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gram stain

differential stain -very useful for identifying and classifying bacteria as either gram negative or gram positive.
-primary stain
-decolorizing agent
secondary stain or counterstain

primary stain

crystal violet.
-all bacteria are stained purple by this basic dye


gram's iodine
iodine combines with crystal violet in the cell to form a crystal violet-iondine complex

decolorizing agent

ethyl alcohol or ethyl alcohol-acetone
primary stain is washed of some bacteria while others are unaffected

secondary stain

or counterstain (safranin)
this basic dye stains the decolorized bacteria red

gram negative

those that decolorize easily
E. coli

gram positive

those that retain the primary stain and do not decolorize easily
S. epidermis

staining differently

bacteria stain differently in their cell walls
gram stain is most consistent when done on young cultures of bacteria
because the gram stain is usually the first step in indentifying the bacteria


old gram-positive cells stain gram-negative bcuz cell wall starts to deteriorate and takes in the color
iondine cannot be added before primary stain bcuz it is not a stain itself but it attaches to crystal violet
-safaranin can be ommitted bcuz if gram + still purple, gram - will be colors less , so u can still tell the difference
gram positive and gram negative differ for antibiotics

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