Bacteriology 102:


John L's BacteriologyPages >
Bact.102 Website–Fall 2006 >
Media Review Questions

On this page is a set of questions on mediaformulation.
Links are provided for answers for the firstthree.
Some general review questions can be found here.
The lab manual referred to hereinis referenced here.

All of thesequestions have been given in tests or quizzes
in Bacteriology 102 in recentyears.

Click here for ashort essay on "media programming."
Click here for aspecial page on Kligler Iron Agar.

1.  Relevant to formulation ofselective-differential isolation media and Experiment 14:  You wish toexploit certain properties of the difficult-to-isolate bacteriumExcalibacterium (an enteric) in order to help you detect and isolate itfrom samples which are highly contaminated with other enterics. You decide tostart with MacConkey Agar which you know contains lactose as the onlyfermentable sugar. Peptone is another medium ingredient which you recall; itcontains a mixture of various amino acids – none in any especially highamount. Following is a table showing important genera to consider in thissituation:

genusfermentation ofdecarboxylation of
other enterics+++ or –+ or –+ or –+ or –+ or –

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

2.  Another question relevant todifferential media and Experiment 14:  You are in an enteric lab outin the real world, and you are picking colonies off plates of selective-differentialisolation media for further testing. These plates had been inoculated withenvironmental samples, and we expect a variety of enterics to be present. Wealso expect some colonies of that pesky Pseudomonas to be presentalso.

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

organismfermentation ofdecarboxylation ofH2S
other enterics++++ or –+ or –+ or –+ or –+ or –
Pseudomonas+ or –?

As part of the enteric isolation routine, you plan on picking colonies into ascreening medium, such as Kligler Iron Agar (KIA) before doing a lot of specifictests. Before you do that, you decide you have time to make a modification ofKIA 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
pictoralrepresentation of the setup which can be found here.

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

3.  Here is an easier KIA-related question:  You have aKligler Iron Agar (KIA) slant which you inoculate with an enteric that fermentsglucose (the sugar that is in the relatively low concentration in KIA) but notlactose (the sugar in the high concentration). The organism also deaminatessome of the amino acids in peptone (as do all enterics), and it alsodecarboxylates lysine but does not produce H2S.

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

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


glucose fermentation++++++
lactose fermentation+
mannitol fermentation+++
H2S production++
ornithine decarboxylation+ or ++ or + or

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

strictly aerobic
able to reduce nitrate to nitrite (therefore capable of a form of anaerobicrespiration)
able to use nitrate as a source of nitrogen, and starch asa 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