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Exciting brain teaser problem

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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Exciting brain teaser problem

Postby Samsowl » Wed Nov 18, 2009 12:57 am

Hi Guys, I'm new to this forum
I'm wondering whether anyone can give me their opinion or know an answer to this question.

I was given a broth culture and told it contained 2 normal flora and 1 pathogen.

I've been able to seperate what i believe is S.aureus, E.coli, Strep and Salmonella...One of them, i'm guessing either E.Coli or Strep isnt right...

So far, I've seperated them out and my positive for S.aureus was growing it on Manitol Salt Agar (MSA) which gave yellow collonies on yellow agar, very clear i think conclusive result. Gram stain also showed Gram +ve Cocci in clusters... VERY sure it's S.Aureus :) - Now i believe that S.epidedimis is normal flora, however S.Aureus isnt? is that right?
So mabye this is the pathogen?

Other tests i did, I used MacConkey's Agar (MAC) for what i'd seperated as E.coli and grew dark pink colonies on pink agar, classic result for E.coli. And i also had gram negitive rods in the initial gram stain i did with the original broth. Also i grew the same bacteria on CLED Agar and got yellow collonies, on yellow agar (colour change from blue to yellow).

I grew the strep out on Horse blood agar (HBA).

I took colonies from each of these agars replating a few times to try and get pure cultures.

I then did an API20E test on the sample i had from the MAC agar.... on the nutrient agar i put the final pure cultures on...it looks kinda fishy... I dont know what to think about it, I dont think it did look like E.coli like i'd wanted it to be!!
I got results that it was Salmonella... they were pretty conclusive results too...

Anyway, so i'm left with this!
I'm supposed to have 2 normal flora and 1 pathogen,
Has anyone got any idea what kind of thinking i can do next, I need to write a report on the pathogen, I cant do any more testing, we ran out of lab time... Please help!!

Thankyou lots for even taking the time to read this!
Thanks,

Sami
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Postby canalon » Wed Nov 18, 2009 1:41 am

S. aureus would generally be considered a pathogen although it is very often carried without any problems by a large part of the population. S. epidermidis is very common normal member of the skin flora, but IIRC they should stay red on MSA.
E. coli is definitely a part of the normal flora, but it can also be a deadly pathogen, depending on the strain and the host. In your case I think it would be part of the normal flora.
Salmonella enterica would also definitely be considered a pathogen by most standard, although healthy carriage is not unheard of.
And streptococci are quite common inhabitants of the human skin, although given the opportunity they can easily cause quite some problems. But considering your starting point I would put them in the normal flora.
Which leaves you with 2 pathogens and 2 commensal.
I do not know your lab and the conditions, but if I had to suspect contamination, S. aureus would be my favourite guess. You could also decide that the S. enterica was an accident during the transfer to NA, since the MAC and CLED both showing that you have a lactose fermenting bacteria in your hands is really atypical (although not impossible) for a S. enterica sample.
You have to make the final call I guess and write your report on one of those. You could start a paragrpah stating your problem and how you decided in the end. At least if you were wrong, if I was your instructor, I would give a good chunk of the mark for a correct ID, even if something went wrong.
Patrick

Science has proof without any certainty. Creationists have certainty without
any proof. (Ashley Montague)
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Re:

Postby kolean » Wed Nov 18, 2009 2:30 pm

canalon wrote:
I do not know your lab and the conditions, but if I had to suspect contamination, S. aureus
would be my favourite guess. You could also decide that the S. enterica was an accident during the transfer to NA, since the MAC and CLED both showing that you have a lactose fermenting bacteria in your hands is really atypical (although not impossible) for a S. enterica sample.
You have to make the final call I guess and write your report on one of those. You could start a paragrpah stating your problem and how you decided in the end. At least if you were wrong, if I was your instructor, I would give a good chunk of the mark for a correct ID, even if something went wrong.


I would second this part. An investigator scientist would always look at the mistakes in the results. This is usually how things get discovered, so it is a good habit to get into.
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