SIM agar

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The SIM Agar is a semi-solid nutrient medium for the differentiation of bacteria from the family Enterobacteriaceae and can in a colorful row are used for their identification. This medium is recommended by the American Public Health Association for the microbiological examination of food. With the SIM agar three features can be assayed simultaneously: the formation of S chwefelwasserstoff (H 2 S), the I ndol-forming and active movement ( M otilität ) of bacteria. The nutrient medium is in test tube used as a high-layer tube and with the aid of an inoculating needle with the pure culture is inoculated.

Mode of action

Escherichia coli cultivated in SIM agar, with a negative result for H 2 S formation, with a positive result for the indole test (after adding Kovac's indole reagent), and with a positive result for motility.
Cosenzaea myxofaciens (classifiedas Proteus myxofaciens until 2011) cultivated in SIM agar, with a positive result for H 2 S formation, with a negative result in the indole test, and with a positive result for motility.

The SIM culture medium contains the inorganic compound sodium thiosulphate . Some representatives of the Enterobacteriaceae, such as B. Salmonella and Proteus are able to reduce such sulfur compounds to hydrogen sulfide (H 2 S) . The H 2 S formed reacts with the iron salt ammonium iron (III) citrate , which is also contained in the medium, to form iron sulfide , which is precipitated as a black precipitate. Other nutrient media that use the same principle to demonstrate the formation of hydrogen sulfide are Kligler agar (iron two-sugar agar according to Kligler ) and TSI agar ( triple sugar iron agar ).

As part of the nutrient medium, peptone provides the bacteria with the amino acid tryptophan . Some types of bacteria have the enzyme tryptophanase and can break down tryptophan into indole , pyruvate and ammonia . The indole formed is detected in the indole test with Kovacs reagent . However, this test is only carried out after incubation and evaluation of the other characteristics by adding the reagent manually. A cherry-red coloration of the solution indicates a positive result for indole formation. If the solution remains colorless or turns yellow, the bacterium is indole-negative.

Furthermore can the mobility ( motility , in English motility ) of bacteria recognize this movement is active in bacteria by flagella caused. The finished SIM medium contains less agar than usual with a solid nutrient medium. The bacteria can spread in this so-called soft agar , provided they are mobile. This leads to a clouding of the medium by the bacteria outside the branch duct, while in the case of immobile bacteria only growth and thus clouding can be observed in the inoculation duct. The tests for mobility and indole formation can also be carried out with the help of a MIO tube .

Test results

Results of the reactions with the aid of SIM agar as part of Enterobacteriaceae diagnostics
Microorganism H 2 S formation Indole formation agility
Escherichia - + +/-
Enterobacter - - +
Klebsiella - - -
Salmonella + - +
Shigella - +/- -
Proteus vulgaris + + +
Proteus mirabilis + - +

Typical composition

The nutrient medium usually consists of (data in grams per liter ):

See also

Individual evidence

  1. ↑ Don F. Splittstoesser (Ed.): Compendium of Methods for the Microbiological Examination of Foods . 3. Edition. American Public Health Association 1992, ISBN 0875531733 .
  2. ↑ a b c Roland Süßmuth, Jürgen Eberspächer, Rainer Haag, Wolfgang Springer: Biochemical-microbiological internship. 1st edition. Thieme Verlag, Stuttgart / New York 1987, ISBN 3-13-685901-4 .
  3. ↑ a b c Technical information SIM nutrient medium from Merck KGaA, accessed on March 2, 2013.
  4. ↑ Michael T. Madigan, John M. Martinko, Jack Parker: Brock Microbiology. German translation edited by Werner Goebel, 1st edition. Spektrum Akademischer Verlag GmbH, Heidelberg / Berlin 2000, ISBN 978-3-8274-0566-1 .