Page 55 - MNUmicrobiology practical 2025
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General Microbiology & Immunology (PM 401)    second level    Semester 4       2024/2025


                  •  When counterstain, positively charged safranin, is added, it interacts with the free

                     negatively charged components in Gram-Negative cell wall and membrane and

                     bacteria  becomes  pink/red.  Whereas,  there  is  no  space  to  enter  inside  the

                     dehydrated Gram-Positive cell wall due to CVI complex and dehydration. Hence,

                     safranin can’t stain them red or pink and Gram-Positive bacteria reveal the purple

                     or violet color.






































                  Gram staining requirements



                  1- Stock culture containing organism #1

                  2- Stock culture containing organism #2

                  3-  Clean microscope slides

                  4- Gram’s staining reagents

                  -Gram’s crystal violet [primary stain]



                  General Microbiology & Immunology (PM 401)   Practical note                         Page 44
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