where agar was developed by scientists.

The selective medium is Phenyl Ethyl Alcohol (PEA). It is selective because phenyl ethyl alcohol is in the medium-based which inhibits the growth of most (but not all) Gram-negative organisms and encourages the growth of (not all) Gram-positive organisms.

Mannitol Salt Agar (MSA) is a differential, selective medium that promotes the production of salt-loving (Halophytes) species. Because of its high concentration (7.5%) of sodium chloride, MSA is a selective medium that impedes the growth of most species. During the presence of the sugar mannitol and the pH indicator Phenol Red, MSA is a differential medium.

So, the answer is:

Type of Agar Plate	Non-selective Media	Selective Media	Differential Media
Nutrient Agar (NA)	X
Tryptic Soy Agar (TSA)	X
TSA with 5% Sheep Blood (BAP)			X
Chocolate Agar Plate (CAP)			X
MacConkey (MAC)		X	X
Eosin Methylene Blue (EMB)		X	X
Hektoen Agar (HE)		X	X
Phenyl Ethyl Alcohol (PEA)		X
Mannitol Salt Agar (MSA)		X	X

Answer 2

stop cheatin you slack bastards