Question

Identification of unknown bacterium:

In Step 1: You received an unknown broth culture in a tube containing a gram positive and gram negative microbe. You proceeded by inoculate the sample on a Soy Agar isolation streak plate.  RESULTS =  Separate smooth, gray isolates and Separate pinpoint, yellow/gold isolates

In Step 2: Isolation of pure cultures on separate agar slants. RESULTS = gray colonies were pure on the slant AND yellow colonies were pure on the slant.

In Step 3: Gram staining method to determine morphology and which is positive and negative. RESULTS = gray slant is Gram negative Bacilli AND yellow slant is Gram Positive Cocci.

PART A) The gram+ sample is either: S. Aureus, L. Lactis, or M. Luteus; what test should be permed next and why? should it be:

- Carbohydrate lactase fermentations, Dextrose,  Sucrose, MR Reaction, Citrate Use, Urease Activity, Catalyse activity, Gelatin Identification, or Lipid Hydrolysis?

PART B) The gram- sample (gray) is either: E. coli, E. aerogenes, K. pneumoniae, S. dysenteriae or Salmonella typhimurium. Which test will you do next and why?

- Carbohydrate lactase fermentations, Dextrose,  Sucrose, H₂S Production, Citrate Use, Urease Activity, Oxidase activity, Gelatin Identification, Starch Hydrolysis, or Lipid Hydrolysis?

Answer 1

1. Gram positive bacterial culture

Out of the given  three suspected cultures, S.aureus and M. luteus produce yellow colonies on Soy agar. Since the results of the first step shows the presence of yellow colonies,  the sample may be either of these two organisms.This can be further confirmed by making use of catalase test, which is positive for both of these organisms (S.aureus & M.luteus). A negative catalse test indicates the presence of L. lactis. So by conducting catalase test one can rule out the posiibility of the presence of L.lactis in the sample. Further to distinguish between S.aureus & M.luteus, an oxidase test could be carried out. A positive test indicates the presence of M.luteus and a negative reaction indicates S.aureus

2. Gram negative bacilli

The gram negative bacteria can be initially differentiated based on lactose fermentation. E .coli., E. aerogenes and K. pneumonia ferments lactose wheras S.dysentriae and S. typhimurium are non fermenters of lactose. The lactose non fermenters can be further differentiated based on H2S production. S. typhimurium produce H2S wheras S. dysentriae does not. Lactose fermenters (E .coli., E. aerogenes and K. pneumonia) can be further differentiated based on citrate test, which is positive for E. aerogenes and K. pneumoniae and negative for E.coli. Further differentiation of E. aerogenes and K. pneumoniae can ber done by urease test.E. aerogenes is urease negative and K. pneumonia is urease positive