Question

Introduction: Selective and differential media are used frequently in Microbiology Pathology Labs to identify a known group of bacteria from a complex mix of other bacteria. The medium is designed to select against as many of the background organisms as possible by using a medium in which they will not grow but the organisms under investigation can grow hence the "Selective" part of the name. Any assayable characteristic that can be used to differentiate a specific group of interest from the selected group of organisms can be incorporated into the medium to distinguish that group. The aim of this Lab is to understand use of selective/differential media and the interpretation of their results. Mannitol salt agar Contains: 7.5% NaCl included with the sugar mannitol, a source of protein, pH indicator (phenol red) Selection: The high salt concentration selects against salt intolerant organisms and for halophiles (salt tolerant organisms). Describe your results Eosin methylene agar Contains: EMB contains: peptone, lactose, dipotassium phosphate, eosin Y (dye), methylene blue (dye). Selection: Eosin and methylene blue together inhibit the growth of gram positive bacteria and encoutage the growth of gram negative bacteria including coliforms Differentiation: Lactose is a fermentable carbohydrate that differentiates between organisms that ferment lactose (e.g., E. coli) and those that do not. In our plate Escherichia coli went through fermentation because it produced a metallic green sheen. The other samples produced pink colonies, which means they were slow or weak fermenters MacConkey agar: Contains: Protein source, lactose, bile salts, and the dyes crystal violet and neutral red. Selection: Bile salts and crystal violet make the medium selective by inhibiting the growth of gram-positive bacteria but allowing many gram negatives (including coliforms) to grow. Differentiation: Lactose is a fermentable carbohydrate that differentiates between organisms that ferment lactose (e.g., E. coli) and those like Pseudomonas and Klebsiellosis that do not. In our plates we did not yield any results because the plates were covered with a thin layer of MacConkey agar. Describe your results Triple sugar iron agar Contains: Carbon sources Three sugars, glucose (0.1%), sucrose (1%), and lactose (1%) and nentone as a source of protein. Selectinn Bente. Tienen
can somrone help me write an introduction for each agar. thank you ?

Answer 1

1. Mannitol Salt Agar:- It is a selective and differential media used to grow bacterial cultures. The high salt concentration makes it very selective towards gram-negative and gram-positive bacteria. Phenol red acts as the indicator and detects mannitol fermenting bacteria

Results:- Yellow color colonies with yellow zones are formed by Staphylococcus aureus bacteria. Other bacteria of genera staphylococci produced small pink-colored colonies with no color change in medium.

Mechanism;- Bacteria having the ability to ferment mannitol into an acidic byproduct turn phenol red into yellow color. The species of staphylococci can tolerate this huge salt concentration but other pathogenic bacteria cannot tolerate such high salt concentration due to this it is used for selective isolation of staphylococcus species.

2. Eosin Methylene Agar:- It is a selective medium for the isolation of gram-negative bacteria. it is widely used for the isolation of coliform bacteria like E.coli. differentiation between bacterial colonies is solely based upon the color of colonies

1- Bacteria forming colored colonies in EMB agar are usually lactose fermenters.

2- Bacterias forming colorless colonies in EMB agar do not ferment lactose and are called as Non-lactose fermenters.

#Eosin and methylene dye used in EMB agar inhibit the growth of gram-positive bacteria and promote the growth of gram-negative bacteria hence is a selective media for gram-negative bacteria like salmonella and shigella.

Results:- Gram-negative bacteria- lactose fermenter turns colonies dark purple in color with a green metallic background.

Lactose non-fermenting bacteria form either colorless colonies or light lavender color colonies.

3-MacConkeys Agar:- It is a selective and differential medium used to isolated gram-negative bacteria. It differentiates gram-negative bacteria based upon their ability to ferment lactose

# Presence of bile salt and crystal violet dye inhibits the growth of gram-positive bacteria and makes this medium selective towards gram-negative bacteria.

#pH indicator used as an indicator is neutral red. The indicator turns pink in color in case of lactose fermenting bacteria at pH below 6.8

Result:- Lactose fermenting gram-negative bacteria form pink-colored colonies whereas the lactose non-fermenting bacteria form colorless colonies on MacConkeys Agar