1)Urea broth was prepared and sterilized by autoclaving. Bacteria was inoculated

Question

1)Urea broth was prepared and sterilized by autoclaving. Bacteria was inoculated and after an overnight incubation, the color was unchanged. What conclusion can you make from this result?

2)When un-inoculated Simmons Citrate media was incubated overnight, it turned blue. Explain this color change.

3)For a unknown, your results are the following: Gram negative rod; turbidity and yellow color in Phenol Red Lactose; Turned blue in Simmon’s Citrate agar; Urea broth turned pink; Methyl Red negative; Vogues-Proskauer positive; Kovac’s reagent negative; non-motile on motility tube. What is this unknown?

4)  According to Table 39.3 in the Lab Manual, K. pneumonia bacteria is citrate Positive. Explain how and why a strain of K. pneumonia would result in a citrate negative result.

Explanation / Answer

1st- Urease positive organisms convert urea into ammonia which makes the environment alkaline . Following this the pH indicator color changes from pale yellow to pink. However if an organism is urease negative , the medium dosent changes the color.

2nd question : It may be because the agar is inoculated with other carbon sources like glucose which is giving the false positive results. For this the agar should be diluted in saline before the medium is inoculated.

3rd question : It is clearly Klebsiella pneumonia

4th question : Such conditions are called as false negative . This is usually seen in following conditions :

1- When you inoculate large amounts of the inoculum.

2- inoculum is from broth culture and not pure overnight culture.

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