We devised a Citrate-Acetate (CA) medium for rapidly differentiating Shigella. The medium consisted of 3.0 g of sodium citrate, 2.0 g of sodium acetate, 0.2 g of glucose, 1.0 g of dipotassium phosphate, 1.0 g of mono ammonium phosphate, 0.2 g of magnesium sulfate, 5.0 g of sodium chloride, 0.08 g of brom thymol blue, 15.0 g of agar, and 1000 ml of distilled water. An evaluation was made of the CA medium, for the rapid differentiation of 23
Shigella strains, 129
Escherichia coli strains and 130 isolates, that formed colourless colonies suspected to be
Shigella on SS agar plate, from feces of healthy people.
The results obtained were as follows
1) On the CA medium, all
Shigella strains did not grow and there was no change in colour.
2) Positive growth rates of
E. coli strains after incubation for 24 hr at 37° on CA medium, sodium acetate medium (Acet) and Christensen citrate medium (C-Cit) were 96.0%, 95.2% and 28.0%, respectively. Therefore, the positive growth rate of E. coli strains after incubation for 24 hr on CA medium was significantly higher (p<0.01) than that on C-Cit medium.
3) Positive growth rates of isolates after incubation for 24 hr at 37°C on CA medium, Acet medium and C-Cit medium were 95.4%, 83.1% and 71.5%, respectively. Therefore, the positive growth rates of isolates after incubation for 24 hr on CA medium was significantly higher (p<0.01) than that on Acet medium and C-Cit medium.
4) The genus of tested isolates were
Escherichia, Enterobacter, Hafnia, Pseudomonas, Citrobacter, Aermonas, Alcaligenes and
Morganella.
5) It was found that sodium acetate, and sodium citrate and glucose incorporated into the CA medium had enhanced the growth of
Escherichia and
Hafnia, respectively.
From the above results, the CA medium is recommend as a differential medium for use in the rapid differentiation of
Shigella and organisms that formed colourless colonies suspected to be
Shigella on SS agar plate in the routine examinations.
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