Help with this microbiology question (from USMLE Rx)

[medpack87]

Patient with signs and symptoms of subacute bacterial endocarditis. Blood cultures reveal gram positive, catalase negative organism. Which of the following culture conditions would aid in identifying the causative organism?
a) Absence of colonies when grown with penicillin
b) Bile solubility
c) Growth in 6.5% NaCl
d) Growth in the presence of optochin
e) Growth in chocolate agar

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[Jonari]

Patient with signs and symptoms of subacute bacterial endocarditis. Blood cultures reveal gram positive, catalase negative organism. Which of the following culture conditions would aid in identifying the causative organism?
a) Absence of colonies when grown with penicillin
b) Bile solubility
c) Growth in 6.5% NaCl
d) Growth in the presence of optochin
e) Growth in chocolate agar

what's the most common cause of subacute endocarditis - there is your clue...you can immediately cross out:

B - bile solubility is associated with G+ streptococcus species
C - that's associated with Enterococcus, iirc
D - grows in optochin meaning it's "optochin resistant" this is associated with the beta-hemolytic G+ strep
E - choco agar - Neisseria species

 

[tiedyeddog]

Patient with signs and symptoms of subacute bacterial endocarditis. Blood cultures reveal gram positive, catalase negative organism. Which of the following culture conditions would aid in identifying the causative organism?
a) Absence of colonies when grown with penicillin
b) Bile solubility
c) Growth in 6.5% NaCl
d) Growth in the presence of optochin
e) Growth in chocolate agar

Ok, I pretty sure about this:

For starters, what bacteria most related to subacute endocarditis (slow onset, low fevers, etc)? You should immediately think Strep. Viridans. Let's eliminate things first.

E. is not correct. You use chocolate agar to identify Hib. Not associated with subacute endocarditis.

C. is not correct, that is used for Enterocci or S. Bovis.

A. won't tell you much. Many of the G+ bacteria are killed by penicillin.

B. Again, used more for E. Coli or other G - enterics or for differentiating enterococcus/S. Bovis.

D. This is the correct answer. Viridans is a A-Hemolytic. It is optochin resistant.

To answer these, I would start with memorizing some kind of bacterial flow chart, either Pholston's or the ones of pages 127 or 133 of FA.

 

[tiedyeddog]

sorry for the double post. :\

 

[Jonari]

sorry for the double post. :\

nvm im an idiot

 

[tiedyeddog]

what's the most common cause of subacute endocarditis - there is your clue...you can immediately cross out:

B - bile solubility is associated with G+ streptococcus species
C - that's associated with Enterococcus, iirc
D - grows in optochin meaning it's "optochin resistant" this is associated with the beta-hemolytic G+ strep
E - choco agar - Neisseria species

not to nitpick, just want to make sure you don't get this wrong on the real deal!!!
but choco is for HiB. Neisseria is Thayer-Martin (VPN)

lol im a potato...growth would mean resistant right...not sensitive...FML

haha, yeah, sorry Jonari

 

[medpack87]

I like your answer. Thank you very much. This question is straight from USMLE-Rx. I feel microbiology is one of my strong areas, and I have a good summary of this very high yield topic.

When I answered, I went for choice B. Bile solubility. I also thought of the causative organism being S. viridans. I know that S. viridans is bile soluble (meaning it doesnt grow in bile right?), while other streps like S. faecalis and S. Bovis grow in bile (and in the case of faecalis, NaCl too). The answer was wrong. FA official explanation for this was: S. pneumoniae is bile soluble but not a common cause of subacute endocarditis. (I thought s. viridans was also bile soluble, can someone confirm this?)

Choice D, growth in the presence of Optochin, is also wrong. I actually crossed this one out because I tripped and thought S. viridans was SENSITIVE, so... i figured it wouldnt grow, now I remember its resistant. FA's official explanation for not going for this choice is the following: Both viridans and faecalis are optochin resistant, so it would not help you differentiate between these 2 strains. :/ (11% chose this answer)

The answer, according to the FA people is C. Growth in 6.5% NaCl. They explain the signs/symptoms of subacute bacterial endocarditis, and that organisms that cause this are is S. viridans AND S.faecalis. They both are gram + cocci, optochin resistant and bile soluble (I THOUGHT FAECALIS COULD GROW IN BILE, AND BILE SOLUBLE MEANS IT CANT WTF?), but S. viridans cannot grow in NaCl, in contrast to S. faecalis. (27% chose this)

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This is one of those typical WTF questions by USMLE Rx. Sometimes it can contradict your own notes taken from FA!!! 😱
So we were all wrong, even though I feel we all knew these concepts pretty well.

Can someone clear the following for me please?
-Bile solubilty. I understand it means the organism CANNOT grow in bile. True?
-Strep. faecalis. I understand this organism CAN grow in bile and salt (therefore its NOT bile soluble) someone confirm me this please. I dont know what to believe for now.
-Strep.bovis, i understand it can grow in bile, but CANNOT grow in NaCl. And that is the way to tell it apart from Strep. faecalis.


Man, Im supposed to have these concepts straight already, and I thought I did. As a matter of fact, I think I have, but this FA question shook my brain

 

[beatsbydre]

I like your answer. Thank you very much. This question is straight from USMLE-Rx. I feel microbiology is one of my strong areas, and I have a good summary of this very high yield topic.

When I answered, I went for choice B. Bile solubility. I also thought of the causative organism being S. viridans. I know that S. viridans is bile soluble (meaning it doesnt grow in bile right?), while other streps like S. faecalis and S. Bovis grow in bile (and in the case of faecalis, NaCl too). The answer was wrong. FA official explanation for this was: S. pneumoniae is bile soluble but not a common cause of subacute endocarditis. (I thought s. viridans was also bile soluble, can someone confirm this?)

Choice D, growth in the presence of Optochin, is also wrong. I actually crossed this one out because I tripped and thought S. viridans was SENSITIVE, so... i figured it wouldnt grow, now I remember its resistant. FA's official explanation for not going for this choice is the following: Both viridans and faecalis are optochin resistant, so it would not help you differentiate between these 2 strains. :/ (11% chose this answer)

The answer, according to the FA people is C. Growth in 6.5% NaCl. They explain the signs/symptoms of subacute bacterial endocarditis, and that organisms that cause this are is S. viridans AND S.faecalis. They both are gram + cocci, optochin resistant and bile soluble (I THOUGHT FAECALIS COULD GROW IN BILE, AND BILE SOLUBLE MEANS IT CANT WTF?), but S. viridans cannot grow in NaCl, in contrast to S. faecalis. (27% chose this)

-----
This is one of those typical WTF questions by USMLE Rx. Sometimes it can contradict your own notes taken from FA!!! 😱
So we were all wrong, even though I feel we all knew these concepts pretty well.

Can someone clear the following for me please?
-Bile solubilty. I understand it means the organism CANNOT grow in bile. True?
-Strep. faecalis. I understand this organism CAN grow in bile and salt (therefore its NOT bile soluble) someone confirm me this please. I dont know what to believe for now.
-Strep.bovis, i understand it can grow in bile, but CANNOT grow in NaCl. And that is the way to tell it apart from Strep. faecalis.


Man, Im supposed to have these concepts straight already, and I thought I did. As a matter of fact, I think I have, but this FA question shook my brain

I thought this was a pretty straightforward question.

Check out the bottom of this page for your questions: http://en.wikipedia.org/wiki/Streptococcus_pneumoniae

ANd I think you should not think too much about it. For example, don't try to wonder if viradae would be bile-soluble or not. Just remember viradae is with optochin and it's resistant. That's all you need to know.

 

[MLT2MT2DO]

I thought this was a pretty straightforward question.

Check out the bottom of this page for your questions: http://en.wikipedia.org/wiki/Streptococcus_pneumoniae

ANd I think you should not think too much about it. For example, don't try to wonder if viradae would be bile-soluble or not. Just remember viradae is with optochin and it's resistant. That's all you need to know.

He said the correct answer was "C" in which case USMLErx is incorrect or just playing headgames. Strep Bovis can additionally cause subacute endocarditis albeit in someone with colon cancer, additionally that would be that growth in bile could be just as correct.

I still believe that "D" is the MOST correct answer if this is word for word how it is explained


According to the CDC Strep Viridans doesn't grow in NaCl...this answer is straight wrong.
http://www.cdc.gov/ncidod/biotech/strep/strep-doc/section2.htm

 

[Zzmed]

Guys it can be C or D.
Maybe in the question stem there are some clues (i.e. prior GI surgery) --> feacalis --> C

 

[MLT2MT2DO]

Guys it can be C or D.
Maybe in the question stem there are some clues (i.e. prior GI surgery) --> feacalis --> C

While I agree, with the info given I don't think we can assume that. Additionally in the answer description it says Viridans can grow in NaCl and that's why it's the best answer because it covers both Viridans and Feacalis.

The BEST answer is still D

 

[Zzmed]

While I agree, with the info given I don't think we can assume that. Additionally in the answer description it says Viridans can grow in NaCl and that's why it's the best answer because it covers both Viridans and Feacalis.

The BEST answer is still D

Viridans can't grow in NaCl

 

[Jamiu22]

While I agree, with the info given I don't think we can assume that.....

The BEST answer is still D

👍👍

When they give you a question about subacute endocarditis, for step 1... it's def the more likely Viridans... it's more common - after missing a whole bunch of questions - I realized that always choosing the more common disease is a better choice - unless a there's a hint screaming at you from the question stem. When seeing subtle hints, you are more than likely reading into the question....

Go with your gut!! and a more likely cause of the disease

 

[coolforschool]

This is straight from king Phloston himself:

"I've seen in a question where they wanted to know how to differentiate between S. viridans and Enterococcus endocarditis (via culturing only, not pt Hx).

Both grow well in bile, but only S. viridans is optochin-resistant. So not only does optochin resistance distinguish S. viridans from S. pneumoniae, but it also distinguishes it from Enterococcus."

There you go.

 

[MLT2MT2DO]

This is straight from king Phloston himself:

"I've seen in a question where they wanted to know how to differentiate between S. viridans and Enterococcus endocarditis (via culturing only, not pt Hx).

Both grow well in bile, but only S. viridans is optochin-resistant. So not only does optochin resistance distinguish S. viridans from S. pneumoniae, but it also distinguishes it from Enterococcus."

There you go.

There isn't an answer for "Grows well in Bile"...there is only Bile Soluble (which is the opposite of "grows well in bile")

 

[MLT2MT2DO]

Viridans can't grow in NaCl

Agreed, but the description is saying it can and as such why they say C is a better choice than D.

 

[coolforschool]

There isn't an answer for "Grows well in Bile"...there is only Bile Soluble

I think what he meant was that both S. viridans and Enterococcus are bile insoluble, meaning they can grow in bile because they aren't lysed by it.

 

[MLT2MT2DO]

I think what he meant was that both S. viridans and Enterococcus are bile insoluble, meaning they can grow in bile because they aren't lysed by it.

Look at the reasoning in post #7 on the thread.

 

[tiedyeddog]

👍👍

When they give you a question about subacute endocarditis, for step 1... it's def the more likely Viridans... it's more common - after missing a whole bunch of questions - I realized that always choosing the more common disease is a better choice - unless a there's a hint screaming at you from the question stem. When seeing subtle hints, you are more than likely reading into the question....

Go with your gut!! and a more likely cause of the disease

I agree, I never in a million years would have gotten this right using the logic presented by USMLErx. I am always just going to go with what I know on these, what else can you do?

 

[coolforschool]

Look at the reasoning in post #7 on the thread.

http://en.wikipedia.org/wiki/Streptococcus_viridans shows that S. viridans is bile insoluble compared to S. pneumoniae, which makes the explanation from Rx wrong (I think they meant to say bile insoluble).

From everything I've read, Enterococcus is also optochin-resistant, which makes Phloston wrong.

The best test is the growth in 6.5% NaCl because Enterococcus can grow in that while S. viridans cannot.

Edit: Couldn't find a good source for Enterococcus being optochin-resistant, but they say that here http://books.google.com/books?id=xzIsZo44GkoC&pg=PA718&lpg=PA718&dq=enterococcus+optochin+resistant&source=bl&ots=zS-OEMdwBr&sig=lYRIO59fj55fiWBodwZv-BDbdEE&hl=en&sa=X&ei=QtqXUdGlLIWg9QShoIDQCg&ved=0CEkQ6AEwBA#v=onepage&q=enterococcus%20optochin%20resistant&f=false in the paragraph titled "Susceptibility to Optochin."

Information about S. viridans not growing in 6.5% NaCl in the second paragraph under "Identification." http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3020876/.

 

[wahaha]

Patient with signs and symptoms of subacute bacterial endocarditis. Blood cultures reveal gram positive, catalase negative organism. Which of the following culture conditions would aid in identifying the causative organism?
a) Absence of colonies when grown with penicillin
b) Bile solubility
c) Growth in 6.5% NaCl
d) Growth in the presence of optochin
e) Growth in chocolate agar

A. Streps are sensitive to penicillin so would not help narrowing down the organism
B. Bile solubitily is used for Streptococcus Pneumo, it would only help in identifying strep pneumo which does not cause endocarditis
C. Growth in in NaCl differentiates your Group D entercoccus from your Non-Group D streptococcus
d. Optochin resistance with a zone of at less than t 14 mm is indicative of an alpha strep like Strep Viridans group(common cause of subacute endocarditis)

E. Chocolate grows everything. Would not help in narrowing down the organism.

Strep Pneumo is bile soluble meaning the colonies are flatten out once Bile( or sodium desoxycholate) is added. Other organisms will retain their shape and are therefore bile insoluble.

 

[MLT2MT2DO]

http://en.wikipedia.org/wiki/Streptococcus_viridans shows that S. viridans is bile insoluble compared to S. pneumoniae, which makes the explanation from Rx wrong (I think they meant to say bile insoluble).

From everything I've read, Enterococcus is also optochin-resistant, which makes Phloston wrong.

The best test is the growth in 6.5% NaCl because Enterococcus can grow in that while S. viridans cannot.

Edit: Couldn't find a good source for Enterococcus being optochin-resistant, but they say that here http://books.google.com/books?id=xzIsZo44GkoC&pg=PA718&lpg=PA718&dq=enterococcus+optochin+resistant&source=bl&ots=zS-OEMdwBr&sig=lYRIO59fj55fiWBodwZv-BDbdEE&hl=en&sa=X&ei=QtqXUdGlLIWg9QShoIDQCg&ved=0CEkQ6AEwBA#v=onepage&q=enterococcus%20optochin%20resistant&f=false in the paragraph titled "Susceptibility to Optochin."

Ok so basically the question is saying "what's the best way of differentiating between the 3 subacute bacterial endocarditis'"? If that's the case I would agree seeing as how Strep Bovis is also Optochin resistant and Bile Insoluble

 

[coolforschool]

Ok so basically the question is saying "what's the best way of differentiating between the 3 subacute bacterial endocarditis'"? If that's the case I would agree seeing as how Strep Bovis is also Optochin resistant and Bile Insoluble

Yeah, I think the question is pretty straightforward having read it again, but the explanations with errors in them threw everybody off.

 

[MLT2MT2DO]

Yeah, I think the question is pretty straightforward having read it again, but the explanations with errors in them threw everybody off.

I don't know about straight forward, I don't like the way it was asked. They gave a subacute endocarditis and no prior Medical Hx...I'd still probably pick Optochin if I saw the question without more context.

Maybe "Which of the tests would help differentiate the different POSSIBLE causative organisms." Would be much better worded

 

[wahaha]

Also Enterococcus and Alpha Strep are differentiated based on a test called PYR. Entercoccus is positive and Alpha Strep are negative. You'd perform a PYR before an Optochin. So if you got far enough to an optochin you're considering Strep Pneumo vs Strep viridans group.

 

[MLT2MT2DO]

Also Enterococcus and Alpha Strep are differentiated based on a test called PYR. Entercoccus is positive and Alpha Strep are negative. You'd perform a PYR before an Optochin. So if you got far enough to an optochin you're considering Strep Pneumo vs Strep viridans group.

Hell, you'd look at hemolysis on the Blood agar before anything, but they don't seem to think that's viable information in this question.

 

[wahaha]

Hell, you'd look at hemolysis on the Blood agar before anything, but they don't seem to think that's viable information in this question.

Well enterococcus can be alpha, beta, and gamma so if it happened to be alpha hemolytic it wouldn't have helped much.

 

[MLT2MT2DO]

Well enterococcus can be alpha, beta, and gamma so if it happened to be alpha hemolytic it wouldn't have helped much.

Do you have a source? I've only ever heard of Bovis/Fecalis as being gamma

 

[Suncrusher]

Do you have a source? I've only ever heard of Bovis/Fecalis as being gamma

microcards, levinsons, etc. It's definitely variable hemolysis

 

[MLT2MT2DO]

microcards, levinsons, etc. It's definitely variable hemolysis

Yup in FA, was going off my RL micro knowledge. Can honestly say I've never seen feacalis anything other than gamma, surprised by this.

 

[wahaha]

Do you have a source? I've only ever heard of Bovis/Fecalis as being gamma

"Gamma Streptococcus": Enterococcus faecalis (24 hours, non-hemolytic). "Gamma streptococci" are usually non-hemolytic after 24 hours of incubation, but many eventually display weak alpha hemolysis. (The genus Enterococcus was once a part of the Streptococcus genus, was considered a "gamma Streptococcus species," and usually reacts as Lancefield group D). (Rebecca Buxton, University of Utah, Salt Lake City, UT)

http://www.microbelibrary.org/compo...d-other-catalase-negative-gram-positive-cocci

 

[Suncrusher]

i missed a qbank question because of it, so now it is forever ingrained in my mind >: [

 

[coolforschool]

I don't know about straight forward, I don't like the way it was asked. They gave a subacute endocarditis and no prior Medical Hx...I'd still probably pick Optochin if I saw the question without more context.

Maybe "Which of the tests would help differentiate the different POSSIBLE causative organisms." Would be much better worded

"Patient with signs and symptoms of subacute bacterial endocarditis. Blood cultures reveal gram positive, catalase negative organism. Which of the following culture conditions would aid in identifying the causative organism?"

I see what you mean, but in my mind, when I read that question, I think, "Okay, the patient has subacute bacterial endocarditis." That for me tends to rule out S. pneumoniae as a possible cause because it rarely causes endocarditis, so the optochin sensitivity test wouldn't really tell you much. In contrast, I've seen a lot of endocarditis questions dealing with S. viridans (dental procedures) and Enterococcus (urinary procedures). I'm not sure how common they are in real life, but they definitely come to mind whenever I see endocarditis on a question, along with S. aureus.

 

[medpack87]

Wow, hot topic.

"Patient with signs and symptoms of subacute bacterial endocarditis. Blood cultures reveal gram positive, catalase negative organism. Which of the following culture conditions would aid in identifying the causative organism?"

I see what you mean, but in my mind, when I read that question, I think, "Okay, the patient has subacute bacterial endocarditis." That for me tends to rule out S. pneumoniae as a possible cause because it rarely causes endocarditis, so the optochin sensitivity test wouldn't really tell you much. In contrast, I've seen a lot of endocarditis questions dealing with S. viridans (dental procedures) and Enterococcus (urinary procedures). I'm not sure how common they are in real life, but they definitely come to mind whenever I see endocarditis on a question, along with S. aureus.

Yes, this seems to be the best reasoning to go with in this question. How do you differentiate between Strep. viridans, and Strep. faecalis. And the answer would be (according to FA) the NaCl test.

Just to clear the doubts, I summarized the question stem directly to the diagnosis of subacute bacterial endocarditis. The patient history was of many lines, describing someone with previous rheumatic fever disease many years ago and now signs and symptoms of subacute bacterial endocarditis. No history about dental procedures, urinary tract procedures or colon cancer. It was one of those 2-step questions in which you need to come up with a dx and then think about what they are really asking. To make things easier I just put the dx here directly in the thread, because the main problem was the bile, NaCl and optochin properties.And I double checked to avoid any typos during my post (ie soluble vs insoluble).

 

[WilliamsEph]

I thought it was C:

E. Faecalis can cause subacute endocarditis and it grows in 6.5% NaCl.
Group D Strep Bovis also causes endocarditis (mainly in colon cancer pts.), but it does not grow in 6.5% NaCl.

 

[medpack87]

I thought it was C:

E. Faecalis can cause subacute endocarditis and it grows in 6.5% NaCl.
Group D Strep Bovis also causes endocarditis (mainly in colon cancer pts.), but it does not grow in 6.5% NaCl.

That is the answer. But mainly to differentiate from Strep. viridans which doesnt grow in 6.5% NaCl and E. faecalis which does.

E.bovis causes endocarditis, but I that in USMLE you ojnly consider that answer if there is some history of colon cancer

 

[Corbac]

http://en.wikipedia.org/wiki/Streptococcus_viridans shows that S. viridans is bile insoluble compared to S. pneumoniae, which makes the explanation from Rx wrong (I think they meant to say bile insoluble).

From everything I've read, Enterococcus is also optochin-resistant, which makes Phloston wrong.

The best test is the growth in 6.5% NaCl because Enterococcus can grow in that while S. viridans cannot.

Edit: Couldn't find a good source for Enterococcus being optochin-resistant, but they say that here http://books.google.com/books?id=xzIsZo44GkoC&pg=PA718&lpg=PA718&dq=enterococcus+optochin+resistant&source=bl&ots=zS-OEMdwBr&sig=lYRIO59fj55fiWBodwZv-BDbdEE&hl=en&sa=X&ei=QtqXUdGlLIWg9QShoIDQCg&ved=0CEkQ6AEwBA#v=onepage&q=enterococcus%20optochin%20resistant&f=false in the paragraph titled "Susceptibility to Optochin."

Information about S. viridans not growing in 6.5% NaCl in the second paragraph under "Identification." http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3020876/.

I think This is the best explanation .
All the comments are also good .

In this type of questions we should use the chart flow for COCCI .
For alfa hemolytic OPTOCHIN is S. S . Pneumoniae and R . for Viridans .
For beta hemolytic ( complete hemolysis ) BACITRACIN S. is Gr A (S . Pyogenes) and R. is Gr B ( S agalactie ) .
For Gamma Hemolytic Groop D ( usually there is no Hemolysis ) , Growth in 6.5% NaCl is E . Fecalis and Negative or Na Cl intolerant ) is S Bovis .
The stem question is scarce in Hx however there are key words like SBE and Catalase negative .
Following the multistep reasoning and the explanation provided in the article which was excellent , I chose C as the correct answer .

Pls correct me if I am wrong .