11. Complete this diagram with these three species: Escherichia coli, Enterobacter aerogenes, andStaphylococcus aureus.YesMetallic-green coloniesSpecies:Gram stain:Growth on EMBThick pink, mucoid coloniesSpecies:Gram stain:NoSpecies:Gram stain:Growth on MSAYes, with yellow haloNo

A dichotomous key is a visual tool. As the name suggests, at each stage of the key, there is one question that may be answered in affirmative or negative (yes or no), creating two options (di-chotomous). This scientific tool is used in many areas, including microbe identification, where a series of tests can lead to identifying each of the organisms in a group.In the given dichotomous diagram, two microbial culture media are used to primarily identify the three given organisms. The presence or absence of growth on these media initiates the key, and the type of growth characteristics help separate the organisms. The media used are selective as well as differential.A brief description of the media used is as follows:EMB Agar – Eosin Methylene Blue Agar, as the name suggests, has the dyes eosin and methylene blue incorporated into its ingredients. These metachromatic dyes will inhibit gram-positive bacteria while selectively growing gram-negative bacilli. It also helps differentiate lactose fermenters from lactose non-fermenters.Selective Components: This medium is specifically designed to discourage the growth of Gram-positive organisms, making it a selective medium for Gram-negative organisms.Differential Components: The presence of dyes also incorporates color into colonies that can ferment lactose and leaves colonies that cannot ferment lactose colorless or pale. The organisms that produce excess acids during lactose fermentation get a deposit in the growth and display a metallic sheen.Growth Appearance (+): Escherichia coli will give colonies with greenish-black metallic sheen due to metachromatic nature of the dyes included; other lactose fermenters will give dark purple-centered coloniesGrowth Appearance (-): whereas non-lactose fermenters give pale, colorless colonies on EMB agarMSA Agar - Mannitol Salt Agar is another commonly used medium for quick identification of pathogenic bacteria. This too is selective only for certain groups of bacteria while inhibiting others; at the same time, it helps differentiate those that grow on the basis of mannitol fermentation.Selective Components: 7 - 10% NaClDifferential Components: Mannitol and Phenol Red indicatorGrowth Appearance (+): Staphylococcus aureus - Yellow colonies as it ferments Mannitol, producing acidGrowth Appearance (-): Staphylococcus epidermidis - Pink colonies as it does not ferment Mannitol and rather decreases pH by using amines and peptides in the mediumThe three given organisms are identified as follows:Escherichia coli is a gram-negative, lactose-fermenting bacillus.Enterobacter aerogenes is a gram-negative lactose-non-fermenting bacillus.Staphylococcus aureus is a gram-positive mannitol-fermenting coccus.From the growth on selective and differential media, the organisms are identified, as shown in the image below: