The Man of Many Hats

I was thinking about pathogens for a new blog post while biting into a delicious, 100% angus beef burger the other day and thought about the kinds of organisms that could be hiding within. Of course the chances are remote but I thought about infections of Escherichia Coli...that's the last time I'm going to try spelling that word! As a result, I decided to post a little bit about this bacteria. Crazy enough, I read on the news today that 14 people have died and hundred are sick in Germany and across Europe due to contaminated cucumbers so this post fits perfectly with a current event.


What few recognize is that e.coli isn't this horrible pathogen that inflicts disease whenever you have it within your gut. In fact, everyone has billions of these organisms sitting in their colon right now and they're definitely NOT making anyone sick. More importantly these bacteria are supposed to be there to help digestion by breaking down cellulose from plants that humans eat. Humans can't digest this material (aptly named "insoluble fiber" on the nutrition facts on the product label) so the bacteria that are harbored in your gut are beneficial to you. Disturbing this balance can increase the risk of other types on conditions, but I'll save those for another post.

So what gives e.coli such a bad rap? The answer to that starts with an incident that happened in 1982 with a restaurant food chain. The hamburgers that were served harbored a dangerous strain of e.coli that was ingested by customers leading to deadly gastrointestinal problems combined with kidney failure. Some of those infected died and e.coli suddenly became a bacteria to be feared. Isn't the media a wonderful thing?

Ok, great, so what made that bacteria so deadly compared to the ones in your gut right now? The best way to answer this is seen in the 6, yes that's right 6, kinds of pathogenic e.coli. I'll only explain 4 of them because little is known about the last two and these four and are the primary cause of disease in humans. The difference, the presence or absence of various virulence factors that mediate disease.

The first in Enterotoxigenic e.coli or ETEC (E-teck). This causes what many call "Traveler's Diarrhea" or, if you're in Mexico, "Monteczuma's Revenge." It's the leading cause of bacterial diarrhea in developing countries as well due to contamination of drinking water with run-off from waste. This form of e.coli has acquired two virulence factors from horizontal gene transfer, a nifty trick that also deserves its own post. At some point in the bacteria's evolution, it acquired the genes coding for two toxins called the heat-labile toxin (LT seen below with fancy colors) and the heat-stable toxin (ST). The LT acts very similarly to the cholera toxin providing further evidence for an acquired virulence factor from a different bacteria. The LT functions by adding a ADP-ribose molecule (right), which is modified from NAD (the top part of the molecule on the right) used in normal cellular metabolism, to a special kind of protein within the host cell membrane called a G protein. These G proteins functions to regulate other proteins in the membrane that further act upon ion channels using a molecule called cAMP. What happens here is that the addition of this ADP-ribose molecule causes the G Protein to become constantly activated. This eventually deregulates the ion channels by having way too much cAMP around, which leads to more salt ions in the intestinal lumen of the host organism. In order to balance the increase in salt, water flows out of the cell and into the intestinal lumen which leads to diarrhea...super fun! The ST works in a similar manner but instead of an increase in cAMP, there is an increase in cGMP but the result is the same.

The next is the one that has caused the raucous. It's known as Enterohemorragic e.coli or EHEC. The model system for these strains is e.coli O157:H7 and this is the same bacteria isolated from the patients that ate at the food chain in 1982. Here again, the culprit is the aquisition of a virulence factor from a different bacteria. In this case the bacteria acquired the Shiga toxin (left)  from a closely related bacterial pathogen called shigella. What separates this from ETEC is the fact that EHEC actually kills the intestinal cells they infect. The shiga toxin cleaves 28s RNA (what the hell is that!?).  28sRNA is a necessary part of a ribosome which translates proteins from the mRNA the cell makes from DNA. Another rule in biology : If you (the cell) can't make protein, you (the cell) die! So by getting rid of 28sRNA the cell not longer produces proteins and dies which leads to the destruction of the instestinal tract and the bloody diarrhea found in these patients. It is also associated with a condition called hemolytic uremic syndrome wherein blood cells are destroyed leading to acute kidney failure which is very bad.


The third form of e.coli is Enteropathogenic or EPEC. This strain is super cool. Intestinal cells don't have the necessary receptor to mediate proper attachment of the bacteria. This isn't good for the bacteria so it cleverly has a gene that codes for the its own receptor called TIR. The bacteria then uses a syringe-like molecular machine that injects the receptor directly into the host cell! The receptor then migrates to the membrane and, shazam, the bacteria can now attach to the cell to mediate disease via the intimin protein on the bacterial membrane. Disease is caused by the syringe-like machine injecting other proteins that mess with the host cell membrane, specifically by forming a pedestal on which the bacteria sits (left)...sounds weird but it's true and it's awesome! By screwing with the cell membrane, the intestinal tract loses the tightly bound and organized structure formed by these cells and the result is poor absorption of water leading to diarrhea...super fun!

The final strain is Uropathogenic or UPEC. This strain of bacteria caused urinary tract infection! Here the e.coli has acquired a different set of factors. They have the P pilus which is a protein that is designed to specifically bind to bladder and urethral cells. The result is that these bacteria can ascend the urinary tract and, if given the chance, infect the bladder or even the kidneys. These infections are fought off by the induction of the inflammatory response of the body, and the result is the burning and pain associated with a UTI. Interestingly, this strain has developed a unique way of preventing being eaten by the body's macrophages. The bacteria elongate outward (pic right) and the macrophages can no longer extend enough to take them up! Evolultion ftw! The problem comes when these bacteria infect the kidneys, a condition called pyelonephritis. Inflammation here is very bad!


Wonderful, so what's the point here? Well, first, e. coli is awesome in its diversity. But more importantly, these are the same bacteria which differ only in the set of virulence factors they contain. Not having any factors means the bacteria can't do anything and is harmless. But thanks to horizontal gene transfer between bacteria, e.coli has developed novel ways of causing disease in humans. Increasing the diversity within the e.coli population is evolutionarily beneficial to the bacteria because it allows the bacteria to infect and survive in a different niche within the host, as best shown in UPEC which infects the urinary tract instead of the GI tract. Also, in the case of enteric e.coli (ones that infect the GI tract) diarrhea can also be seen as a potential benefit because it increases the chance of the host organism shedding the bacteria into the environment and allowing uptake by other organisms by the fecal-oral route...gross, but more common than you think!


The moral of the story is simple. The variation of diseases caused by bacteria are caused by the acquisition of virulence factors which can turn a previously avirulent strain of bacteria into a potentially deadly one. It's important to understand this principle if the influence of evolution on a pathogen is to be understood. As you will soon see, bacteria develop ways of swapping bits of genetic information in order to increase fitness and the result is new strains of pathogenic bacteria that were once harmless.

Wool, Soil, and Mail...Er, What Do They Have In Common?

Something funny happened to me last week that I found as inspiration for my first pathogen article. I forgot completely that four years ago at the end of my senior year of high school, I wrote a letter to myself to be delivered four years later. I received the letter and read it and laughed at how naive I was at that age. What was more surprising was that I received another letter in handwriting that was not my own. I opened it in curiosity and there was simply one word haphazardly written in the middle of a piece of torn out notebook paper. It read "Anthrax." Of course, based on the friends I hung out with, I assumed it was both referring not only to my aberrantly high affection towards bacteria even at that age but the 80's metal band of the same name. When I read this I smiled and gave a laugh. I don't think whichever of my friends sent me that letter in high school had any idea that in four years I would be so amused upon receiving it. It was my final graduation gift, a happy farewell from four years ago and another happy farewell to my undergraduate career four years later. 

This got me thinking about everything I had learned about bacillus anthracis, the causative agent of anthrax. Of course, anthrax gained national attention during 2001 when several letters filled with b. anthracis spores were sent to several high profile people including Tom Brokaw. But the history of bacillus anthracis (that's pronounced anth-ray-cis) goes back far before these incidences. Robert Koch, the father of bacteriology, used b. anthracis in 1877 to show that bacteria cause diseases. This was the first time that the bacterial basis for disease had ever been proven beyond a doubt and it became the fundamentals of Koch's Postulates which are still used today as a means of determining if a pathogen is the causative agent of a disease. Basically, he's the man AND he has a pretty sweet beard.

Interestingly anthrax was primarily a disease in people who sorted wool. Animals would become infected with the bacteria after picking it up from the soil where it normally resides in the environment. Humans would become infected as a result of close contact with the infected animal. The disease became known as Woolsorter's Disease and it proved to be rather fatal. 

There are three forms of the disease which depend on where contact with the bacteria is made. This includes cutaneous anthrax (located on the skin as black, necrotic lesions...gross), gastrointesinal (in the stomach after ingesting spores), and pulmonary (inhalation, this is the worst as you will soon see...). Immediately what is interesting here is that the virulence of the disease depends on location of the infection. Cutaneous and gastrointestinal forms are far less severe as a pulmonary infection which suggest that the bacteria has developed mechanisms specifically designed for lung infections and this is exactly the case.

Infections in humans can only be accomplished through the spores. This is a unique adaptation to some bacteria that essentially causes the bacteria to "hibernate" when environmental stresses are high. These spores are incredibly resistant to heat, UV, and antibiotics. Pretty wicked, huh? Anyway, when these spores are inhaled they obviously enter the lungs where they are taken up by macrophages in the lungs. This is where the story gets interesting. A mechanism designed to eliminate invaders is used by the bacteria. The spores are not destroyed within these macrophages and the macrophages unknowingly harbor a dangerous enemy within. The macrophage does what it does, which includes traveling to a lymph node bringing the bacteria with it. At some point the spore reenters the bacillus form and replicates VERY rapidly. The bacillus form is VERY replicative unlike the spore. My professor once said that it could grow in spit! The bacteria secrete a toxin which kills the cell and the bacteria emerge from the macrophage within the lymph node. From here they have a one way ticket into the bloodstream. Thanks to a poly-D-glutamic acid capsule (isn't that catchy?) that surrounds the bacteria they can not be taken up my any other macrophages in the blood. As you can imagine, this is not good for the host which leads to an overload of bacteria in the bloodstream, a condition called bacteremia. This leads to...you guessed it...another kind one way ticket...

So this brings up an interesting evolutionary adaptation that many bacteria employ. They use the defense systems of the host in order to establish a successful infection. This is pretty clever. They wait until the body's innate defenses do what they do and then emerge before the body knows what hit it. Not only this, but there is some unknown temporal trigger that tells b. anthracis to emerge and replicate at the right time. This employs a virulence factor known as the Lethal Toxin, which kills the macrophage, but it's only released when it is most advantageous for the bacteria! This won't be the first bacteria to use this kind of pathway. Evolution has selected for these types of organisms to develop ways of directly invading immune cells, essentially eliminating the struggle of the pathogen to survive the innate immune system...the picture speaks for itself!

Luckily, the government is working hard to develop vaccines and new antibiotics against the bacteria. The University of Illinois at Chicago recently received a government grant of  $14 million to help develop novel drugs to use against the bacteria. Obviously, the problem is getting it to either kill the spore or kill the bacillus itself, both very difficult due to the traits of the bacteria. In my opinion a vaccine against the poly-D-glutamic acid capsule may be useful, but it's definitely a tricky disease to try to combat given the interesting way it causes disease. However, we should all sleep easy because it's super unlikely that any of us will ever become infected with b. anthracis. Isn't biology fun!?

Microbiology 101: The Red Queen Rules All

Hello, and welcome to my blog. Nothing has fascinated me more than the field of microbiology. From viruses to bacteria, each organism had shown to be an incredible piece of nature. To me, I'd even take it as far as saying that they are works of art. At first glance many bacteria look the same, either a rod or a bunch of circles.  But when you get down to understanding what they do and why they do it, they turn into something incredible. You see that in the realm of evolution and with the struggle to survive comes the advent of some of the most clever and interesting evolutionary adaptations imaginable. This blog aims to enlighten my readers to these mechanisms in a simple way, but still provide that "wow" factor that has driven me to learn as much as I can about these microbes

First off, there are several things that must be explained. I am not a master in this field and the information I provide is only what I have gained through four grueling years of study. Next, this blog is meant to look at some of these adaptations from the perspective of evolution. If you are a person that doesn't believe evolution is real then I encourage you to read through my articles with an open mind. Either way some of these pathogens will lead you to say "How do they do that?" regardless if you believe evolution is the source or God (or whatever deity you may believe in) is the source. Look at and admire the beauty of these pathogens and you will appreciate nature more. Finally, question what I write and respond. As a writer I'd love to engage is conversations about the topic I write about. Some will be my own opinion and I am more than welcome to see what everyone else thinks about it...Oh yeah one more thing, the images in this blog are taken from Google except the background image which I took using PyMOL freeware.

Now that that's out of the way I'll get right to the basics of microbiology. The simple rule: EVERYTHING IS RULED BY EVOLUTION. The way by which pathogens survive are all ruled by the principles of natural selection. The most fit will continue to proliferate while the least adapted pathogens will die. It's no surprise then that bacteria replicate many times in a short time-frame. By increasing the turnover rate of bacteria there is an increase in the chance of variation that can lead to more adaptive mechanisms. It is also no surprise then  that certain pathogens take advantage of this genetic variation by increasing the amount of mutations in their genomes by getting rid of repair mechanisms or by recombining genomes with other bacteria.

This leads us to the Red Queen Hypothesis. This is basically the hypothesis that best explains why pathogens act the way they do in the human body.In order to understand disease pathogenesis caused by bacteria and viruses, it it necessary to understand this hypothesis. In Lewis Carroll's Through the Looking Glass the character of the Red Queen says "It takes all the running you can do, to keep in the same place." In microbiological terms this translates into an arms race between the pathogen and the host. As the bacteria develops ways of infecting the host in order to survive, the host develops ways to combat the infection. Conversely, as the host develops these defenses, the pathogen develops new ways of infecting the host. The result is an evolutionary stalemate, a battle to stay in balance with neither the host nor pathogen gaining ground in the battle of survival. Disease happens when this homeostasis is disrupted and the pathogen gains a new trait that leads to disease.

As a result of this arms race the pathogen develops virulence factors, traits that cause disease in a host organism. It is the virulence factors that are the focus of pathogenesis and what I find most interesting about how these "bugs" cause disease. These factors will be the primary focus throughout my posts. This also raises a bigger question of why in some bacteria these traits are expressed but in avirulent (non-disease causing strains) bacteria they are not...But more on that stuff later. For now, enjoy the journey and appreciate how these small beings have continued to persist within the human population despite our best attempts to eradicate them entirely.