Will a EMB plate result change color after too much time spent in the fridge?

[audacityxx]

Working on finding out an unknown bacteria for my Micro class. I'm pretty sure I remember seeing a growth with green metallic sheen on my EMB plate the week after preparing the plate. Weeks later and going over my stuff for the last time, the growth turned to a dark purple.

 

Could this be possible? If so, I can go ahead with my guess being E. Coli and chalking up the color change to the plate being left in the fridge for way too long.

 

Here are my unknown test results.

 

PRS: Positive for fermentation (acid) + gas

PRL: Positive for fermentation (acid) + gas

PRG: Positive for fermentation (acid) + gas

Citrate: Negative (was a green color)

MR: Positive (all red)

VP: Negative (no red color)

Nitrate: Positive w/o adding zinc (nitrate to nitrite)

Thioglycollate Broth: Shows Facultative Anaerobic

SIM: Negative for H2S (no black), Positive for indole and motility

Nutrient Gel: Negative (stayed gelatin in cold water)

Urea: Negative (no color change)

Motility: Positive

Litmus Milk: Negative for Casin (no curd), positive for acid, neg for Litmus Reduction (turned pink)

Catalase: Positive (bubbles)

Oxidase: Negative

TSI: Positive for Glucose, Lactose and/or sucrose fermentation, negative for gas

Pheny: Negative (no color change)

Tryptone Broth: Positive for Indole

 

Plates:

Spirit Blue: Negative

PEA: Negative, no growth

NA: Positive for growth, no color

Mannitol Salt: Negative, no growth.

 

I only looked at my Gram Stain once, which was hard to see and I recorded that it was Gram Positive rods. But, im beginning to think I read the stain wrong since all my results match up to E. Coli perfectly. In retrospect, should of repeated the Gram Stain but I can't now.

 

Anyway, does anything else grow with a green metallic sheen on EMB? My professor said that other organisms grow and not to assume you have E. Coli just because you see green, but I'm beginning to think she was just trying to throw us off since despite a lot of research, I haven't found any other bacteria that grows with a green metallic sheen.

[CharonY]

Weird, I thought I had already typed an answer, but anyway, the colour is due to strong acidification of the medium. How that coloration appears depends a bit on high vigorous they ferment lactose and also a bit on colony density. I.e. growth rate and motility may change the hue a bit, which may range from very characteristically green to some degree of brown-purple-pink (often within the same colony with gradation towards the fringe). So from that the prof. is obviously right, E. coli is not the only Gram- that fits the bill.