Question

3. For all the various media types and tests used in this lab (not including the pre-enrichment an enrichment media), state what reation indicates a positive result for Salmonella and for E. co. 4. Why are the various foodstuffs used in this lab typically found to have high levels of bacteria an more specifically Salmonella? 5. Briefly describe the basis of serological identification and agglutination. 6. Why are there so many serotypes of Salmonella enterica? 7. Why was the Rappaport-Vassiliadis Soya broth incubated at 42.5°C, whereas the nutrient broth f pre-enrichment was incubated at 37'C 8. What was the purpose of adding lodine to the Tetrathionate Brilliant Green broth? 9. What is Lysine Iron agar (LIA) used for? Why does the LIA agar turn purple when lysine decarboxylated? 10. Suggest 2 media that would successfully distinguish Salmonella enteric and Shigela spp. Why wou you expect them to be similar or different?

Answer 1

3. Salmonella and E. Coli both are gram negative rods and belong to the enterobacteriaceae. They share quite a few common biochemical characteristics. Some of the reactions shown by them are as follows:

a. TSI Agar test. Bacteria produces H2S with TSI (triple sugar iron) agar which consists of glucose, sucrose, lactose, phenol as pH indicator and ferrous sulfate. H₂S produced in the medium reacts with iron to form ferrous sulfide which appears as black precipitate. Positive for Salmonella but Negative for E. coli.

b. Lysine Iron Agar test. Bacteria decarboxylates lysine (by the enzyme lysine decarboxylase) and produces H₂S which appears as a black precipitate in the butt. As a result of decarboxylation the medium turns alkaline which is indicated by appearance purple color. Positive for Salmonella and E. coli.

c. Urease test. Used to test the ability of a bacteria to produce urease and hydrolyze urea. Urea agar (medium) used for the test consists of urea and phenol red as pH indicator. The medium changes color to yellow in an acidic environment and fuchsia in an alkaline environment. Urease production hydrolyzes urea to ammonia and carbon dioxide and turns the medium fuchsia. Negative for both Salmonella and E. coli.

d. Citrate test. Citrate test is used to check whether a microbe can utilize citrate as a source of carbon and energy in the lab or not. The medium consists of bromothymol blue as pH indicator (green - neutral pH, yellow - acidic pH, blue - basic pH). Citrate utilization is indicated by color change of the medium from green to blue (pH change from neutral to basic). Positive for Salmonella but Negative for E. coli.

e. Oxidative Fermentation test. This is the used to test the ability of a bacteria to utilize carbohydrates under aerobic and anaerobic conditions. In this test 2 tubes are filled with medium (mannitol/glucose). One of them is sealed with paraffin oil layer acting as oxygen barrier. Bromothymo blue is used as pH indicator. Positive reaction turns the medium yellow (acidic pH) as a result of fermentation. Positive for Salmonella but Negative for E. coli.

f. Motility Indole test. It is a composite test to check Motility and Indole production. Mannitol motility (MM) medium can be utilized to test motility and mannitol fermentation. Phenol red is used as pH indicator. Color change is red to yellow (acidic medium). Motility test is Positive for both Salmonella and E. coli. Indole production is positive for E. coli but negative for Salmonella.

4. Laboratory characteristics not given in the statement but assuming that that lab has a large umber of enriched and enrichment media and considering the aerobic as well as facultative anaerobic nature of salmonella and wide temperature range of growth it can grow in a variety of medium and food stuff, even in canned sea foods. This is primarily the reason why salmonella is so abundant in lab foodstuff.

5. Slide agglutination test: The test employes use of antisera or antibody to identify salmonella and shigella by causing agglutination of the organisms. Antisera directed against the cell wall O antigens of salmonella and shigella are used for diagnostic purpose. On the other hand, Public health labs use antisera against flagellar antigen H and capsular Vi antigen of salmonella.

6. The reason behind the existence of numerous serotypes of salmonella enterica is the variations in the surface antigens (H & O) and Phase (V-W & S-R) variations.