Question

62. During staining, the smear is heat-fixed in order to                                                                                                 

A. kill the organism so that dyes will penetrate                                                                                                      

B. attach the organism firmly to the slide                                                                                                                     

C. kill the organism and attach the organism firmly to the slide                                                                                                                 

D. neither kill the organism nor attach the organism firmly to the slide

63. Which of the following is true about negative staining and positive staining?                                                                               

A. Negative staining is used with live organisms while positive staining is applied on dead organisms.                                                                                                                                                                          

B. Negative staining colors the background only while positive staining colors the specimen.                                                                                                                      

C. Both increase the contrast between the specimen and the background.                                                                      

D. Negative staining uses acidic dyes while positive staining uses basic and acidic dyes.

E. All of the above

64. Let's assume that you were teaching the Gram stain to 5 high school students, and each student did one thing wrong during the Gram-staining procedure, as shown in the following list. All five students were staining slides containing a mixed culture of a Gram-positive rod and a Gram-negative coccus. Of the 5 students listed below which one will probably get a CORRECT Gram reactions (assuming everything else was done correctly)?                                                                  

A. One put the alcohol on first instead of when it should have been applied.                                                                                 

B. One used basic fuchsin (a red dye) instead of safranine as a counter stain.

C. One forgot the alcohol step completely.                                                                                                            

D. One forgot to use a counter stain.                                                                                                                                        

E. One left the alcohol on for 20 minutes instead of 20 seconds

Answer 1

62)

C.Kill the organism and attach the organism firmly to the slide.

Heat fixing is done to attach organism to the slide this also inactivates the enzymes that can degrade the stain. If due is added to a live organism , the membrane wouldn't take up the dye efficiently.

63) E. All of the above.

64)B.One used basic fushcin instead of safranin as counter stain.

Changing counter stain doesn't change the result obtained, gram negatives appear red and gram positives appear purple.When alcohol is added first, the outer membrane of gram negative bacteria degrades exposing peptidoglycan layer. When crystal violet is added, it takes up the dye. Like gram positives peptidoglycan layer don't loose the dye when counter stained. Hence both appear violet.