Question

Suppose your preformed a gram stain on a sample from a pure culture of bacteria and observed a field of red and purple cocci. Adjacent cells were not always the same color. What would you conclude? Human cells can be stained with crystal violet or safranin. So why can’t human cells be gram stained?

Answer 1

Both purple and red colour bacterial cells were obtained because the pure culture given to us was actually contaminated with some other type of bacteria. Therefore, the contamination led to the entry of bacteria from other source thereby giving both purple and red colour.

During Gram staining technique purple colour is given by Gram positive bacteria and red colour is given by Gram Negative.

Human cells can be stained by safranin and cannot be gram stained. This is because Gram staining technique is used for the the staining of peptidoglycan which is present in the cell wall of bacteria. This peptidoglycan is not present in human cells and therefore human cells cannot be stained using gram staining.

Although human cells can be stained at one time with Crystal Violet which will give purple colour and at other time with safranin which will give red colour but both of them cannot be used simultaneously or one after the other.

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