Black coloration of sheep blood agar

[triton]

Hi
I wonder if someone can tell me what causes black coloration of sheep blood agar caused by bacterial grow? At first I though it was an alpha hemolysis, but I think it is not since it should be yellow and green, not black. the black coloration I have observed looked like that: http://microculture.tumblr.com/post/166 … numbers-of

I can tell you exact composition of medium that was used, if it is important.

The second question is: is expression of hemolysin specific for certain conditions? I can see hemolysin encoded in the genome, but I fail to observe hemolysis on plates.

Thank you so much.

[biohazard]

Hmmh… as far as I can tell that looks quite normal non-hemolytic growth typical to Bacillus species, though the angle and light are not ideal for spotting it. I am not sure what causes the darker coloration of the agar (I’d call it deep red rather than black from what I see), but it is a common phenomenon possibly related to the use of other nutrients in the medium and to the change of pH or something. Bacteria break down and secrete various compounds, so I would not be surprised if some of those cause the darkening of the agar. Admittedly, I do not know the exact answer so if someone does, let us know. And the dark spot seems to be created by the administration of bacteriophages, as indicated in the text.

The answer for the second question depends on the hemolysin in question, but most are expressed only in specific conditions. For example, the presence of iron inactivates some types of hemolysin expression in bacteria that use it to obtain iron from red blood cells. Thus, on standard blood agar these bacteria (e.g. Streptococcus pyogenes) virtually always cause hemolysis, since it does not contain free iron. Some other hemolysins are targeted against other cell types (e.g. leukocytes) and their expression patterns are different and related to the presence of these cells.

[triton]

quote biohazard:

Hmmh… as far as I can tell that looks quite normal non-hemolytic growth typical to Bacillus species, though the angle and light are not ideal for spotting it. I am not sure what causes the darker coloration of the agar (I’d call it deep red rather than black from what I see), but it is a common phenomenon possibly related to the use of other nutrients in the medium and to the change of pH or something. Bacteria break down and secrete various compounds, so I would not be surprised if some of those cause the darkening of the agar. Admittedly, I do not know the exact answer so if someone does, let us know. And the dark spot seems to be created by the administration of bacteriophages, as indicated in the text.

The answer for the second question depends on the hemolysin in question, but most are expressed only in specific conditions. For example, the presence of iron inactivates some types of hemolysin expression in bacteria that use it to obtain iron from red blood cells. Thus, on standard blood agar these bacteria (e.g. Streptococcus pyogenes) virtually always cause hemolysis, since it does not contain free iron. Some other hemolysins are targeted against other cell types (e.g. leukocytes) and their expression patterns are different and related to the presence of these cells.

Thank you
The example I showed is Bacillus and I chose it, because it was close to appearance of my plates. I will attach photo of my bacteria (when I find some camera).

My bacteria are Enterobacteriaceae, namely Biostraticola tofi. See the description paper for methabolic capabilities. http://www.springerlink.com/content/k32 … lltext.pdf

We have the genome sequence and I can see several copies of different hemolysins (which is wierd because this should be a free living bacterium). Thats why I try to find out if it can lyse blood cells.

Do you have any references which types of hemolysins are turned on in specific conditions? i.e. hemolysin XY is targeted against ZZZZ cells and it is expressed under conditions WWWWW

[biohazard]

Sorry, I don’t have any specific references at hand currently.

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