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NBME 22 Answers

nbme22/Block 1/Question#48
A 1-year-old boy is found to have an ...
Staphylococcus aureus🔍

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submitted by thirdaid(7),

Phagocytes need to produce hydrogen peroxide from oxygen to be able to undergo the oxidative burst that will kill bacteria.

Hydrogen peroxide can come from:

  • A reaction catalyzed by NADPH oxidase (missing in Chronic Granulomatous Disease)
  • Production of hydrogen peroxide by own infecting bacteria

Hydrogen peroxide can be degraded by catalase in catalase-positive organisms. Without hydrogen peroxide there is no creation of hydroxyl halide radicals and no destruction of the pathogen.

There are two catalase positive organisms in the answers: E. coli and S. aureus. We are more commonly in contact with S. aureus and it is the more common pathogen so that's the answer.




This Question its about respiratory burst

Patients with NADPH deficiency=chronic granulomatous disease (CGD)

Even though patients with CGD can't make Superoxide, they can use it from the bacterias and convert it to bleach HCLO and kill the bacterias.

BUT bacterias with catalase enzymes neutralize their own superoxide and thats why the CGD patient can't kill them.

Catalase positive bacterias: S. aureus - Aspergillus

thomasburton  I thought E.coli was catalase positive too? Why can that not be correct? +4  
mb10  (FA 186) Catalase (+) microbes, especially S aureus +4  
makinallkindzofgainz  @thomasburton - because First Aid said so, so suck it +  



This Question its about respiratory burst

Patients with NADPH deficiency=chronic granulomatous disease (CGD)

Even though patients with CGD can't make Superoxide, they can use it from the bacterias and convert it to bleach HCLO and kill the bacterias.

BUT bacterias with catalase enzymes neutralize their own superoxide and thats why the CGD patient can't kill them.

Catalase positive: S. aureus - Aspergillus




 +0  upvote downvote
submitted by hello(184),

please help -- If catalase-positive bacteria neutralize their own superoxide, why isn't it the case for catalase-positive bacteria to infections in everyone?

I'm not understanding the connection to NADPH oxidase deficiency.

hello  to cause** infections in everyone +  
bmd12  Bc everyone isn't NADPH deficient, meaning they can produce their own superoxide without needing to rely on the superoxide produced by the bacteria. +  
bmd12  so even if catalase positive organisms neutralize their own superoxide, our body is producing its own and not relying on the ones produced by bacteria. You only begin to rely on the superoxide produced by bacteria if you are NADPH deficient. +  



Question is asking about encapsulated organisms infecting CGD patients. E.coli is also encapsulated. Can anyone expand on this?

keycompany  Step pneumonia is the most common pathogenic organism in CGD, and the most common cause of pneumonia, otitis media, meningits, and sepsis. While CGD are at an increased risk of encapsulated E. coli infections, however, they are at MOST risk for S. pneumo. This is kind of just a memorization fact that you need to know about S. pneumo. +  
keycompany  Sorry english is clearly not my shit, but you get the point +  
biliarytree220  CGD is susceptible against catalase-positive organisms (FA 109), of which S. aureus is the one to look out for. It's not about encapsulated organisms, like I had it confused in my head. +6  
.ooo.   You are completely right about E.Coli being encapsulated and is also a CAT+ organism and patients with CGD would have an increased risk of infection for both S. Aureus and E. Coli. How you narrow down the two is the most common infections are S. Aureus and Aspergillus (FA 109 like mentioned above) and also using the pneumonic "Cats Need PLACESS to Belch their Hairballs" (FA 128) Nocardia, Pseudomonas, Listeria, Aspergillus, Candida, E.Coli, Staphylococci, Serratia, B cepacia, H pylori +4  



Note: the questions stated "respiratory burst" suggesting an URT infection.

1) this rules out anything but respiratory infection (non rep infection: E. coli, E. faceium)

2) G6PD deficiency more susceptible to catalase positive organisms -- this rules out (all strep organisms)

3) Left with H. influenzae & Straph. aureus (BOTH are catalase positive)

4) Encapsulated organism are most concerning when there is asplenia.

imnotarobotbut  Respiratory burst has nothing to do with a respiratory infection. It describes the process of phagocytosing a bacteria and using NADPH oxidase/ROS to lyse it +3  
belleng  Aspergillus is still in the running, it is catalase positive as well...but not a choice +  



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