Question

A.)There are four important steps in a Gram stain. List the specific stains/chemicals used in each step, and the color of both Gram positive and Gram negative bacteria after each step.

B.)Describe 2 reasons why heat fixing a specimen before viewing under a microscope is important.

C.)Describe how the design of the swan - necked flasks in Pasteur's classic experiment helped disprove spontaneous generation. What were the results of the experiment? How did it counter the "vital force" argument believed by many scientists.

Answer 1

1.Four important steps in gram stain are:

a.primay stain b.iodide treatment c. Decolourisation d.counterstaining

Where primary stain is crystal violet which is taken up by the bacteria than fixation with iodine takes place so that crystal violet is retained in the bacteria and after washing aa alcohol which acts as a decolourising agent which distrupts the cell membrane and after adding counterstain saffranin they gives pink colour.

Gram positive bacteria contains more that is thick peptidoglycan layer so that the rupture of cell wall is no so easily so saffranin cannot enter because of no cell wall distruption in gram negative bacteria due to less cell wall there is uptake of saffranin.Hence gram positive gives violet colour and negative pink colour.

2.Heat fixing is important because during washing the slide after each step there is a chance of bacteria gets washed off because of which heat fixation is necessary to retain the bacteria on slide .

And it makes the bacteria to kill so that we can study the arrangement of bacteria and killing the bacteria makes to take the dye easily.

3.swan theory is that he took a bottle neck jar and added the medium so that to prove that the fermentation is due to germ theory that is when flask and sterile media is placed due to presence of small tube air could not pass easily and medium remains sterile.when we remove or tilt tube the media is with microorganism and growth occur so that he proves that spontaneous generation is not cause of growth of bacteria . It is due to air flow where bacteria exists.

So he counted the vital force and made the scientists believe is that spontaneous generation is not cause of life.where life only comes from the other organisms not spontaneously.