Question

You have obtained samples from a patient who is in the ICU at your hospital. You perform the Gram-Stain and IMVIC tests, and see the following results: Gram-Stain Upon examination under the microscope you view pink colored rods. IMVIC Indole: After the addition of Kovac's Reagent, you see no color change at the top of the media. Methyl Red: After the addition of methyl red, your broth immediately turns a red color. Voges Proskauer: After the addition of VP reagents A and B, your broth does not change color. Citrate: Following incubation, you observe the your Simmons Citrate slant is green in color, but after further research, you learn that you may also observe a blue color. H2S: You see black precipitate in your media. I Starch Hydrolysis Following incubation, you add iodine to your starch agar plate, and do not observe any clearing around the growth of your culture. The plate remained a blue-black color throughout. Catalase After incubation, you obtain a colony from your culture, and place it on a slide. You then add hydrogen peroxide, and see bubbles appear. Oxidase After incubation of your culture, you obtain a sample with a swab, and you add the oxidase reagent. You do not observe any color change. Nitrate Upon incubation of your culture in the nitrate media, you add nitrate reagents A and B, and immediately observe a red color,

Answer 1

Gram stain : It is commonly used to differentiate the gram positive and gram negative bacteria. In the above scenario the Gram stain results in gram negative bacteria(pink colour).

For gram positive the results will be Purple colour(crystal violet), for gram negative bacteria the results will be pink colour due to treatment with counter stain(safranin) thus this test helps in determining the positive or negative bacteria.

IMVIC TESTS:

Indole test: it is done to identify the use of tryptophan by the microorganism by using enzyme tryptophanase which gives the end product as indole.

The indole positive is determined by a red colour layer at the top of medium in tube and negative by no change.( In the above scenario the microorganism is negative for Indole test)

Methylred : to determine the microorganism uses the glucose as source and produces acid subsequently as end product. (In the above scenario, the appearance of red colour indicates the positive result. The negative will be no colour change medium remains yellow)

Voges Proskauer : it is to determine the glucose use but for the different end product (acetoin) not the acid. (In the above scenario, there was no change indicating negative result, the positive result is indicated by a light pink colour intensifies (reagent reacts with acetoin) at the top) Note: this test should be done carefully any disturbance or error will lead to falsified result.

H2S : the black colour indicates positive result, that is the sodium thiosulfate in the medium is used by the bacteria and produces H2S as colour less on treating with Ferric ammonium sulfite it will produce black colour.

Citrate : this is to test the alkanie by product after citrate use. The citrate agar is green in colour the positive result will be shown up by royal blue colour at the slant.

Starch hydrolysis: to determine the convertion of starch to maltose (reducing sugar test) the blue-black colour indicates the microorganism does not hydrolyse the starch. Negative for reducing sugars.

Catalase test: catalase positive produce O2 bubbles( in above scenario) the microorganism is aerobic.

Oxidase test : no change represent negative result. It is done for identification of gram negative bacteria. (Anerobic respiration)

Nitrate test: it is to determine the ability of microorganism to reduce the nitrate to nitrite or furthur to N2 gas. Ability of microorganism to reduce nitrate to nitrite in anaerobic conditions. The red colour indicates the positive for test.

So from gram stain we found it is Gram negative bacteria, by IMVIC we found it is Indole - negative, Methylred - Positive, VP - Negative, Citrate - Negative, H2S - Positive), for Starch hydrolysis - Negative, For Catalase - Positive, For oxidase - negative indicate (anaerobic), Nitrate - Positive (anaerobic).

So its is Gram negative Anaerobic microorganism. Suggesting Enterobacter species.

Thankyou...