All strain of s. Typhi can able to utilize citrate
Answers
Citrate = oxaloacetate + acetate
oxalacetate = pyruvate + CO2
Citrate Utilization Test
Citrate Utilization Test
Further metabolic breakdown is dependent upon the pH of the medium.
A. Under alkaline conditions, pyruvate is metabolized to acetate and formate.
pyruvate = acetate + formate
B. At pH 7.0 and below, lactate and acetoin are also produced.
pyruvate = acetate + lactate + CO2
pyruvate = acetoin + CO2
The carbon dioxide that is released will subsequently react with water and the sodium ion in the medium to produce sodium carbonate, an alkaline compound that will raise the pH. In addition, ammonium hydroxide is produced when the ammonium salts in the medium are used as the sole nitrogen source.
Growth usually results in the bromothymol blue indicator, turning from green to blue. The bromothymol blue pH indicator is a deep forest green at neutral pH. With an increase in medium pH to above 7.6, bromothymol blue changes to blue
Citrate utilization test A: Negative B: Positive
Citrate utilization test (Image source: Microbelibrary)
A: Negative
B: Positive
Procedure of citrate utilization test:
Inoculate simmons citrate agar lightly on the slant by touching the tip of a needle to a colony that is 18 to 24 hours old.
Incubate at 35oC to 37oC for 18 to 24 hours. Some organisms may require up to 7 days of incubation due to their limited rate of growth on citrate medium.
Observe the development of blue color; denoting alkalinization.
Expected results in citrate utilization test:
Citrate positive: growth will be visible on the slant surface and the medium will be an intense Prussian blue. The alkaline carbonates and bicarbonates produced as by-products of citrate catabolism raise the pH of the medium to above 7.6, causing the bromothymol blue to change from the original green color to blue .
Citrate negative: trace or no growth will be visible. No color change will occur; the medium will remain the deep forest green color of the uninoculated agar. Only bacteria that can utilize citrate as the sole carbon and energy source will be able to grow on the Simmons citrate medium, thus a citrate-negative test culture will be virtually indistinguishable from an uninoculated slant
Quality Control strains used in citrate utilization test
Positive: Klebsiella pneumoniae
Negative: Escherichia coli
List of Bacteria which gives positive citrate utilization test
Klebsiella pneumoniae
Enterobacter species (minority of strains gives negative result)
Citrobacter freundii
Salmonella other than Typhi and Paratyphi A
Serratia marcescens
Proteus mirabilis (minority of strains gives negative result)
Providencia
Citrate Test: variable (different strains give different results)
Proteus vulgaris
Vibrio cholerae
Vibrio parahaemolyticus
Citrate test: negative
Escherichia coli
Shigella spp
Salmonella Typhi
Salmonella Paratyphi A
Morganella morganii
Yersinia enterocolitica