3.13.3: True/False

1. The selective nature of the MacConkey agar is due to the presence of crystal violet and bile salts, which inhibits Gram-positive bacterial growth.

2. The catalase test should have the same result whether the bacteria are on a slant, on a nutrient agar plate, or have been transferred to a glass slide.

3. A phenol red result that gives an alkaline result but shows gas production in the Durham tube is most likely a result of reversion.

4. All bacteria have the ability to ferment any sugar available.

5. A phenol red dextrose tube can be incubated on Friday and then observed the following Monday.

6. The methyl red test would be highly useful in differentiating Staphylococcus and Streptococcus species of bacteria.

7. The oxidase test is used to distinguish enteric bacteria from non-enteric bacteria.

8. In the catalase test, if possible, you should not test colonies from a blood agar plate because they may give a false positive due to the erythrocytes having catalase activity.

9. Phenol red will turn yellow in the presence of acid.

10. When looking at a blood agar plate with bacterial growth, beta hemolysis is complete lysis of the red blood cells, alpha hemolysis is partial lysis, and gamma hemolysis is no lysis.

11. In order for an organism to give a positive result in a methyl red test, it must produce enough acid from fermentation to overcome a phosphate buffer.

12. The urease test is used to test for gram-positive streptococci.

13. A nitrate test that turns red after the addition of nitrate reagents A and B and the addition of zinc and HCl would be negative for nitrate reduction.

14. Bubbles are expected to form if hydrogen peroxide is introduced to a sample of Staphylococcus epidermidis on a microscope slide.

15. The ingredient added to the agar for the casein test that allows differentiation between positive and negative results is skim milk.

16. A sample can safely be concluded to be negative if it remains clear after the addition of sulfanilic acid and alpha-naphthylamine in the nitrate reductase test.

17. Escherichia coli bacteria would produce transparent colonies when growing on a MacConkey agar plate.

18. A positive result for starch hydrolysis is observed when halos are observed around the bacterial cultures after the addition of iodine to the starch plate.

19. Bacteria capable of lactose fermentation will change the media of a MacConkey agar to a pink color.

20. The addition of Kovac’s reagent onto a SIM agar that has been incubated with a bacterial species capable of producing the enzyme tryptophanase would result in a cherry red color change.