Tag Archives: protozoa

Laveran’s eye

The military hospital of Constantine, Algeria was a fitting place to view what must have seemed the Devil in microscopic form.

Stages of the malaria parasite drawn by Alphonse Laveran

Phylogenetic humility

Just a welcoming sentiment from Robert M. Sapolsky in his collection of science essays, Monkeyluv. Sapolsky is a professor of biology and neurology at Stanford, and his essays, often humorous, delve into neuroscience, hormones, and human behavior. Oliver Sacks has called him “one of the best scientist-writers of our time,” and while I wouldn’t put Sapolsky on the same pedestal as I do Gould, or Levi, or Thomas when it comes to prose and insight, the man certainly has his moments, which manifest in me dog-earring a corner of a page. This one stood out to me this morning on the 3 train to work while reading an essay titled ” Bugs on the Brain.” As someone who gets excited over pathogenic protozoa and animal behavior, I muttered “Toxoplasma gondii” upon seeing the heading.

Like most people who come across any mention of Toxoplasma (it gets a fair amount of press; e.g., via Carl Zimmer), Sapolsky is interested in the precision of how the protozoa can manipulate behavior. The organism relies on a simple and common two-host system to complete its life cycle: the predator-prey interaction of rodent and cat. Rodents ingest the protozoa, which encysts in the mammal, with particular affinity for denning in the brain. When rodents are consumed by felines, Toxoplasma can reproduce, new organisms are shed through feces, which happen to be a food source for rodents and thus how the life cycle comes full circle. Many pathogens that rely on multiple hosts influence behavior, and there is a bevy of literature that describes just this, particularly with tapeworms. In the case of Toxoplasma, the protozoa interferes with a rodent’s natural aversion to feline pheromones; interferes is a weak term–the organism makes the rodent attracted to feline odor, increasing the probability that it becomes successful prey.

What interests Sapolsky so about this host-parasite interaction is that a rodent infected with Toxoplasma gondii otherwise behaves normally. As he notes, infected rodents maintain their social status within the system’s hierarchy, they continue to mate and thus sense pheromones of the opposite sex, and their recognition of other vertebrate odors isn’t tampered with in the least. The protzoa is simply able to manipulate the recognition of and reaction to the pheromones of one predator, that of the definitive host. To Sapolsky (and most of us interested in the long-term interplay between parasites and their host), this is evidence of how counter-intuitive and beautiful evolutionary process can be. Sapolsky takes the opportunity to highlight Toxoplasma gondii as a correction to teleological interpretations of evolution: its processes aren’t directional, aren’t progressive.  As he notes, “We are certainly not the most evolved species around, nor the least vulnerable. Nor the cleverest.” But the punctum of his message, to borrow a term from Roland Barthes (that which “pierces” the viewer/reader), is the statement, “we need phylogenetic humility.”

The painted lives of ciliates and schistosomes

Art has always been one way to mediate tensions, tensions such as those between the logic-driven mind of scientific inquiry and the subjective experience of the non-human, what Jakob  von Uexküll called an organism’s umwelt. Thomas Nagel famously argued that we can never know what it is like to be a bat, or any non-human organism, but whether through experimental-minded writings on what the world might be through a tortoise’s point of view  or through watercolor paintings, the artistic hopes to bridge various umwelten more so than a declaration of scientific understanding—the difference lies within the distinction of this is how a starling sees the spectrum of light and thus the world (science) and this is how a starling might see the world (art). Might opens possibilities, a window into creative endeavor.

The paintings of Emilie Clark might be one answer to Nagel. Clark, a painter based on Brooklyn, NY, has worked on a series of projects involved in life on a microscopic scale. In a 2004 gallery showingPondering the Marvelous, Clark responds to the writings of Mary Ward, a 19th-century Irish natural historian and painter, specifically Ward’s A World of Wonders Revealed by the Microscope. In imagining Ward’s writings as personal letters to the artist, Clark produced two series of her own watercolors—the first based on Ward’s description of Ireland’s microscopic landscape, and the second on Clark’s own collection.

Untitled MW-#50. Painting by and courtesy of Emilie Clark.

Untitled MW-#12. Painting by and courtesy of Emilie Clark.

The paintings are not meant in their entirety to be illustrations of these organisms’ umwelten, and nor do they achieve this ideal. But these paintings play with the possibility of “what if?” And it is this play that creates an opening in our imagining of the umwelt of other species. Perhaps best said by the poet–immunologist Miroslav Holub, the act of play allows us, simply, to “avoid the aridities of rationalism.” Yet this is not Clark’s first foray into toying with the lifeworld of other microorganisms.

In a previous post, I briefly touched on the topic of cover art for scientific journals—in this case, a watercolor of a stag beetle by Albrecht Dürer for a 2005 issue of Emerging Infectious Diseases. One of Emilie Clark’s projects, which one can find on her webpage, is likewise producing watercolor medical illustrations, many of which have found their way onto the covers of The Journal of Experimental Medicine. The JEM, since its beginnings in 1896, publishes original research on the physiological, pathological, and molecular mechanisms that are encountered by or reactions of the host in response to disease. In the case of a November 2005 issue of the journal, the target pathogenic organism of Clark’s illustration was Schistosoma mansoni, one of three causative agents of human schistosomiasis.

From the cover caption of JEM 2005; 202 (10). Emilie Clark's watercolor of S. mansoni eggs. The eggs secrete a chemokine binding protein, thereby suppressing the inflammatory response.

Schistosomes are blood flukes (trematodes) that belong to the genus Schistosoma. In addition to S. mansoni, the other two members of this genus that cause disease in humans are S. hematobium and S. japonicum. The disease itself, caused by human contact with water home to schistosome cercaria, is a definitive chronic condition whereby the mature schistosomes, after reaching the final stage of their life cycle, migrate to the mesenteric or rectal veins and begin to mate, thereby producing up to 300 eggs per day for the rest of their reproductive lives—which can be as long as 4–20 years. A proportion of these eggs will become lodged in the target veins, where they mature and secrete antigens that elicit an intense immune response in the host. It is this immunological reaction, which can continue as long as the mating worms and the eggs continue to exist in the body, that characterizes schistosomiasis. It was the point of the primary research communication by Philip Smith et al., the inspiration for the choice of Clark’s watercolor, to demonstrate one way in which S. mansoni modifies the human host to tolerate decades-long chronic infection without causing death. In particular, the researchers demonstrated that S. mansoni eggs secrete a protein into host tissues that binds certain chemokines—proteins that induce directed chemotaxis, how certain cells direct their movements according to particular chemicals in their environment, in nearby responsive cells—and inhibits their interaction with host chemokine receptors and their biological activity.

Now, compare Clark’s interpretation of the organisms and this phenomenon with a direct realistic representation through a microscope. Continue reading

The umwelt of a paramecium

On days when it rains and I am stuck inside at a desk, I often find my thoughts return to a single thematic idea: how does a single-celled organism perceive the world? Having recently read Devin Johnston’s Creaturely and Other Essays, I was struck by the author’s same general thought with regard to the higher vertebrates—in this case, the starling: “As science discovers the spectral sensitivities of birds, their sensory world proves alien to ours, their consciousness recessed from us.” Unlike that of humans, the eye of the starling does not filter out the ultraviolet spectrum of light. The organism sees the world with a fourth dimension attached—its world is, in essence, unknowable to us.

Season: organic/plant motifs and structures of microorganisms. Print by Yellena James (www.yellena.com)

As a sensory experience of one’s environment, this seeing is subjective, what the German biologist Jakob von Uexküll called each organism’s umwelt—what in German literally means “environment,” but which is typically taken as “subjective universe.” The term stands against a typical assumption of modern ecology that all organisms in an ecosystem share the same environment. Instead, von Uexküll argued that the subjective perception of organisms drives ecological interactions—parasitism, mutualism, etc. The entomologist/molecular biologist Alexei A. Sharov, who himself moved from ecology into the emerging field of biosemiotics, contextualizes the theory best with an example from plant ecology:

Uexküll thought that organisms may have different umwelts even if they live in the same place. A stem of a blooming flower is perceived differently by an ant, cicada-larva, cow, and human. Umwelt is not an ecological niche because niches are assumed to be objective units of an ecosystem which can be quantified using external measuring devices. On the contrary, umwelt is subjective and is not accessible for direct measurement for the same reason that we have no direct access to perceptions of other people.

Pistil. Photograph by author.

von Uexküll argued we cannot know the precise, quantified experience of the ant, cicada, or cow, just as Johnston struggles against studies of animal behavior that claim to have understood the way a starling sees. Each organism’s umwelt exists in a reciprocal exchange between phenomenological experience and the biophysical world—one of von Uexküll’s main ideas from the umwelt theory is that each component of this subjective universe has functional meaning to the agent. The stem of a blooming flower may be food, shelter, landmark, etc, depending on the species and context of the interaction. Each organism actively participates in the production of umwelt through these repeated interactions. In Sharov’s words,  the organism “simultaneously observes the world and changes it; the phenomenon which Uexküll called a functional circle.” Because these interactions are tied up with functional use and subjective experience, von Uexküll’s approach to animal behavior could not separate subjective (experience) from objective (biophysical matter), as modern-day approaches to the subject commonly insist—mind makes the world meaningful, a staple of cultural anthropology. In the related field of the philosophy of science, Sharov allies von Uexküll with pragmatism, the school of thought that argues how objects cannot be separated from interpreters.

Continue reading

Visualizing Plasmodium

Many say the human malaria parasites, five species of the genus Plasmodium, are host to an inherently complex and complicated life cycle. When reading a description of the various stages of the protozoan’s life, this would certainly appear the truth—each form is visually different; home to detailed mechanisms of transformation; subject to alien terminologies, words ending in –cyte, processes like schizogony.

Merozoites of Plasmodium infecting red blood cells. Image courtesy of National Geographic.

As an example, find below a description of the lives of Plasmodium, found in a 2010 review focusing on the history of how the parasite, its transmission via the mosquito vector, and its pathogenesis were discovered.

Infection begins when (1) sporozoites, the infective stages, are injected by a mosquito and are carried around the body until they invade liver hepatocytes where (2) they undergo a phase of asexual multiplication (exoerythrocytic schizogony) resulting in the production of many uninucleate merozoites. These merozoites flood out into the blood and invade red blood cells where (3) they initiate a second phase of asexual multiplication (erythrocytic schizogony) resulting in the production of about 8-16 merozoites which invade new red blood cells. This process is repeated almost indefinitely and is responsible for the disease, malaria. As the infection progresses, some young merozoites develop into male and female gametocytes that circulate in the peripheral blood until they are (4) taken up by a female anopheline mosquito when it feeds. Within the mosquito (5) the gametocytes mature into male and female gametes, fertilization occurs and a motile zygote (ookinete) is formed within the lumen of the mosquito gut, the beginning of a process known as sporogony. The ookinete penetrates the gut wall and becomes a conspicuous oocyst within which another phase of multiplication occurs resulting in the formation of sporozoites that migrate to the salivary glands of a mosquito and are injected when the mosquito feeds on a new host.

The process becomes somewhat clearer with the aid of the following simple cyclical diagram.

Life cycle of the Plasmodium parasite

But even then, there’s still mystique to the organism. We now know that sporozoites are the infective stage of Plasmodium, that they are injected into the human body by the mosquito’s proboscis, and that they become merozoites through exoerythrocytic schizogony in the liver, specifically in the hepatocyte cells.  We now know that these same merozoites invade red blood cells, the erythrocytes, and undergo another process of multiplication known as erythrocytic schizogony. We now know that the replication of merozoites continues in the erythrocytes, and that some of these develop into male and female gametocytes. We now know that these move throughout the bloodstream until they are taken up by another feeding mosquito, and that within the vector the gametocytes develop again into gametes, fertilize, and undergo sporogony, the process of ookinete development and the eventual production of new sporozoites, completing the circle.  But words like exoerythrocytic, ookinete, and schizogony are rather abstract—much in like the anthropological critique of popular reliance on statistics, the abstraction often obscures rather than illuminates form. As statistics seem to hide the individual, a voice, a face, a story, the technical description of Plasmodium‘s journey and fate leaves our imagination empty as to what a sporozoite actually looks like, if merozoites are larger or smaller than their life-history precursor, how the gametocytes move in the blood. In short, we can’t really picture Plasmodium. And if we can’t picture the organism, then all these processes, which are so detailed and meticulous in containing the What’s of each stage, become rather shallow. If we are told how photosynthesis works, but haven’t seen a leaf much less a chloroplast, knowing the process isn’t of much use. Even without considering this protozoa, it isn’t hard to imagine the conundrum of how this visualizing and understanding applies to microorganisms.

Luckily, we have researchers not only hard at work, but also committed to—in contrast to the popular saying—seeing the trees, not just the forest. The following video comes from DNAtube, a fantastic scientific video site, where you can find detailed visuals in motion of a range of biological processes and phenomena. Although the narrator of the video comments that the life cycle of Plasmodium is “very complex,” the visualization  asserts the opposite: the life of the organism is not inherently simple—it is complex is nature—but it can be displayed and explained in simple form, perhaps even in ways that exhibit beauty of sorts.