Streptococcus pneumoniae

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.