Well look at you. Aren’t you a masterpiece of biological evolution? You are a big, ambulatory, autonomous human being. You are separate from the insects on the ground, the birds in the air, and the steer in your burger. As a human, you envision yourself as lord of your surroundings, a unique animal risen and separate from other “lower” forms of life. Look at those clothes! And those thumbs! You are a whole and special individual, a single, isolated member of a species that has dominated and partitioned itself off from “nature” through years of rugged conquest and ingenuity. You could be very smug about all of that.
Except you’d be wrong.
You are not alone. While it is no secret that humans share their bodies with a bunch of microscopic, and smallish macroscopic, guests, the scope of their pervasiveness and impact on our lives is not commonly understood. The role of microorganisms and other parasites in the human experience extends far beyond a bit of armpit odor, bad breath, head lice, and dandruff.
In the perspective of a bacterium, the outside of the human body is an endless substrate, strewn with a ridiculous amount of nutrients and minerals, set on a lumbering factory that just churns out more goodies, making human skin an attractive place to settle down and colonize. The inside of humans is even better! It’s warm and moist and if you’re in the right organ system, new nutrients get delivered right to you! Human bodies are to bacteria as a mystical candy house in the woods is to Hansel and Gretel. It is only by the graces of the immune system provided, non-stop, by vigilant human cells that you and I aren’t eaten from inside out…and also from the outside in…by our plentiful tenants.
And plentiful they are. Humans are made up of a mind-blowingly large number of cells; roughly 10 trillion by adulthood. It is estimated that just the gut flora (“flora” referring to the community of microorganisms inhabiting a certain area of the body) of humans is ten times that amount. Once you factor in bacteria living everywhere else in and on your body, it becomes abundantly clear that humans, on a numbers basis, are little more than scaffolding for empires of tiny organisms. We act as though we run the show and have some sort privileged position over whatever organisms live on or in our bodies, but the reality is that in the human ecosystem, human cells are vastly outnumbered by those of bacteria and fungi. Perhaps we should consider that maybe bacteria don’t live on us, but instead, we live on them.
The density of flora in the human gut is so high that more than half the dry mass of human feces is pure bacteria. So, it’s safe to say that taco night is more than just an invitation for spending quality time with your bathroom, but actually somewhat of a mass eviction. You monster…
The role of bacteria in the gut is thought to be more than just casual leeching of whatever you happen to eat, but is actually closer to a mutual relationship. Under ideal conditions, the gut flora aid in the extraction of certain nutrients from food, produce vitamins like vitamin K and biotin for the host human, by inhabiting the digestive system in such numbers, make it harder for rogue bacteria to take root, multiply, and cause problems. Ironically, a major part of the immune system in the digestive tract is to basically support a sustained ‘infection’ of good guy bacteria. Another benefit given by gut flora is their ability to impact the growth of the epithelial cells in intestines in a way that stabilizes repair and growth, and prevents injury to the gut mucosal lining. There’s also evidence to suggest that relative levels of specific types of gut flora can impact the immune system’s reaction to antigens (the substances that cause allergies). Certain gut flora, early in a human’s life, may “train” the body’s immune system to properly interact with antigens. If this is true, then if you are unlucky enough as an infant to develop an anomalous demographic of gut flora, you may be cursed to a life of Benadryl and/or a blinding fear of PB&J sandwiches. The relatively new found importance to overall health, long-term and short-term, of maintaining a normal and thriving intestinal culture has led many medical doctors to begin embracing the notion of a sort of “bacterial therapy” via probiotics in the wake of events damaging to gut flora populations (like antibiotic use)…something that only decades ago was solely the expertise of your local natural supplements and vitamins shoppe.
Yet, not is all peaches and cream with your foul gut buddies. While the lack of certain denizens can be bad, the overpopulation of normally beneficial species can be disastrous as well. The genera Bacteroides and Clostridium are associated with increased tumor generation in increased numbers, while tumor suppression can be somewhat associated with elevations of Lactobacillus and Bifidiobacteria. Overgrowths of Clostridium difficile can cause a brutal intestinal disease if it manages to outcompete all other bacteria. Most of the endemic bacteria to the gut are only tame in that specific environment. If an injury causing perforations to the gut occurs, and the little guys get out into the body cavity, nasty infections will occur quickly and death can come swiftly without immediate care. Gut bacteria may even have a pivotal role in human obesity. Two phyla of gut bacteria, the Firmicutes and Bacteriodetes, seem to be involved in this. Experiments with mice that either possessed a mutation that made them obese (by impacting lipid metabolism) or were normal, showed that the obese mice had a higher percentage of Firmicutes compared to Bacteriodetes in their guts, and the reverse was true of the normal, lean mice. The same pattern has been observed in obese and lean humans. These ratios even change in real time; if an obese person loses weight, or if a lean person becomes obese, the ratio changes. Lean mice that experienced a transplant of “obese type” flora into the gut end up gaining weight despite a decrease in food consumption. There are fundamental differences in how each group of bacteria assist humans in the absorption of fatty acids and sugars, and the impact of our interior bacterial community may be so entrenched in our ability to take in and regulate nutrition, that just having a certain ratio of gut flora may essentially determine your body weight, and your ability (or inability) to change it.
Some medical doctors and microbiologists are beginning to see the ubiquity of gut flora in our everyday health, and the potential need for something akin to a “microbiota counselor” that helps impact diet and ingestion of certain bacteria in such a way as to increase the overall health of the client. In coming decades, it may be that foods your dietitian may recommend may be dependent on what is currently making up the community in your colon, analysed thoroughly from sampling. Although, misunderstanding of this concept may have unintended consequences.
The flora of bacteria and fungi covering your every single square inch of skin also play an important role in your upkeep. Most just kind of hang out on there, lazily munching on whatever they can. Some of these occasionally, during a slip-up of the immune system, get WAY out of control and cause things like staph infections and candida. Others, however, serve as a regulatory agent and keep the biota in check. One of these is the bacterium Pseudomonas aeruginosa, which produces compounds that strongly inhibit the growth of many other microbes, like the ones that cause diseases like staphylococcal and streptococcal infections. One substance produced by P. aeruginosa is pseudomonic acid, which has been isolated and used in the form of the medicine Mupirocin, and is used specifically against those maladies. The bacterium also stops the growth of Aspergillus, Candida, and Helicobacter pylori, all of which can cause disease in humans. If P. aeruginosa is removed from the skin by an antibiotic rinse, the risk of yeast colonization on the skin goes up dramatically. The other side of all of this is, of course, that an overgrowth of this bacterium is just as bad as any other, and invasions of the body cavity can be life threatening. But, the benefits of this little bacterium in human lives far outweigh the risks.
If you weren’t satisfied with acknowledging that your body is nothing more than crowded cesspool of microscopic procreation and constant war, than consider the big stuff; parasites. While technically bacteria are somewhat parasitic, other, bigger organisms are a huge part of human life, and tend to be more…noticeable. One of the overlooked ones is Entamoeba gingivalis, which you’ll be able to discern, if you know your Latin, is found in the gums. It is an amoeba, and is thus more like humans, dogs, and horses than any bacterium, and it spends its days sliding along your teeth, scrubbing for food particles, as well as bacteria. This organism actually provides some level of benefit, getting the stuff you were too lazy to get with your toothbrush or floss. However, if you manage to get too much bacteria in your mouth from a month of doing nothing but smoking meth and rinsing your gob with Mountain Dew, the population of E. gingivalis explodes, along with all its prey items, and before you know it, you’re dropping teeth like a cage fighter. If a mouth has a high-enough concentration of this amoeba, it can cause an infection in another mouth by being passed through kissing, sharing drinks, etc. At the risk of sounding coy, I’ll also note that the mouth isn’t the only place adorned with mucous membranes that can harbor a population of this stuff, although that is where it always starts. So, you know, take that information and run with it.
When one mentions ectoparasites, or parasites that live on (or sometimes within the outer layer, depending on the definition of the term) the outer layers of skin/fur/scales/feathers, the images that are typically conjured up are those of fleas, ticks, lice, mites, chiggers, bedbugs, leeches, etc. Those are all well and good, but agonizingly familiar to those residing in the so-called “developed” world, which has only existed for a century or two. Humans actually have a long history associated with ectoparasitism, and it is only in recent centuries that those in Western civilization have eradicated a larger diversity of offenders native to the areas in which its citizens inhabit. However, unfortunately, many of these parasites are still an issue throughout the tropics and warm temperate latitudes, which house a very large number of the 7 billion people on this planet. While those in the northern latitudes haven’t lived with the reality of rasping, ever-present parasites in generations, many others have not had that luxury, partially because of proximity to equatorial latitudes, and partially due to a long history of colonialism, slavery, exploitation, war, and oppression, culminating in divergent trajectories in levels of economic and resource development…but that is lengthy and involved discussion for another time I suppose.
One example of these parasites still very much a part of the tropical world is the human botfly. Botflies, along with blowflies, parasitize large mammals…often this means livestock, but it also includes humans. Lucky us. Ranging from Mexico to Argentina, is Dermatobia hominis, the human bot fly. Basically, the parasitization scenario goes like this: an impregnated bot fly, heavy with eggs, tackles and grapples a mosquito mid-flight. It then deposits some eggs on the back of this mosquito, and lets it go. The mosquito, undoubtedly traumatized, seeks to drown its woes in warm blood, and eventually lands on a human and begins to feed. The body heat from the human skin signals the eggs to hatch and the larvae to fall onto the skin. The mosquito flies off, the tiny maggots then enter the skin through the bite entry hole of the mosquito, and hide out there for eight weeks or so, growing and feeding on human blood and tissue. Over this period, the larvae grows to be as wide as your pinky, and is anchored in by rows and rows of recurved spines, with a breathing tube extended out through the skin. What results is a tender, slow-developing boil. If allowed to mature completely, it will simply squeeze itself out and drop to the ground, where it pupates in the soil over about a week and eventually emerges as an adult. It leaves behind a gaping, horrific hole which easily becomes infected. A common practice for removing botfly larvae is to cover the affected area of skin with nail polish or duct tape as to asphixiate the larva via its breathing tube and weaken it, and then to gently pull it out by its end, careful not to rip it in two, killing the larva and leaving a chunk inside to get the wound infected. If you managed to get through that part, and somehow haven’t fainted from fluid loss from excessive vomiting, application of antibiotics is a must.
The video below goes over an example in this process in exquisite, gory detail. This is fair warning: for some, the video will be potent nightmare fuel and an effective deterrent from eating for the rest of the day. If you aren’t a calloused and disturbed biology-minded soul like myself, you may not want to watch this.
And the award for Most Unique Souvenir goes to…
A particularly nasty parasite that straddles the line between exterior and interior parasitism is guinea worm, Dracunculus medinensis, which causes dracunculiasis. Basically, larvae start off living in stagnant areas of water in the tropics, where they infect copepods (water fleas). Humans who drink from this stagnant water, unfiltered, ingest the infected copepods. Those copepods are digested, but the larva inside remains intact and mates with any members of the opposite sex that were also taken up with the water. After this, the male will die and be reabsorbed. The female, however, bores deeper into the body and settles in the connective tissues near long bones and joints, where she waits and develops for about a year. When she’s ready, a blister develops near the foot, and within a few days, it bursts and the guinea worm begins to emerge from the flesh. A mature female guinea worm can be as thick as a spaghetti noodle and measure up to three feet long. The pain associated with this disease is intense, and mimics that of a burning sensation. Evolution, being the crafty and cruel force it can be, was the cause of this particular symptom; the burning sensation encourages sufferers to soak afflicted areas in water to soothe the pain. As soon as water surrounds the end of the exposed guinea worm, she releases hundreds of thousands of eggs into the water, completely contaminating the water supply and inevitably continuing her genetic line. The worst part of this disease is that there is no vaccine or medicine that can kill the worm. Once a person has the parasite, they must wrap it, incrementally once it emerges, around a twig until it is completely dislodged. Anything faster than this may break it into pieces and cause it to migrate away from its exit site, prolonging the incredibly painful infestation and removal process.
Fortunately, guinea worm can be prevented with access to filtered water. Implementation of portable drinking filters and some minor changes to water sources over the past 25 years have almost completely eradicated a disease that at one time, ravaged the tropics of Asia and Africa. The number of reported cases of the disease have dropped from 3.5 million in the mid-80s, to no more than 3,000 a couple years ago.
So, haughty human, remember that you are not isolated from nature. The barbarism of life on this planet is on you, in you, and an inherent part of you, in the proximate medical sense, and as a part of millions of years of evolutionary heritage. By measure of cellular number, you are more commensalist bacterium than human. Despite a plethora of perfumed soaps, antibiotic cleaning agents, and sterilized living areas, the grit and grime of the world is never going to go away. It can turn on you in an instant if you aren’t careful, and the “charismatic” wild parasites are still around, still heavily adapted to human exploitation. While you and I are certainly special and unique to each other, to scores of hungry, mindless invertebrates and microbes, we are all mountains of resources upon which they can engorge themselves. Sadly, with all our errored, perceived differences and biases, perhaps it is only in the eyes of our loyal parasites that we are all equal.
© Jacob Buehler and “Shit You Didn’t Know About Biology”, 2012-2014. Unauthorized use and/or duplication of this material without express and written permission from this blog’s author and/or owner is strictly prohibited. Excerpts and links may be used, provided that full and clear credit is given to Jacob Buehler and “Shit You Didn’t Know About Biology” with appropriate and specific direction to the original content.