Gram Negative Bacteria: Pseudomonas aeruginosa and Escherichia coli

Gram Negative Bacteria: Pseudomonas aeruginosa and Escherichia coli

Assalamu alaikom, welcome to the first part of gram-negative bacteria. In this video we’ll take a look at some of the
important gram-negative bacilli The first one we’re gonna call him Pseudomonas aeruginosa He’s an obligate aerobes, and a cool
mnemonic to remember this is: aeruginosa is aerobic. He’s also non-lactose
fermenting, but is positive for oxidase, and this is particularly important for
lab interpretation, and we’ll discuss this more in the second part of this
topic. And in culture he has the scent of grapes
Moreover, the culture would look blue to greenish and this is because he produces
a blue-green color from the pigments pyrovirdin and pyocyanin. When it comes to his virulence factors he has some mighty weapons, such as: phospholipase C which damages cell membranes, endotoxin which is common between all gram negatives. And if you remember from our first video: the introduction to microbiology, we
mentioned that this is what causes the fever and shock in patients with
gram-negative infections. And then he has exotoxin a, which inhibits EF-2 or
elongation factor 2 by adding an ADP ribose moiety to it, and thus, not
allowing the cell to make proper elongated proteins.
For the clinical importance a high yield point is that it can cause nosocomial
infections, but that’s not all; in fact, he can cause otitis externa: which is
inflammation of the external ear, pneumonia: which is inflammation of the
lung, osteomyelitis or inflammation of the bones, and many other infections. The
point here is that he affects many parts of the body, and can even cause something called rapidly progressive necrotic cutaneous lesions, known as ecthyma
gangrenosum, which is typically only really seen in immunocompromised
individuals. And that’s pretty much all you have to do about Pseudomonas. So
let’s move on the second gram-negative bacteria that we will discuss is: E. coli.
It’s one of the most high-yield bacteria that you need to know about. So
let’s begin with its distinguishing features: E.coli is a gram-negative rod,
facultative anaerobe, and he is part of our normal GI microbiota, and like Pseudomonas, he’s oxidase negative. He’s the only lactose fermenter out of all the
bacteria we’re going to discuss in this video (and please keep this in mind when
we get to the lab diagnosis part in the second video. To culture E.coli we can
use macConkey’s agar, and eosin methylene blue agar. On macConkey’s agar, lactose
fermenters will grow in pink colonies, where lactose non-fermenters will grow
in colorless colonies. So E.coli will grow in pink colonies. On eosin
methylene blue agar, it produces a metallic green chain.
Now let’s discuss its virulence factors First of all, he is catalase positive, so
patients with chronic granulomatous disease or CJD, are at increased risk of
getting this infection and to explain why that happens, we need to know that the
neutrophils and macrophages use reactive oxygen species or ROS, to kill the
bacteria, and people with CJD have a defect in the enzyme NADPH oxidase which is important to generate the ROS; however, if a bacteria invades a patient
with CJD, the patient can borrow hydrogen peroxide from the bacteria and
use it to generate ROS and kill the bacteria. But if the bacteria has the
catalase enzyme, then this borrowing mechanism will not work, since catalase
enzyme will break down any hydrogen peroxide in the bacteria, which will make
the patient susceptible to catalase positive bacteria. E.coli is also
encapsulated: meaning it has a capsule, and it also has flagella, and pili for
movement and adherence. Lastly he has endotoxin, since he’s a gram-negative
bacteria. And also e.coli is very genetically diverse, and can be
differentiated into serotypes based on differences between three antigens: the
first antigen is o-antigin, which is related to the lipopolysaccharide, and
remember from our introduction to microbiology, that all gram-negative
bacteria has lipopolysaccharide in their outer membrane. Then it has H-antigen,
which is associated with the flagella, and K-antigen which is associated with
the capsule. The first clinical importance here is:
gastroenteritis, and we have two main types of E.coli that are particularly
important here: the first one is Enterotoxogenic E.coli, which affects the
small intestine and causes watery diarrhea. And then we have Enterohemorrhagic, which affects the large intestine and causes bloody diarrhea. Let’s begin with
Enterotoxogenic E.coli or, ETEC, for short It’s transmitted by water and is
associated with travelers diarrhea, and it’s called so because of the classical
clinical presentation which is: traveling to Mexico where the patient drinks the
contaminated water. Travelers diarrhea is also known as Montezuma’s Revenge.
Fun fact Montezuma was an Aztec emperor back in the 16th century who used to
rule Mexico. ETEC produces heat labile toxin and heat stable toxin. The heat
labile toxin which is similar to cholera toxin increases cyclic AMP, whereas
the heat stable toxin increases cyclic GMP.
Now let’s discuss enterohemorrhagic E.coli or, EHEC. EHEC is most commonly
transmitted by eating undercooked meat such as: undercooked hamburger, you can remember that from the H in its name EHEC toxin is Shiga-like toxin, that
inhibits the 60s subunit of the ribosome. And lastly remember that EHEC can
cause HUS or hemolytic uremic syndrome Other than gastroenteritis
E.coli can cause other diseases including: UTI, urinary tract infection. Actually,
E.coli is the most common cause of UTI and it uses the pili to adhere to the
urinal surface. It’s more common in females because they have a shorter
urethra. And E.coli can also cause neonatal meningitis and this is because
of the capsule it has. So remember that the k-antigen is associated with the
capsule so mostly the serotypes that contain k antigen are the cause of
meningitis. Lastly E.coli is the leading cause of
gram-negative sepsis because of the endotoxin it has.
This is all you need to know about E.coli. And this concludes our video, but stay tuned for the next part 😉

17 thoughts on “Gram Negative Bacteria: Pseudomonas aeruginosa and Escherichia coli”

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