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About microscopic forms of life, including Bacteria, Archea, protozoans, algae and fungi. Topics relating to viruses, viroids and prions also belong here.
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Can micrococcus form H2S or black coloration on media?

Mon Feb 27, 2012 4:31 am

IF so, which type of micrococcus can? From what I know m. luteus forms yellow colonies but not sure about M.varians, I think it's white colonies. But can any type of Micrococci form black color (possibly be H2S) on media? The reason I ask is because all the tests I've done so far point to my gram positive organism being micrococcus BUT all the plates and media I used (whether it was agars, tubes, slants, or whatever) all turned black. Only my PEA agar didn't turn fully black but was slightly discolored. Everything else did though so that really confuses me. I've done the tests many times and always get the same results. So what kind can? I cant find anything on this...

My unknown organism I am trying to figure out is Gram positive cocci (it was in "tiny" purple clusters when I observed it under microscope). So far I've done the following tests below. Which bacteria or microorganism would match the following:
- Gram positive cocci, in clusters
- Catalase positive (+) (meaning that it forms bubbles/gas on catalase test)
- Mannitol negative (-) (on MSA-mannitol salt agar- it doesn't grow or ferment salts)
and -Fructose negative (-) (So, that means it doesn't ferment fructose)

-- I also want to mention that from my agar plates I used all turned to a black/brownish coloration but I'm unsure if this would indicate it's due to H2S (hydrogen sulfide) and that's the main thing that throwing me off. However I've done the streaking sevral times, and seem to have a pure culture according to my gram stains so I don't know if that shoudl be happening. THe gram stains came out fine, mainy in 1 color.

Here are some other details about the media I used:
--Also, the colonies for my gram positive are mostly little, grayish/clear dots. I used blood agar (which had really dark black color around the tiny colonies), chocolate agar and Macconkey agar. Only my PEA agar didn't come out with the black color but all the other ones did. I find it strange my Mac agar came with the color since I looked it up and have been told that too.

:arrow: Does this sound right? Could my organism still be a micrococccus or staph (since my tests point to those) even with the black color on my plates? And if so what type would produce the H2S or black pigmentation??

Also, from these tests I know that the organism is a type of Staph (or it may be a Micrococcus?), but it isn't Staph aureus since it doesn't ferment mannitol, but am not sure what else it could be or narrowed down to. There was absolutely NO growth at all on my MSA plate so that makes me unsure if it could be Staph epidermidis since I read that grows on MSA but doesn't ferment the manitol so wouldn't be yellow like S. aureus would be. But what type of Staph wouldn't grow at all on MSA?? IS it taht my organism actually isn't a Staph? (all my tests seem to point that it is though so I don't know.) What about Micrococcus species-do those not grow at all on MSA?

That confuses me since isn't staph supposed to be halophiles, that is, salt loving organisms? that's what I always thought anyways :/

Please explain your answer or if you used a link if you could put it in your answer. Sorry for all these questions. am just confused about my results especially the black color. This is for my unknown lab project by the way. I've been looking this up but am having hard time finding what it is. Thanks for all answers!!

Mon Feb 27, 2012 2:27 pm

Look up Micrococcus spp and you'll see the next test you should have run rather than MSA. There is a citation consistent with melanin and Micrococcus - you can fnd it on PubMed but it sure is obscure and doubtful as an unknown.


Mon Mar 05, 2012 10:53 am

JorgeLobo wrote:Look up Micrococcus spp and you'll see the next test you should have run rather than MSA. There is a citation consistent with melanin and Micrococcus - you can fnd it on PubMed but it sure is obscure and doubtful as an unknown.

hey thanks. You were right about it being "obscure and doubtful as an unknown". But u couldnt see my plates I was using so you'd not have any idea other than whatever i wrote but yeah I was way off and on the wrong track. But luckilly my teacher so nice she gave me another chance (I didn't confirm it with her what my organism is but just asked her if I was on the right track. if i did say it was micrococci i probly wouldnt have been able to do it again) anyways. So I was supposed to do a different test since it should've been catalase negative apparently. She hinted to me that black color shouldn't even be there in first place and that there was hemolysis on my plate. It was just hard to see before. Now I've done most the other tests and looks like it will either be Strep pneumoniae or Strep mitis.

And yeah I know I was wayyy off! Anyways I just wanted to tell u this since u try helping me out. However, what I still don't get is where all that black coloration would've came from. She said it's probably was contaminated but couldn't say for sure. Also, the alpha hemolysis was pretty hard to find since the plate was darkened. Have any idea what it could've been? (I doubt it would be H2s since strep don't make that..or do they? Also, this blackening was on all media used!!)
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