bluestar
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A 35 year old woman presents to her gyn doctor with complaints of burning on urination for the past 2 months. Dipstick test of her urine demonstrates markedly positivity for leukocyte estrase, but no reactivity for nitrite. Urine culture later grows out large numbers of organisms. which of the following bacteria are most likely to be responsible for this patient's infection?
A, enterobacter sp,
B, Enterococcus faecalis
C, E. Coli
D, Klebsiella Pneumoniae
E, Pseudomonas aeruginosa
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| mash
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enterococcus fecalis
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| bluestar
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then what about Pseudomonas? it doesn't reduce nitrates as well.
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| Sakaki-
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Doesn't P. aeruginosa reduce nitrates (but with denitrification), still leading to a negative result with the nitrite dipstick test?
I suppose that it has something to do with the setting; P. aeruginosa is mainly acquired in a nosocomial setting or in immunocompromised patients.
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| ghizou
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do you have the right answer? because i think is E. coli: it is the most commun agent causing cystitis in yong women.
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| Sakaki-
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Yeah, but E. coli reduces nitrates to nitrites.
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| bluestar
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The answer given is B, but with the explanation that Pseudomonas shows positve result by the dipstick for nitrites. This explanation is apparently wrong since pseudomonas is strict aerobic and doesn't ferment or reduce nitrite. I think both B and D is right. any other thoughts?
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| mash
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i think pseudomonas does reduce nitrates to nitrites but it reduces it further so, dipstick test is negative but if u add Zn after dat it wont change color which implies nitrate was reduced(coz Zn itself reduces nitrates)
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| bluestar
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mash, what's your reference? I read medical microbiology and immunology and it says P. aeruginosa doesn't ferment glucose or reduce nitrate. btw, what exactly is the reaction of nitrite --> nitrate?
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| mash
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P aeruginosa uses nitrate as electron acceptor..
nitrate reduction test is to differentiate organisms that possess the enzyme nitrate reductase
Napthylamine and sulfanilic acid reagents combine to produce red diazonium dye when reacting with nitrite
All negative reactions must be confirmed by adding metallic zinc
Zinc catalyses the reduction of nitrate to nitrite
A red colour demonstrates the presence of residual nitrates and thus a negative reaction
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| Sakaki-
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Mash is right; gas is formed in the Durham tube, indicating reduction to dinitrogen gas if you include it in the nitrate reduction test (which Mash explained) (actually, it could be any gas including ammonia... but Pseudomonas aeruginosa performs denitrification).
If you want to bother with oxidation numbers, it's just that Pseudomonas reduces the nitrate (NO3-) (N has an oxidation number of +5) to nitrogen (N2) (N has an oxidation number of 0 here). So it does reduce nitrates... just past nitrites (and going through it).
Anyways, I don't know if that is a typo, but it goes from nitrates -> nitrites rather than the other way around (the other way is an oxidation).
(the chemical equation, if you're into that kind of stuff is:
NO3- + 2e- + 2H+ -> NO2 + H20)
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| bluestar
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Ok, guys, I found this on the web :"Although the bacterium is respiratory and never fermentative, it will grow in the absence of O2 if NO3 is available as a respiratory electron acceptor. " So that explains everything. But frankly, this question is too tricky, I'd never have made it right were this on the real exam.
Thank you both for the detailed explanation. oh, sakaki, I did made the typo. sorry... and you made one as well: nitrate is NO3- and nitrite is NO2-
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| bluestar
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Oh, btw, go you guys know what other bacteria has nitrate reductase apart from, say, E. Coli, Salmonella, Shigella, Klebsiella, Enterobacter, Serratia, Proteus and dear Mr. Pseudomonas?
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| Sakaki-
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Haha--that is very weird; I've always thought of Pseudomonas as an aerobe. But even then, that statement doesn't necessary mean that nitrite is the final product. ^^ Further action of nitrite reductase could again yield a negative result for nitrite.
Heh--and I did make a typo. ^^
"All Enterobacteriace [except] certain biotypes of Pantoea (Enterobacter) agglomerans and certain species of Serratia and Yersinia <does not specify which>, reduce nitrate to nitrite."
-Koneman's textbook of diagnostic microbiology
I think your list is complete (except for Edwardsiella, Citrobacter, Morganella and Providencia - but really...ignore these, except maybe Citrobacter).
I really don't remember them using the nitrate reductase test in the lab for anything besides Pseudomonas (oxidase positive, O-F positive, motile workup).
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| bactitech
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One of the problems that comes up in the laboratory is that docs sometimes order a urine this way: "perform culture if urinalysis is positive"
Every lab has different criteria for this statement. Our Ph.D. has stated concern that enterococci do not produce positive nitrate in a urinalysis test. If a positive nitrate is a criteria for positivity, then enterococci would be missed.
I don't know about Pseudomonas, but I would place my bets with enterococci. This is a very interesting question.
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| mani
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i agree with enterococci even if we leave the discussion of nitrite reaction bcoz psuedomonas causes nosocomial infection as somebocy already pointed out
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| sarangi2020
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hey...there is also another clue given regarding leukocytr esterase which is +ve with E. faecalis....so i think is the answer.....am i right?
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| bactitech
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Please understand that leukocyte esterase is an indicator of white cell response. It is not a chemical reaction for the identification of enterococci. The body produces white cells, and hence leukocyte esterase, in response to infection.
In a patient with low white cells, i.e. someone undergoing chemotherapy or otherwise immunocompromised, LE may not be the indicator it would be in other patients.
Nothing is cut and dried here. I know you are trying to study for board registry questions, but please realize that microbiology is not like that in all instances.
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| Ig F
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well this one was way too tricky.............
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| SmokyWaters
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the question is about NON NITRITE REDUCTASE containing organisms  simple
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