Is it normal that all my agar plates(or media)turned black??

[SweetSarah]

For my microbiology class we are now doing our unknowns which are definitely the hardest part of the class :/

Anyways after incubating it, my plates all turned out with some black pigmentation or color around the colonies. I’m thinking this is because it produces Hydrogen sulfide (or if not it’s something along the way) But no one elses turned like that in my class. And it confuses me that all my plates had this black color on it, even my MacConkey plate which everyone elses turned pink or something like that but not black! What confuses me even more is that we haven’t been working with any bacteria in class that does something like this, so it looks like I have a harder one because now I’m unsure like what kind of tests I need to do and all. In my book and I’ve even been searching online, the Mac agar plates don’t look like this, nor does the blood agar (which I know tests for hemolysis, but mine doesn’t seem to have anything on it but just black around the colonies too!)
Could it be that I did something wrong or am i right in my thinking so far? 🙁

Also if the organism does produce the H2S (hydrogen sulfide) and that is what causes the black coloring on my agar, should I also see this color come up on my other lab tests? (I will be doing Tuesday or at least that’s the plan). Like for example my TSI tubes will they also turn black color??

I saw a list of organisms that produce H2S when doing the TSI and I am wondering if mine may be one those. Like salmonella was on the list. Anyone else know any other organisms it may be??

Please know that I haven’t done much so far, I’m still waiting for my gram stain results but will be doing these tests soon hopefully if I can finish the other parts. I just want to see if I am doing it correctly and if I’m on the right track. I may be completely wrong so that’s why I’m asking to see what your all thoughts are. Any other tests I should consider doing?? Any advice as what I should do if it is wrong??
Thanks. 🙂

[canalon]

It is hard to answer without looking at your results and without knowing exactly what you did, but the presence of black pigments on McConkey agar is quite surprising. How was your medium prepared? Was it you? from a premixed powder or from separate ingredients? What recipe did you use in the later case?

[JorgeLobo]

Hydrogen sulfide production per se does not produce black pigmentation – excess free iron in the medium forms FeS and that is the basis for the color. MacConkey agar is not so supplemented so it would not produce black colonies with a bug producing higher levels of hydrogen sulfide. Could you be looking at precipitated bile salts ( http://www.austincc.edu/microbugz/macconkey_agar.php )?

[SweetSarah]

quote JorgeLobo:

Hydrogen sulfide production per se does not produce black pigmentation – excess free iron in the medium forms FeS and that is the basis for the color. MacConkey agar is not so supplemented so it would not produce black colonies with a bug producing higher levels of hydrogen sulfide. Could you be looking at precipitated bile salts ( http://www.austincc.edu/microbugz/macconkey_agar.php )?

hmm I thought it was because of H2S. Maybe you’re right though. am gonna look up the bile salts thing you’re talking about then. thanks for the heads up 🙂

[Author]

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