Question

1)What color would a Gram-negative cell be if we skipped the iodine? What color would a Gram-positive cell be if we skipped the same step? Why?

2)Are all bacteria either Gram-positive or Gram-negative? If not, what is an example of a different type of bacteria? How does it differ from a Gram-positive/negative cell?

Answer 1

1. Both gram negative and gram positive would appear as pink if we skipped iodine.

Iodine here serve the purpose of a mordant in Gram stain.Modrant is an agent that fixes the primary stain (crystal violet) to the bacterial cell wall. Iodine enhances the affinity for crystal violet in cell(Make the stain difficult to remove). As a result,a crystal violet iodine complex is formed in gram positive bacteria due to it's thick peptidoglycan layer. But the gram negative bacteria is unable to retain the crystal violet because of it,s thin petptidoglycan layer in cell. So ,when using a decolorizing agent such as alcohol, gram positive bacteria able to retain it's color, but gram negative bacteria looses it's color and become colorless. On using counter stain(safranin or Basic fuchsin) gram negative bacteria accepts it's color and turn pink.

But in this case, iodine is not added. So, both gram negative and positive bacteria would remove the stain by crystal violet when a de staining or decolorizing agent is added. So, both gram positive and gram bacteria accept the secondary stain safranin and appear as pink.

2.There are bacteria that are neither gram positive nor gram negative called Atypical bacteria.They are not detectable on gram staining .This kind of bacteria do not accept any color on gram staining and remain colorless.

They are also known an incomplete bacteria.

Chlamydiaceae and the Mycoplasmataceae (including mycoplasma and ureaplasma)   are examples of atypical bacteria.

They are lacking the peptidoglycan layer , so they cannot retain crystal violet of safranin. So, they remain colorless.