Streptococcus pneumoniae – causes, symptoms, diagnosis, treatment, pathology

Learning medicine is hard work! Osmosis makes it easy. It takes your lectures and notes to create
a personalized study plan with exclusive videos, practice questions and flashcards, and so
much more. Try it free today! Streptococcus pneumoniae or Strep. pneumoniae can be broken down into strepto-
which means chain, -coccus, which means round shape, and pneumoniae, which refers to the
fact that it causes pneumonia – no surprises there. So, Strep pneumoniae are round bacteria that
tend to grow in chains, usually in lancet-looking pairs called diplococci. They’re the most common cause of community-acquired
pneumonia – meaning pneumonia acquired somewhere other than the hospital. Ok now, a little bit of microbe anatomy and
physiology. Strep pneumoniae has a thick peptidoglycan
cell wall, which takes in purple dye when Gram stained – so this is a gram-positive
bacteria. They’re non-motile and don’t form spores,
and also, they’re facultative anaerobes, meaning that they can survive in both aerobic
and anaerobic environments. Finally, they’re catalase negative – which
means they don’t produce an enzyme called catalase. Ok, now, when cultivated on a medium called
blood agar, Strep pneumoniae colonies cause alpha hemolysis, also called green hemolysis,
because they produce hydrogen peroxide, which partially oxidizes initially red hemoglobin
in the blood agar to green methemoglobin. Other Streptococcus species, like Strep viridans,
are also alpha hemolytic. So, an optochin test is done to distinguish
Strep pneumoniae. That’s where a few drops of optochin are
added to the culture. Strep pneumoniae are optochin sensitive, meaning
the bacteria dies after adding optochin, whereas Strep viridans are optochin resistant – meaning
they survive. Now, Strep pneumoniae has a number of virulence
factors, that are like assault weaponry that help it attack and destroy the host cells,
and evade the immune system. So first, Strep pneumoniae is encapsulated,
meaning it’s covered by a polysaccharide layer called a capsule. The capsule has pili and fimbriae, which are
hair-like extensions that help it attach to a host cell. Once attached to a mucosal surface like in
the nasopharynx or the middle ear, Strep pneumoniae can multiply and produce biofilms. A biofilm is basically a layer of goop-like
material made of exopolysaccharides or EPS, within which Strep pneumoniae bacteria live
and reproduce. Comparing a biofilm to strawberry jam, the
seeds would be the bacteria and the rest of the jam would be the EPS. The bacteria in the biofilm hide from the
host’s immune system and antibiotics, and even exchange resistance genes. Sneaky… As if that wasn’t enough, Strep pneumoniae
also produces toxins, and the most important ones are IgA protease and pneumolysin. IgA protease destroys Immunoglobulin A or
IgA, which normally binds invading bacteria, so neutrophils can destroy them. That’s like a computer virus first taking
over the antivirus software! And then there’s pneumolysin, which activates
the host’s complement system – a set of plasma proteins involved in immunity. This results in a local inflammation which
not only destroys the bacteria, but also the host tissues – like alveolar capillaries and
pneumocytes. And here’s the most interesting part. Strep pneumoniae can actually peacefully colonise
the nasal cavities and sinuses, where it doesn’t do any harm so long as the immune system keeps
them in check, restricting their growth and preventing them from spreading somewhere else
in the body. Problems arise in individuals with weaker
immune systems, like infants and the elderly. Other immune-weakening conditions include
an HIV infection, diabetes, malignancy, or alcohol abuse. Additionally, some toxic compounds in cigarette
smoke can weaken the local respiratory defense mechanisms, making individuals more susceptible
to Strep pneumoniae infections. Strep pneumoniae can cause a number of infections. One example is rhinosinusitis, which is when
the mucous membrane lining the nose and the paranasal sinuses gets inflamed, resulting
in fever, facial pain, and headaches. Sometimes, when the walls of the paranasal
sinuses are very thin, and Strep pneumoniae can get into the cranial cavity – resulting
in meningitis. With meningitis, people develop a fever, neck
stiffness, and a headache. Strep pneumoniae can can also invade the Eustachian
tube, and cause an infection of the middle ear – or otitis media, which causes pain and
an earache. Chronic otitis media can spread to the mastoid
antrum behind the ear, where it causes mastoiditis. Finally, from the mastoid antrum, the infection
can spread to the cranial cavity, which is another way Strep pneumoniae causes meningitis. However, the most common disease caused by
Strep pneumoniae is pneumonia. Strep pneumoniae sticks to the alveolar cells
– the pneumocytes, and releases pneumolysin, which leads to destruction of the pneumocytes
and alveolar capillaries. Pneumonia causes fevers and chills, chest
pain, and cough. In severe cases, Strep pneumoniae can cross
the alveoli-capillary wall and enter the blood, resulting in bacteremia. Eventually, bacteremia can cause serious problems,
like sepsis – which can lead to hypoperfusion of vital organs. Sepsis caused by Strep pneumoniae is particularly
common in people with spleen problems – because the spleen plays an important role in immunity,
especially against encapsulated bacteria. So Strep pneumoniae infections are typically
worse in individuals who have undergone a splenectomy, which is when the spleen is removed,
or in those with sickle cell disease, who have functional asplenia. Once, in the bloodstream, Strep pneumoniae
can also cause pneumococcal endocarditis, forming clumps called vegetations that damage
the valves. It can also infect the pericardium, resulting
in purulent pericarditis which is where pus collects in the pericardial sac. Strep pneumoniae can also spread to a joint
resulting in septic arthritis, or it can cross the blood-brain barrier and cause meningitis
as a complication of bacteremia, rather than invading directly as part of a primary disease. Once it has crossed the blood brain barrier,
it can also cause an intracranial or spinal epidural abscess. Finally, Strep pneumoniae can spread from
the bloodstream to the peritoneum, causing spontaneous bacterial peritonitis. This is more common in individuals with cirrhosis
who develop ascites, or too much fluid in the peritoneal cavity. Diagnosing a Strep pneumoniae infection relies
on cultures from biological samples like pus, sputum, cerebrospinal fluid or peritoneal
fluid. Alternatively, PCR can be done to detect Strep
pneumoniae DNA. Individuals at risk for a strep pneumoniae
infection should get vaccinated. This includes young children below 2 years,
and old people over 65 years. Other high-risk groups include those with
chronic diseases such as diabetes, HIV, cancer, most importantly those with sickle cell disease
and those whose with functional asplenia. Currently, vaccines against Strep pneumoniae,
are divided into two main groups based on their mechanism of action. The first one is the pneumococcal polysaccharide
vaccine, also known as PPV 23, because it contains 23 purified capsular antigens, and
this one is mostly recommended in adults. Another category is the pneumococcal conjugate
vaccine, or PCV13, which contains the capsular polysaccharides specific to 13 Strep pneumoniae
serotypes, bound to a non-toxic recombinant variant of the diphtheria toxin, called CRM197,
which serves as a carrier protein. PCV13 is recommended in young children Finally, treatment for infections with Strep
pneumoniae require antibiotics, and the classic first line of treatment are beta lactam antibiotics,
like Penicillins G and V, or cephalosporins. However, Strep pneumoniae has been progressively
developing resistance against Penicillins and its derivatives such as amoxicillin and
ampicillin. So, alternatives include fluoroquinolones,
the combination of amoxicillin and clavulanic acid, third-generation cephalosporins like
ceftriaxone and cefotaxime. Ultimately Vancomycin and Linezolid can be
used. Alright, as a quick recap, Strep pneumoniae,
is a gram-positive round bacterium that lives in chains. They are non-motile, non-spore forming, alpha
hemolytic, and optochin positive. These bacteria are found asymptomatically
colonizing the nasal cavities and sinuses, but they can take advantage of a weakened
immune system, especially in very young or very old individuals, or in those with diabetes,
HIV infection, cancer, splenectomy, or those with sickle cell disease. They cause diseases such as rhinosinusitis,
otitis media, pneumonia, and meningitis. The treatment of infection with Strep pneumoniae
involves the use of Penicillins, but in the case of resistant strains, fluoroquinolones,
third-generation cephalosporins and vancomycin can be used.


  1. Green Hemolysis??? Hydrogen peroxide??🤔🤔. What if you were gargling with hydrogen peroxide and you ingested a bit, would it form the bacteria anyway? Or would you just get sick?

  2. Great explanation of pneumonia! As a lung specialist, I love the way you explain it. This inspires me to show more examples of CT scans and videos of Bronchoscopy on my channel!

  3. It’s typically a gram positive diplococcus on a gram stain for the doctors and med students listening to this. I think I’ve only seen it in chains from a broth culture, and rarely.

Leave a Reply

(*) Required, Your email will not be published