From Big Medical Encyclopedia

CLARK WEDNESDAY (W. M. Clark, amer. chemist, sort. in 1884) — a differential and diagnostic medium for definition at bacteria of intensity of a kislotoobrazovaniye from glucose (reaction with methyl red) and formations of acetylmethylcarbinol (Foges's reaction — Proskauera). It is applied to identification of enterobakteriya, generally the escherichias allocated at a sanitary bakteriol. research of water, milk, foodstuff.

For preparation To. pages in a mortar mix 5 g of peptone, 5 g of glucose, 5 g of double-base potassium phosphate (K 2 HPO 4 ). Mix is dissolved in 800 ml of a distilled water and within 20 min. warmed up during the stirring on the water bath. Hot solution is filtered via the linen or paper filter, cooled to t 20 °. Adding a distilled water, bring volume to 1000 ml, spill 5 ml in test tubes and carry out fractional sterilization within 3 days in a row. The studied culture is sowed in To. page.

For statement of reaction with methyl red to 5 ml of the four-day culture which is grown up on To. villages, add 5 drops of 0,04% of solution of methyl red. At a strong kislotoobrazovaniye< (pH 5,0) red coloring appears — reaction is designated by Mr+; at a weak kislotoobrazovaniye> (pH 5,0) — yellow coloring (Mr-). It is possible to use spirit indicators which can remain vaguely long time. However it is better to use water solutions of indicators as more sensitive. Keep these solutions in the cold in bottles with ground stoppers.

For statement of reaction of Foges — Proskauera to 5 ml of five-day culture on To. villages add 1 ml of 10% of water solution caustic heat and put in the thermostat at t ° 37 °. Since coloring appears gradually, reaction is read in 18 — 24 hours. At formation of acetylmethylcarbinol pink coloring with yellow flocculation, characteristic of spirit solution of eosine (positive reaction) is observed. In case of a negative take coloring does not appear, Wednesday remains colourless. Reaction yields a positive take at the bacteria fermenting glucose. Reaction turns out more accurate in cultures of those bacteria which give negative reaction with methyl red, i.e. at> pH 5,0. See also Differential and diagnostic environments , Fogesa-Proskauera reaction .

Bibliography: Kozlov Yu. A. Mediums in medical microbiology, M., 1950, bibliogr.; The reference book on microbiological and virologic methods of a research, under the editorship of M. O. Birger, page 60, M., 1973.

I. M. Goncharenko.