Zombie microbiology 101 - Genome engineering: It never ends well

Putting the science in fiction - Dan Koboldt, Chuck Wendig 2018

Zombie microbiology 101
Genome engineering: It never ends well

By Mike Hays

Everybody loves zombies. Okay, maybe not everybody, but zombies have been a source of frightful entertainment for years and their popularity appears to be at an all-time high in today’s media market. Why are we so drawn to them? Robert Kirkman, creator of The Walking Dead comics, believes it is our innate fear of death that draws us in. And, like the relentless nature of death, he says, “Zombies are out to get you, no matter how hard you try.”

Mira Grant, author of the Newsflesh series, thinks zombies hit the human fear chord toward infectious diseases and the potential loss of self and identity caused by a serious infection. The answer probably lies in a little of both fear theories. I lean toward the infectious disease fear side of the fence, though. Maybe the fear of being chased by a relentless and seemingly endless mass of reanimated corpses hell-bent on eating my brain is enough to draw me in, but the fear of losing myself to an unseen microbial Zombie Factor that strikes without warning is what brings me back craving more.

The zombie apocalypse

The world turned to chaos as flesh-eating reanimated corpses shuffled across the planet seeking human brains for a snack, caused by a zombie virus. The zombie virus is a very popular causative agent in today’s zombie media culture. Books, movies, and TV shows feature the zombie-caused-by-virus story. But is it plausible? Maybe, maybe not. Let’s take a look past the decaying flesh, the characteristic zombified gait, and the insatiable drive to consume human flesh, and delve deeper into the potential causative agent of the Zombie Factor.

Viral

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Figure 13.1: Cowpea mosaic virus (Getty Images)

Probably the most popular microbial agent for Zombie Factor is a virus. Viruses are a logical choice. They carry the fear stigma we’ve come to associate with massive outbreaks of diseases, like smallpox, influenza, Ebola, HIV, and rabies. Perfect for the Zombie Factor, right? Well … maybe not. The most dangerous viruses, the ones that spread rapidly, are usually spread by airborne contact. Other modes of transmission like infected animal bites, insect bites, and ingestion or contact with infected body fluids are slower methods of viral transmission, which in turn, slow the spread of the disease.

The Zombie Apocalypse would require a high-velocity transmission virus to be effective against the intelligent, always-prepared human populace. Viruses have limited genome size and complexity. They infect cells and may hijack some cellular processes, but I don’t believe they actually have the genomic power for a massive transformation from human to zombie. It’s a sexy, simple agent on which to base a Zombie Factor, but in my opinion, it just doesn’t have the muster to get the job done alone.

Bacterial

Ten years ago, I would have said “No way” to bacteria as a causative agent of zombification. Back in my scientific youth, bacteria were thought to attach to the target cells, dump their various toxins, and cause the disease. Now we are discovering that bacteria are much more complex than simple toxin dumpers; they secrete proteins that enter the host cell and modulate host gene expression. What does that mean? It means the bacteria can reprogram normal cellular responses. The bacteria can create host mediated changes that help its survival. Now things are getting interesting, huh? Especially when we add another layer of complexity and consider the effects of a whole population of bacteria, called a microbiome.

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Figure 13.2: Escherichia coli under the microscope (Getty Images)

A microbiome is the collection of microorganisms that exist in a particular environment. Part of what we research in the laboratory where I work involves study of the intestinal microbiota and the interactions these microbes have with the host gut in the prevention of infection with enteric pathogens. It is an intricate system where the microbes modulate the cells of the intestine to benefit both the host and the microorganisms of the gut biome. Frankly, I did not put zombification and microbiome together until I received a research alert for an article describing the human death microbiome that the authors called the thanatomicrobiome. The researchers report a characteristic, time-dependent, postmortem shift of the bacteria in human tissues from a predominantly aerobic (oxygen-loving) bacterial population to an anaerobic population (oxygen-hating).

Perhaps a single bacterial pathogen may not have the bang to induce zombification, but how about a unique zombie microbiome signature that triggers a transformation in a nearly dead human host to a member of the walking dead? The zombie bite introduces a zombie bacterial microbiome, which sits resident in the host until the host faces death and then triggers the transformation from deathbed human to walking dead. Sweet, huh?

Parasites

Parasites are potential contributors to the Zombie Factor. Fungus, yeasts, molds, and worms all have the genomic complexity that could potentially cause such a massive transformation. There are documented cases of parasites affecting host behavior, with many of these discoveries rising from the relatively new field of neuroparasitology. Toxoplasma gondii infection in rats makes them less scared of cats and more likely to be eaten by these cats, where the parasite multiplies and is released in the cat’s feces.

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Figure 13.3: Schistosoma mansoni parasite (Getty Images)

The flatworm Euhaplorchis californiensis changes its fish host’s behavior to make it easier to be captured and eaten by birds, which are the reproductive host. Perhaps the most interesting example is of the Ophiocordyceps fungus, coined the zombie-ant fungus. The fungal spore lands on a specific variety of carpenter ants. The spore secretes enzymes that allow entry into the body of the ant where the fungus grows. The fungus releases chemicals to hijack the ant’s neural control and forces it to wander until it finds the perfect leaf. The ant then bites into the leaf, dies, and the fungus grows out of the dead ant’s head. The fungal shoot matures to produce and release more spores that infect more ants. Zombie ants. As you can see, parasites really begin to reveal a microorganism with the potential to be a cause of the behavioral changes associated with zombification and become a potential candidate for the elusive Zombie Factor.

Prions

Prions can be another potential Zombie Factor agent. These infectious proteins are often described as virus-like particles without any nucleic acid (DNA or RNA). In general, these prion proteins appear to be similar to native host proteins, except with slight conformational changes that cause them to spontaneously aggregate, form crystals, and kill host cells. Prion diseases are associated with the degenerative brain disorders, Creutzfeldt-Jakob disease and Kuru in humans, bovine spongiform encephalitis (“mad cow”) disease in cattle, chronic wasting disease in deer or elk, and scrapie in sheep.

The degenerative neurological effects of prion-induced, spongiform encephalitis affect brain function and could potentially induce the characteristics of a zombie: wonky motor skills, speech, and eating disorders. The problem with prions is they are slow moving. It may take years for enough prion protein crystals to aggregate to the point of causing brain damage.

Epidemiology

Can the Zombie Factor move fast enough, be infectious enough, and (this should be a big AND) spread without some sort of intervention through immunity or vaccination, to actually produce an effective army of the living dead hell-bent on live, healthy, human brain consumption?

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Figure 13.4: Ebola virus virion (Getty Images)

Well, it would have to have a high reproduction number (RO value), which is a calculation used by epidemiologists to enumerate how many new individuals become infected per one original infected individual. A more contagious infectious agent would have a higher RO value (measles with an RO of 18) than a less contagious infectious agent (Ebola virus with an RO of 2).

Airborne transmission of the infectious agent also makes it a more contagious agent, which makes it more likely to be spread rapidly and widely. The Zombie Factor would have to be more easily transmittable than through the accepted model of a bite. Infectious agents transmitted through bites are usually slow spreading. Rabies, for example, a virus often associated as the basis for a zombie virus, is a rare disease with an RO value barely over 1. Another thing to consider is the level of care and monitoring of the established public health care system. The rapid recognition, diagnosis, and appropriate isolation of those infected can lower the RO value to almost 0. An infectious Zombie Factor would have to spread very quickly before containment of those infected could be completed in order to be effective.

The zombie factor

In a nutshell, although there are some intriguing, existing microbial agents that have some potential to be the infectious Zombie Factor, they all fall short. Zombie populace, the odds are not in your favor for a global takeover. It doesn’t matter if you are a classic voodoo zombie fan (that’s a whole separate essay on toxins!) or are drawn to the brain-eating side of the zombie fence either by your fear of death or your fear of infectious disease—keep your walking dead fandom going strong. Keep reading, keep watching, and keep writing. And keep those awesome zombie stories coming.