Micavibrio aeruginosavorus (yellow), the “Vampire Bacteria” Source
Halloween will soon be upon us, and we are once again surrounded by stories of supernatural occurrences, violent crimes, monsters, and ghouls (not to mention a few creepy clowns thrown in for good measure). But are these just stories? Are ghosts, witches, and zombies just figments of our imagination, derived from folklore and strange unexplainable incidents? Or do these sinister stories have some truth behind them? As you’ll soon see, microbes are the real ghouls that bring truth to our beloved fiction.
Moldy Bread and the Salem Witch Hunt
Witches are a classic Halloween character, but there might be something a little more terrifying than a cackling old woman on a broomstick—the mold hypothesized to be the culprit behind the Salem Witch Trials. Between the years of 1692 and 1693 more than 200 people were accused of practicing witchcraft in Salem, Massachusetts ultimately leading to the hanging of 20 women. This event was instigated by a handful of girls experiencing unexplainable convulsions, vomiting, hallucinations, and creepy-crawly sensations of the skin—supposedly the handiwork of witches who had pledged allegiance to the devil.
Figure 1. Claviceps Purpurea infection on Rye, the possible source of the accusations that led to the Salem Witch Trials. Source
However, there was probably something smaller at work. The summer of 1691 was warm and damp, ideal conditions for growth of the fungus Claviceps purpurea also known as rye ergot fungus. C. purpurea infects rye, producing a toxic byproduct called ergotamine. This toxin is structurally similar to several neurotransmitters and can bind to receptors of serotonin, dopamine, and epinephrine, acting as an agonist. Ergotamine is also a vasoconstrictor, reducing blood flow, most notably to the intracranial blood vessels. These combined actions lead to ergotamine toxicity symptoms similar to those experienced by the “afflicted girls”: hallucinations, convulsions, vomiting, and skin tingling. Ergotamine poisoning had previously caused mass hysteria in Europe and the number of afflicted persons decreased during the following dry summer in Salem, corroborating the ergotamine hypothesis. Although these “witches” only existed in the minds of the poisoned girls and their community, the real horror of the Salem Witch Trials is the damage and mass insanity inflicted by a simple crop infection.
Figure 2. The vampire- like Micavibrio aeruginosavorus irreversibly attaches to and sucks nutrients directly from prey. Source
Modern-day vampires are famous for their predatory natures and oftentimes theatrical bites to their victims’ necks. But while the bloodthirsty Count Dracula is just a character from folklore, he has a counterpart in the microbiological world. Micavibrio species are predatory bacteria that attack and irreversibly attach to the cell surface of their prey. Like vampires, they survive by leaching nutrients directly from their victim, until it eventually dies. Two species in particular, M. aeruginosavorus and M. admirandus were thought to have very specific prey (Pseudomonas aeruginosa and Stenotrophomonas maltophilia, respectively). But studies show that when these Micavibrio are desperate for a meal, they no longer have a preference for a specific victim species and both begin to prey on a broad range of Gram-negative bacteria.
Research into the genome of M. aeruginosavorus revealed that the vampire bacteria lack the transport machinery to retrieve nutrients from their surroundings and require bacterial predation to survive. They need to suck the literal “life energy” from other microbes earning them the aptly given “vampire bacteria” name. These little cytoplasm suckers aren’t all bad though; they are currently being tested as an antibiotic alternative for treating human infections like cystic fibrosis.
Figure 3. Ophiocordyceps unilateralis (also known as the “Zombie Fungus”) fruiting bodies grow out of the brains of the ants it infects. Source
Zombies are a completely different breed of monster. Reanimated, brainless bodies with limbs falling off, they barely even have any primal instincts left as they meander aimlessly, almost robotically. Are you picturing a human zombie as in Dawn of the Dead? Think again. Think insect.
The classic zombie tale involves a viral infection that takes over the host’s mind, but in the case of real-life zombies, it’s a fungus of the genus Cordyceps that is responsible for the mental demise of its host. This genus includes over 500 species of fungi that attack insects by invading and replacing host tissue gradually with their own mycelium. Just like the archetypal zombie virus we see in movies, several species change and influence host behavior. The species Ophiocordyceps unilateralis specifically infects ants, forcing them to leave their canopy homes and travel closer to the forest floor where the temperature and humidity is more favorable for fungal growth. Then, in their last moments, the ants climb upward once again, finally affixing themselves to the underside of a leaf using their strong mandables. The ants remain there until they eventually die and out of their heads grows a massive fruiting body containing spores. The fungal spores benefit from the height, which increases both their distribution area and their likelihood to infect other unsuspecting ant hosts. And if real-life zombies aren’t creepy enough, how long they’ve been shambling around is striking. Evidence suggests Cordyceps’ ability to control host behavior evolved more than 48 million years ago -way before the first zombie movie came out.
What do you see when you imagine a ghost? An ethereal soul, empty, just a shadow of its former self? Perhaps it met a violent death and remains to haunt the living world with the memory of its being. Or do you scoff and say ghosts don’t exist? Believe it or not, the microbe world has their own ghosts, empty carcasses remaining after a swift bacteriophage lysis, left behind to haunt the cells around them.
Bacterial ghosts are very real. After infecting a Gram-negative bacterium, the bacteriophage gene E forms a lysis-tunnel within the envelope of the living bacteria causing the violent ejection of DNA, proteins and cytoplasmic contents into the environment. What’s left is an empty bacterial envelope, incapable of replication and just a memory of what was once a living bacteria. However, although the prospect of being “disemboweled” is quite unpleasant no matter the phylum, class or kingdom, this is a phenomenon that we shouldn’t be afraid of. In fact, bacterial ghosts can be extremely useful to humans. As a bacterial shell, bacterial ghosts are still able to stimulate the innate and adaptive immune systems without requiring help from additional adjuvants. This makes them excellent vaccine vectors since they lack DNA or transposable pathogenicity islands. These ghosts can also be used for drug delivery and tumor treatment, increasing the specificity of the treatment and lowering the required dosage. Not so spooky after all.
Natural History of Halloween
As you can see, the microbial world has some amazing and terrifying characters that put our legends to shame. Just remember as you suit up for those tricks and treats, the legends of today are not at all legends on the micro scale.