After gram staining a sample from a pure C. acnes colony, the result I get is somewhere between purple and red. Is there a reason why I'm not seeing a clear purple colour for a gram-positive bacteria?
Maybe you have exceeded the duration of the alcohol decolorization step, or the high concentration of alcohol that you use.
Hello dear, I agree with Yacin. Perhaps the alcohol concentration was high, or the time of washing the smear with alcohol was more than the required time
The decolorization of the cells is the most “operator-dependent” step of the process and the one that is most likely to be performed incorrectly. As well as certain gram-positive bacteria lose some of their cell wall properties with age or exposure to harsh or bacterial static/ bactericidal agents causing them to appear gram-negative or gram-variable.
http://www.microbiologynetwork.com/the-gram-stain.asp
https://www.arrowscientific.com.au/index.php?option=com_content&view=article&id=48%3Adifferentiating-gram-negative-and-gram-positive-bacteria&catid=25&Itemid=40
False-negative Gram stains could occur due to inadequate specimen or smear preparation or failure to examine an adequate number of fields.
Article Multicenter Assessment of Gram Stain Error Rates
Dear Salma Walid . See the following useful link: https://asm.org/getattachment/5c95a063-326b-4b2f-98ce-001de9a5ece3/gram-stain-protocol-2886.pdf
Kindly see also the following useful link: https://medlineplus.gov/lab-tests/gram-stain/
Also check please the following useful link: https://www.msdmanuals.com/en-gb/home/infections/bacterial-infections-gram-negative-bacteria/overview-of-gram-negative-bacteria
Grain staining is a delicate procedure that requires attention to detail.
If you exceed the duration of the alcohol decolourization step or a high concentration of alcohol, it will yield a false result.
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