Bacteria 5C: Escherichia, Salmonella, Klebsiella, Shigella, Bacteroides (Distinguishing Features)

Bacteria 5C: Escherichia, Salmonella, Klebsiella, Shigella, Bacteroides (Distinguishing Features)


the last module was a little dry when it
came to lab testing this module has a few more variations which is a good
thing as many of these microbes have similar presentations it’s more
important to be able to differentiate specific bacterial genus for treatment
purposes let’s start with Salmonella it can be tested for in a variety of
manners including many types of body fluids the Widal test or agglutination
test was a classic question regarding this microbe and test for antibodies or
antigen this causes clumping in a test tube as seen in the image the TSI slant
is a specialized agar medium but is positive in a few enteric bugs so its
usefulness varies often patients recover before results for these tests come back
but severe infections might require more specialized options though most of the
fermenting questions are not particularly high yield for medicine
they are more so for enteric microbes the common sugars that we may test for
are glucose lactose sucrose and mannose along with a few others Salmonella also
possesses the H-antigen and K-antigen as well as produces hydrogen sulfide from
sugar fermentation which is seen on TSI testing E. coli is usually not too
difficult to culture however with so many gram-negative rods that cause human
disease simply looking under the microscope isn’t going to be enough to
differentiate this from dozens of other bugs the MacConkey agar is a specialized
agar for lactose fermenting bacteria as shown in this image E. coli is growing at
the bottom section of this plate what is less clear is that Proteus and Shigella
were also cultured and do not show on this medium E. coli’s virotoxin
virulence factor is the cause of HUS in this infection it is called Shiga-like
toxin for its commonalities with Shigella we also discussed how another
organism increases the activity of G-protein via ADP-ribosylation which
microbe does E. coli shared this characteristic with though it has many
other nonspecific characteristics test questions frequently bring up the
aspect of its toxins temperature sensitivity recommendations to properly
cook food helps to prevent disease from this and other microbes we discussed
some bacteriocins in the first module and that bacteria create these to kill
off competing microbes E. coli’s special brand is called colicin Shigella is one
that is easily mixed up with other enteric bugs it has the positive TSI
test like Salmonella as well as slight agglutination testing the Hektoen agar
is the differentiating factor this medium was actually developed for the
purpose of differentiating between these two microbes Salmonella produces
hydrogen sulfide which turns a medium black Shigella which does not turns it
green of course the shiga- toxin has been covered and its virulence factor is
associated with HUS Shigella often uses the 4 F’s
mnemonic for its most likely causes food fingers feces and flies this is also
somewhat different than the meat specific foods we see in Salmonella
though there is some overlap as it also has a very low infectious dose compared
to many other microbes it only takes a few individual bacteria to colonize a
host when it comes to Yersinia it is not a frequently tested for microbe
Y. enterocolitica can be detected in stool and even blood and other fluids but is
usually self-limited and diagnostic testing is unnecessary Yersiniosis
that presents with swollen lymph nodes or respiratory sequelae are more
concerning for Y. pestis a needle aspirate of a bubo can be cultured and
more accurate testing methods are more readily available as well but this is an
unlikely test question for medical exams there’s actually a vaccine for
Yersinia pestis but with the low incidence rate of this disease it is difficult to
know how much protection this actually offers to contemporary strains it can be
described as safety pin or bipolar when seen through the microscope it also
likes to grow in the cold which is in opposition to Campylobacter oddly due to
this bug’s love of iron those with hemochromatosis are
more susceptible to infections for Klebsiella respiratory infections are
often first hinted at by a patient’s presentation that leads to a chest x-ray
to distinguish between different respiratory pathogens a sputum culture
can be attained to help with the diagnosis urease positivity is one of
the unique features that can distinguish this on a test question from most
respiratory microbes with the exception of possibly Pseudomonas genital lesions
are usually diagnosed by clinical presentation alone remember the currant
jelly sputum descriptor for this on test questions a particular demographic in
which it is more common would be those prone to aspiration such as those that
are weak or unconscious also as this is one of the three common MacConkey agar
bugs if that is mentioned in a question it should be a great clue and lastly for
this tier the miscellaneous section let’s reinforce that Proteus is urease
positive what are the other bacteria in the punch mnemonic for urease
positivity what is the name of the microbes kidney stone it also produces
hydrogen sulfide which two microbes did we just discuss in which this factor
helps to distinguish between Enterobacter has a special medium though it’s
so low yield it’s almost not worth mentioning as it ferments many sugars
this also usually doesn’t help to differentiate as well as a single
fermenter Serratia is most notable for its red appearance on culture this is
from a pigment that it expels called prodigiosin bacteroides is an ABC
anaerobe that is found as normal flora in the gut because of some of the
enzymes it possesses when it is displaced into other tissues it can
begin to wreak havoc citrobacter is pretty low yield as well I can also
ferment lactose but really just remember everything here causes UTI hopefully
that wasn’t too brutal there were a fair amount of new tests in this module they
were mainly needed when distinguishing between other bugs in this family but
can sometimes help but those outside of the module as well the patient’s history
and presentation is always the first place to start then you need to think of
what the next best test will be in order to diagnose
the correct pathogen will cover treatment and DDx in the next tier are
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