Bacteriology 102:


John L's Bacteriology Pages >
Bact.102 Website–Fall 2006 >
Media Review Questions
Page Last

On this page is a set of questions on media formulation.
Links are provided for answers for the first three.
Some general review questions can be found here.
The lab manual referred to herein is referenced here.

All of these questions have been given in tests or quizzes
in Bacteriology 102 in recent years.

Click here for a short essay on "media programming."
Click here for a special page on Kligler Iron Agar.

1.  Relevant to formulation of selective-differential isolation media and Experiment 14:  You wish to exploit certain properties of the difficult-to-isolate bacterium Excalibacterium (an enteric) in order to help you detect and isolate it from samples which are highly contaminated with other enterics. You decide to start with MacConkey Agar which you know contains lactose as the only fermentable sugar. Peptone is another medium ingredient which you recall; it contains a mixture of various amino acids – none in any especially high amount. Following is a table showing important genera to consider in this situation:

genus fermentation of decarboxylation of
glucose maltose lactose sucrose mannitol lysine arginine
Edwardsiella + + +
Aquamonas + + ++
Excalibacterium + + +
other enterics + + + or – + or – + or – + or – + or –

A nearly-identical question is presented in Appendix X of the
Bact. 102 lab manual, the answers to which are given here.

2.  Another question relevant to differential media and Experiment 14:  You are in an enteric lab out in the real world, and you are picking colonies off plates of selective-differential isolation media for further testing. These plates had been inoculated with environmental samples, and we expect a variety of enterics to be present. We also expect some colonies of that pesky Pseudomonas to be present also.

Now, the organism you are specifically after is Sorgobacter, an enteric with one or more characteristics that allow it to be differentiated from all other enterics – as seen by the reactions in the following table:

organism fermentation of decarboxylation of H2S
glucose fructose galactose lactose mannitol arginine lysine
Sorgobacter + + +
other enterics + + + + or – + or – + or – + or – + or –
Pseudomonas + or – ?

As part of the enteric isolation routine, you plan on picking colonies into a screening medium, such as Kligler Iron Agar (KIA) before doing a lot of specific tests. Before you do that, you decide you have time to make a modification of KIA that will allow you to decide whether or not you have Sorgobacter, just from the appearance of the modified KIA after incubation.

Work through these questions and check your answers with a
pictoral representation of the setup which can be found here.

To continue the story with some "recently-discovered"
Sorgobacter-like organisms to differentiate further, click here!

3.  Here is an easier KIA-related question:  You have a Kligler Iron Agar (KIA) slant which you inoculate with an enteric that ferments glucose (the sugar that is in the relatively low concentration in KIA) but not lactose (the sugar in the high concentration). The organism also deaminates some of the amino acids in peptone (as do all enterics), and it also decarboxylates lysine but does not produce H2S.

Pending a higher-resolution key, a pictoral
representation of the KIA results is found here.

4.  Here is yet another KIA-related question:  You isolated an enteric from beautiful Lake Splammo, and your Kligler Iron Agar culture of the isolate has a red (alkaline) slant and a yellow (acidic) butt after one day of incubation at 37°C. To begin the identification process, you consider the reactions on the following table:


TEST coliforms Shigella Morganella Providencia Proteus Citrobacter Pseudomonas
glucose fermentation + + + + + +
lactose fermentation +
mannitol fermentation + + +
H2S production + +
ornithine decarboxylation + or + + or + or

5.  Relevant to formulation of selective isolation media and our enrichment-isolation experiments:  You have been commissioned to isolate the following type of bacterium from soil:

strictly aerobic
able to reduce nitrate to nitrite (therefore capable of a form of anaerobic respiration)
able to use nitrate as a source of nitrogen, and starch as a source of carbon and energy
grows at normal incubation temperatures (25-37°C)

Home Page of Bacteriology 102 Web Site
Index of the General Bacteriology Pages  

Page last modified on 8/14/03 at 11:15 AM, CDT.

John Lindquist, Department of Bacteriology

University of Wisconsin – Madison