Expert: Florence M Rollwagen - 7/22/2008

Question
Does e.coli always show on MacConkey as red/pink?  Can it show as colorless colonies as well?

Answer
Hi Renae:  Thanks for your question.

MacConkey agar was originally made with lactose to distinguish Salmonella from coliform bacteria such as E coli.

I’m assuming you mean does E coli grow as clear colonies on traditional MacConkey (with lactose)?  The answer is a qualified yes.  In a student laboratory where we buy our cultures from a supplier, the E coli is non pathogenic and will grow as pink colonies.

In real life, however, enteropathogenic E coli can be isolated from clinical cases of bloody diarrhea and can grow as lactose negative colonies (clear on MacConkey).  http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=266325

Similarly, enterohemorrhagic E coli can grow as clear colonies on MacConkey agar containing sorbital as the carbohydrate.  http://jcm.asm.org/cgi/reprint/33/8/2199.pdf

If you are a micro student, the probability of your getting a pathogenic E coli as your sample is virtually nil.  (We don't give pathogens to students!!) If you’re growing clear colonies on MacConkey (with lactose), I would suspect that you don’t have E coli.

I hope this answer has helped you.  Please write back if you have more questions.

FM Rollwagen, PhD