Question

1. Can iodine be added before the primary stain in a Gram stain? Explain. If iodine was added before the primary stain, what would be the final color of gram positive and gram negative cells in the stain and why?

2. Since you can’t identify bacteria from a Gram stain, why might a physician perform a Gram stain on a sample before prescribing an antibiotic?

3. The cell envelope of Mycobacterium sp. is unlike the envelope of other bacteria. While genetic evidence indicates that these organisms are related to gram-positive bacteria, one could argue that their structural characteristics could place them in place them among the gram-positives or the gram-negatives. Explain.

Answer 1

1. Iodine is used as a mordant in Gram staining. Mordant means trapping agent. It is used for trapping the Crystal voilet molecules to make big sized Crystal Violet Iodine molecules. These molecules struck into the peptidoglycan meshwork and impart purple colour to Gram Positive Bacteria.

So even if we add iodine before Crystal Violet stain, then also big sized Crystal Violet iodine Complexes will be formed and will get struck into the meshwork of peptidoglycan.

The final colour of Gram Positive and Gram Negative bacteria will be same if we add iodine before Crystal Violet. Gram Positive Bacteria will appear purple and Gram Negative bacteria will appear red after staining with safranin.

2. The use of antibiotics depend upon the type of cell wall present in the bacteria. Gram Positive Bacteria can be easily targeted by antibiotics if we target peptidoglycan of these bacteria where as Gram Negative bacteria can not be easily targeted by peptidoglycan targeting antibiotics. Therefore it is very important to know the type of bacteria before prescribing antibiotics to anyone.

Gram staining help the doctor to know about which type of bacteria is present and then prescribe the antibiotics accordingly.

3. Mycobacteria give Gram Positive result after Gram staining. We can make false interpretation like the given bacteria and Gram Positive but not mycobacterium. But the real thing is that mycobacterium and Gram Positive Bacteria, both are two different types of bacteria. The cell wall of Gram Positive Bacteria is rich in peptidoglycan whereas mycobacterium does not contain peptidoglycan. The cell wall of mycobacterium contains mycolic acid.

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