Why is citrate agar selective

Adjust pH to 6. Add agar and Bromothymol blue. Gently heat, with mixing, to boiling until agar is dissolved. The medium may be used either as slopes in test tubes or as a plate medium in petri dishes. In both cases the surface of the medium is lightly inoculated by streaking and, where slopes are used, the butt of medium is inoculated by stabbing. For tubes, dispense 4. Autoclave at degree C under 15 psi pressure for 15 minutes. Cool in slanted position long slant, shallow butt.

Tubes should be stored in a refrigerator to ensure a shelf life of 6 to 8 weeks. The uninoculated medium will be a deep forest green due to the pH of the sample and the bromothymol blue. Limitations of Simmons Citrate Agar It is important not to carry over any nutrients into the citrate medium because this will result in false positive tests. Some organisms are capable of growth on citrate and do not produce a color change.

Growth is considered a positive citrate utilization test, even in the absence of a color change. Tests with equivocal results should be repeated. The inoculum should be from a pure, overnight culture grown on a solid medium and not from a broth suspension.

Colonies may be picked directly off the medium for serological and biochemical tests. It should be noted that Escherichia coli survives on the medium even though it does not usually grow - therefore colonial purity should be established by subculture on to a differential but less inhibitory medium, e.

MacConkey Agar CM The medium is clear and pale pink. Lactose fermenting organisms produce pink colonies and may be surrounded by a zone of precipitated desoxycholic acid, which is due to acid production. The colonies of non-lactose fermenters are colourless, and due to their alkaline reaction they are surrounded by a clear orange-yellow zone of medium.

Escherichia coli - Most strains are inhibited, but the few strains which grow produce pink umbilicated colonies mm in diameter which may be surrounded by a zone of precipitation.

Shigella sonnei - the colonies are 1mm diameter after 18 hours incubation, and 2mm after 38 hours; they are smooth and initially colourless, becoming pale pink on further incubation due to late lactose fermentation. Shigella flexneri - colonies are colourless and similar in appearance to those of Shigella sonnei , but often with a narrow plane periphery round a central dome.

Salmonella paratyphi B - from 1mm diameter after 18 hours incubation to mm on the second day, when they are slightly opaque, dome-shaped, with a central black dot. Find out more article.

Simmons citrate agar test helps in detection of citrate utilizing bacteria. Which is used for the study of those organisms that have the ability to use citrate as a sole carbon source and ammonium ions as the sole nitrogen source. This medium composed of citrate which is the main carbon source of the medium, and ammonium ions as the main source of Nitrogen , and also contains several inorganic ions. This media also contain a pH indicator bromothymol blue , which turns in green at pH below 6.

The citrate utilization ability of a bacteria depends on the presence of a citrate permease that helps in the transport of citrate into the bacterium. Simmons citrate test performed in slants, because O2 is necessary for citrate utilization. In this method, if the test organism started to utilize the citrate for carbon and energy sources, it will grow on Simmons citrate agar.

As a result, it will lead to a rise in pH of the medium, and a pH indicator changes color. The Simmons citrate test helps to identify those bacteria have the ability to utilize citrate as a main carbon source for their energy needs.

Simmons citrate test is used for the detection of carbon utilization bacteria from Simmons citrate agar. In this method, a test microorganism first inoculated with the Simmons citrate agar, which is composed of citrate, ammonium, few inorganic ion for growth, and a pH indicator such as bromothymol blue. The utilization of citrate is occurred by the enzyme citrase. This citrase enzyme helps in breaks down of citrate into oxaloacetate and acetate.

After that, oxaloacetate again break down into pyruvate and carbon dioxide CO2. On the other side, the bacteria started to breakdown the ammonium salts into ammonia, as a result, it increases the alkalinity of medium. This raises the pH, turns the pH indicator to a blue color, and represents a positive citrate test, absence of a color change is a negative citrate test.

This Simmons citrate test is used to identify gram-negative pathogens of Enterobacteriaceae family and environmental isolates. Also used as an instance test kits such as the APIE and Enterotube II include citrate utilization medium as one of the diagnostic tests. Enterobacter aerogenes is a citrate positive where color of the medium changes from Green to blue while E.