So You Want to Know about Pathogens, Eh?

So a recent conversation with a friend sparked today’s topic: viral infections versus bacterial infections.  Even though we’ve all taken high school Biology, I think a lot of people have forgotten what the difference is between the two, and I’m here to change your world ;)

First off, please recognize that I am neither a doctor, nor a member of the CDC, nor an expert in any way, and I will be making generalizations and simplifying things quite a bit.  If you want to learn more talk to a professional or do some of your own research.  Also, say it with me: “viruses suck.”

Why do viruses suck?  I mean, we have preventative vaccines for them (which DO work and WILL NOT give your child autism; please do not believe that crap); can’t we just inject dead viruses into people for every major viral disease and cure viral infections forever?  Not really, no.  For things like polio we’ve done a pretty awesome job with the vaccine, but look at the success rate for the flu or the common cold.  Vaccines work by taking material - sometimes synthetic, sometimes genuine - from the pathogen (virus or bacteria) and sticking it into your body so that your immune system will recognize it and have a plan of attack in case a full-on infection happens.  To understand why we’ve had such great success with some viral vaccines but not others we need to look at how viruses spread.

There are two ways a virus can infect you: a sneaky way, or a Michael Bay way (lysogenic and lytic, respectively).  Don’t worry, I’ll give you an easy way to remember those names so you can wow your friends tomorrow.

See, all viruses start infecting you the same way: a virus attaches to a cell, sticks a tube into the cytoplasm, and spills its guts (see figure A).  By guts I mean genetic material, either RNA or DNA.  Once the genetic material is in, viruses can go into lytic mode or lysogenic mode.

Figure A: A virus inserting genetic material into a cell

Sneaky, sneaky!  (Lysogenic)

Even when viruses aren’t actively making you sick, they’re planning to.  Lysogenic reproduction is when viruses go undercover.  When conditions aren’t good outside for young viruses to be making their big debuts, the viral DNA/RNA will actually attach to the cell’s own genetic material, and then just wait (see figure B).  Every time the cell replicates, the viral DNA/RNA replicates with it.  Lysogenic viruses go so undercover they become a part of your cell.  Then, when the situation outside is better, the viral genes will “turn on” and the cell will start filling up with little viruses it made along with its normal proteins.  See, you don’t get sick when viruses are undergoing lysogenic replication, but it can easily turn into lytic replication, which does make you sick (you’ll see why).

Figure B: "Phage" = virus (also, the genome of the cell is circular because it's a bacterium; animal cells don't have circular genomes)

Hostile Takeover (Lytic)

There are two ways lytic reproduction can occur: right out of the gate, or after lysogenic production.  If conditions are favorable, viral DNA/RNA won’t even bother to attach itself to the cell’s genome; it will just take over the cell’s machinery and start making copies of itself on its own.  Pretty soon the cell is full of virus copies.  And what happens when you fill something past the point of capacity?  If you said, “it explodes,” you’re correct.  Violent, but correct (figure C).  The cell will rip apart, releasing all the newly-formed viruses into the world to infect as many new cells as possible, ideally in the exact same lytic way.  If the virus has gone through lysogenic reproduction it does the exact same thing, it just lets the cell make its own doom instead of taking over the machinery.

Figure C: MASSIVE EXPLOSION!

“Spotalotamus,” you cry, “how will we ever remember those names and processes?”  Don’t worry, baby birds, I’ll feed ya.  See, ‘lys’ and ‘lyt’ come from the same Greek root meaning “to loosen,” but in Biology we take it to the next level and they both mean “to break.”  So, both lysogenic and lytic are out to break apart your cell, but lysogenic takes longer.  Eh?  Longer word, longer process; you get it?  OK, maybe it won’t be so easy to remember the words, but I tried :P

So, why do some viral vaccines work great and others don’t?  Keep in mind that with every reproduction of genetic material, there’s usually a slight mutation in the genome.  With viruses replicating at such a massive scale, there’s bound to be some viruses that are stronger than others (as well as weaker ones, or non-viable ones; evolution works both ways).  The thing is viruses – like everything with genetic material – are constantly changing.  The easier they are to spread, the more mutations they will have.  Polio hasn’t changed enough to make the vaccines worthless, so we still have a preventative measure.  With the cold and flu though, there are TONS of mutations.  The flu vaccine they put out every year is usually made up of a few of the most common variations seen, so if you get a virus that wasn’t in the vaccine, you’re screwed.

And that is why viruses suck.  The best we can do in the fight against viruses is use preventative measures and treat the symptoms.

Bacteria, on the other hand, are the good guys.  For now.  It’s been estimated that the average human has ten times more bacteria in their body than human cells.  Pause.  Rewind.  Play.  The average human has TEN TIMES more bacteria in their body THAN HUMAN CELLS.  Needless to say, we usually get along with these guys pretty great.  You’ve even got some good-guy E. Coli hanging out in your digestive tract at this very moment, just doin’ its thang.  AND bacteria are alive, which means they need things like cell walls and working organelles, and that, my friends, is why bacteria can be great.

While there are some bacteria that have gone to the dark side with the viruses and get inside your cells to infect you, the majority stay outside of your cells, so we’re going to focus on those.  See, bacteria reproduce very fast as well, and they mutate just as much as viruses, but WE CAN KILL THEM.  I love that!  Since bacteria reproduce just by splitting in half for eternity, we’re going to focus more on the killing aspect.

There are two main types of bacteria: Gram-negative and Gram-positive (figure D).  The names allude to one Hans Gram who discovered that if you used a certain dye on bacteria, you could figure out how they protected themselves. 

Gram-negative
These bacteria don’t absorb the dye, and are seen as a pinkish color.  The dye isn’t absorbed because these bacteria have an awesome line of defense: two membranes, a cell wall, and an outer capsule for extra protection.  Needless to say, these are the harder ones to kill with antibiotics.  Usually the antibiotics you’re given for these bacteria work on destroying that membrane so it’s vulnerable to the environment around it.

Gram-positive
These bacteria do absorb the dye, and they’re seen as a purple color.  The main defense of these puppies is just a thick cell wall, and we have tons of antibiotics that rip that sucker apart.  Yay penicillin!

Figure D: The two types of bacteria

You may have noticed earlier that I said bacteria are the good guys “for now.”  And the main reason their villain applications are in is that people don’t understand the difference between viral and bacterial infections.  Bacterial, you can treat.  Easy.  However, if you take antibiotics for a viral infection (a very common practice in the past, since people would just pester their doctors until they gave them something), you’re setting yourself up for problems.  Remember how you have more bacteria in your body than human cells?  Well, if you put antibiotics into that sea of bacteria, you’re going to start your very own natural selection.  Recently there have been more and more cases of antibiotic-resistant bacteria, and it’s due to people taking antibiotics when they don’t need them. 

So be smart.  If you’re told it’s a virus, go home.  Drink water.  Rest.  Take a cough drop, if you want to.  Don’t pester your doctor for antibiotics.  If it’s bacterial, shout for joy because that sucker is going down. 

YAY SCIENCE!