Mission Impossible IV: Covert Alveoli Infiltration!

“And the closing act for tonight’s show “Winter Fun” is the one and only: pneumonia! A talented performer who typically stars in films with his colleagues the cold and the flu, pneumonia has been known to make rare solo appearances every so often.” 

In tiny air sacs in our lungs, called alveoli, oxygen from the air we breathe moves into the bloodstream as carbon dioxide is flushed out. Pneumonia strikes when these alveoli fill with fluid or pus thanks to an infection courtesy of one of our friendly neighborhood microbes. 

Pneumonia can be caused by any of microbiology’s big three: fungi, bacteria, or viruses! But since fungal and viral forms tend to be less common and less severe, the real showstopper here is bacteria, specifically Streptococcus pneumoniae. 

Image of S. pneumoniae provided by AJC1. 

Hence its classification as a microbe, a bacterium is just one cell, and this cell is typically 10-20 times smaller than any of our own 30-some trillion cells. But in the “smallest splash” competition, viruses still have bacteria beat by a factor of 10-100 times! But unlike “zombie” viruses, since a bacterium is cellular (and doesn’t eat brains, of course), it is considered alive. 

And bacteria are also independent (setting them apart from both viruses and teenagers)! Not only can a bacterium turn its food into something edible, namely ATP, and carry out other metabolic tasks to take care of itself, but it can also copy its own DNA and make its own proteins without mooching off a host cell. 

Of course, having to do so many chores means you have to be a bit more complex than the one-gene rhinovirus! In fact, the circular DNA molecule of Strep. pneumoniae codes for more than 2,000 different proteins. 

So, when bacteria can take care of themselves, why and how do they cause disease? 

Photo provided by Yale Rosen.

A bacterium is an extracellular pathogen. In other words, bacteria just hang around, rather than going behind enemy lines and infiltrating our cells.

 Strep. pneumoniae specifically, has decked its capsule out with different proteins that help it grab onto the mucous membranes in our throats. It is also bejeweled with pili, or what is basically a forest of stubby hairs for sticking to membranes. And it can be a quiet, or even suave companion, hanging out in your throat without causing any mischief. But don’t let your guard down; S. pneumoniae is up to no good! 

Just as soon as your immune system gets worn-out (since we’re a bunch of overly stressed, sleep-deprived college students), these bacteria launch their covert mission to finagle their way into your lungs, eventually dropping anchor and setting up shop in the skin cells lining the alveoli. Here S. pneumoniae comes out of hiding and begins a coup d'état, replicating en masse, and giving our immune system a run for its money! 

Not only do our immune defenses inflame our tissues, but the proteins, sugars, and fat molecules in the bacterial cell wall and membrane also antagonize our cells. And to make matters even worse, S. pneumoniae secrete enormous volumes of hydrogen peroxide and different toxins. Its like the itchy sweater on top of the blistering sunburn and extreme poison ivy! The cells in our alveoli don’t stand a chance! 

That is, without the help of our handy-dandy antibiotics! Luckily, these beauties can typically clear pneumonia up in a jiffy, so be sure to see a doctor if you think you might have pneumonia. Otherwise, this not-so-friendly bacteria could bring even nastier acts to the stage! 

For a complete list of symptoms associated with pneumonia see Mayo Clinic or WebMD’s webpages. 

And as always, best of luck staying healthy this winter!