Expert: Florence M Rollwagen - 2/16/2009

Question
Why will old gram-positive cells stain gram-negative?

Answer
Hi Torra:  Thanks for your question.

The gram stain differentiates between different kinds of bacteria and is the key test in any bacterial identification.

The first step, staining with crystal violet, may not take if the bacteria have a damaged cell membrane.  This occurs if they are dead, old or damaged.  

Then when the second stain is applied (safranin) the (not)blue cells will take it up and appear gram negative.

Some bacteria give a gram-variable pattern: a mix of pink and purple cells. Breakage of the cell wall can occur in Actinomyces, Corynebacterium, Mycobacterium, and Propionibacterium which will give gram-negative staining of these gram-positive cells.

In cultures of Bacillus and Clostridium a decrease in peptidoglycan thickness during growth coincides with an increase in the number of cells that stain gram-negative.

Hope this helps.  Write back if you have more questions.

FM Rollwagen, PhD