Assalamu alaikom everyone, and welcome to another ATP video. In the last two
videos we talked about Pseudomonas, E.coli, and Shigella. Today we're gonna discuss
the fourth gram-negative bacteria in our list and that is: Salmonella!
Salmonella is a gram-negative rod, she is motile, non-lactose fermenter,
oxidase negative, and can produce hydrogen sulfide. Salmonella species
grows in colorless colonies on macConkey's agar since they can't ferment lactose
And since Salmonella produces hydrogen sulfide, Salmonella species will grow in black
colonies on Hektoen's agar and TSI agar. Remember that I told before that Shigella
does not produce hydrogen sulfide, and so Shigella species grow in green
colonies on Hektoen's agar. Now let's talk about the virulence factors. The first
one is endotoxin, and by now you should figure out why she has an endotoxin. And
the second one is Vi capsule, which allows the Salmonella to survive the
complement mediated damage. Okay let's move on to the clinical part now. First
we have salmonellosis. Now there are two types of presentation depending on the
species of the infecting salmonella, which are: typhoidal or non-typhoidal
Typhoidal salmonellosis, also known as typhoid fever or enteric fever, is caused
by salmonella typhi, and it's characterized by: fever, constipation,
followed by diarrhea, and occasionally rose spots rash. When S.typhi is
ingested in large amounts, she begins the infection at the ileocecal region, and
results first in constipation. Salmonella typhi has a special mechanism that
allows her to get into the intestinal cell, but we will not go into the details
of that today. In the first week of infection salmonella typhi invades the
bloodstream, which will cause systemic symptoms, mainly fever,
remember that's why it's called typhoid fever, and may also result in gross spots
rash. During this period the blood culture usually will be positive.
It is important to know how S.typhi spreads through the bloodstream. This
happens by invading macrophages. and once inside the macrophage S.typhi escapes the
killing mechanism and replicates. After that S.typhi leaves the macrophages and
infects the biliary tract, or the liver and gallbladder in other words, and from
there S.typhi enters the intestinal tract again through the bile, and all of
this happens during the third week of infection, and at this time S.typhi
causes diarrhea and the stool culture is usually positive at this point. Now
there's also a milder form of typhoid fever, which is called: para-typhoid fever,
and this is caused by another species called S.para-typhi. And that's it for
typhoidal salmonellosis, now let's move on to the non-typhoidal type. There
are two common non-typhoidal species: S.enteritidis, and S.typhimurium,
it's pretty hard to pronounce these names :D These species
invade the ileocecal region, and after the invasion they will cause
inflammatory response by the immune system and that'll be associated with an
increase in cyclic AMP. This will cause watery diarrhea first, but also
these species can cause shallow ulcers which will eventually lead to bloody
diarrhea. Last thing we want you to know about in the clinical relevance is that
Salmonella is considered the most common cause of osteomyelitis in sickle disease patients,
not the trait. To complete our discussion about Salmonella. We
need to talk about the transmission and vaccination. For Salmonella typhi it's
only found in humans, so it's transmitted by fecal-oral route. For the non-typhoidal
one, they are found in animals as well as in humans, and they are mainly
transmitted through chicken products, but they can be also transmitted by reptile
pets like snakes and turtles. Lastly we have live attenuated vaccine for
salmonella typhi but, on the other hand we don't have a vaccine for the
non-typhoida type, yet. So maybe in the future you will find the vaccine for it.
Finally, it's time to put all of this together and understand
how exactly we can form an algorithm that will put us on the right path of
diagnosing which species is infecting the patient. To start we check if the
organism is capable of lactose fermentation or not. If so we're most
likely betting on E.coli, if not we need to do more tests. Next we check if the
organism is capable of oxidation. Therefore requiring the presence of
oxidase enzyme. If so we're betting on Pseudomonas, if not we need to perform
one last test. We check if there is any hydrogen sulfide production on TSI agar.
If so, it's Salmonella, and keep in mind that the clinical presentation would
help in differentiating S.typhi from other Salmonella species. If not, then we're
betting on Shigella. Lastly remember what we said about
Hektoen's agar. Salmonella will grow in black colonies and Shigella will grow in green
colonies. Alright, and this brings us to the end of this video. If you enjoyed it
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