Predatory Bacteria Are Fierce, Ballistic and Full of Potential
In 1962 Heinz Stolp, a researcher in Berlin, was searching for new viruses when he ran out of the filters that sifted them from these samples. So he substituted filters with slightly larger holes: 1.35 microns instead of 0.2 microns. No viruses, which normally reproduce very quickly, grew on the glass plates he had coated with bacteria to use as virus chow, and at that point, the contents should have been tossed.
But he didn’t get around to it, and a few days later, tantalizing signs appeared: holes in the lawn of bacteria. Something very small—but not a virus—was eating his bait. Stolp, no dummy, pounced on the chance to identify a new predator.
Under his microscope was a pack of Bdellovibrio bacteriovorus. No bacteriologist had ever encountered what Stolp was now seeing: a dedicated, active and refined killer.
Bdellovibrio microbes are bacteria-seeking torpedoes sometimes called “the world’s smallest hunters.” Only one fifth the size of a typical bacterium, they punch above their weight. An individual Bdellovibrio darts about until it happens to smash into prey. When it does, the impact is so violent that the victim reels several cell lengths and stops moving within seconds.
Affixed to its victim ballistically, Bdellovibrio bores inside and cleans out the place like a cross-country team at a post-10K buffet. Its systems for relieving prey of their innards are so evolved (one author termed the process “exquisite molecular dissection”) that when it is finished eating, nothing is left but ghosts and membrane fragments.
Most bacteria eat detritus or produce their own food. But since Stolp’s discovery of the hunter Bdellovibrio, scientists have come to realize predatory bacteria are diverse, important and potentially useful. They rival killers such as amoebas and viruses as killers of the vast bacterial herds of the soil, and scientists hope that they could also be put to work in humans and animals as dynamic antibiotics.
Once scientists figured out what to look for, they discovered Bdellovibrio-like organisms just about everywhere, including bird and mammal feces and possibly even human blood, where short nucleic acid sequences with predatory bacteria signatures have been found. Like viruses that prey on bacteriathey could be inside you right now.
Most bacterial predators don’t attack their prey like that Bdellovibrio does, but their strategies can be no less inventive. Lysobacter you specialize in demolition on contact and can blow up not only other bacteria but also green algae, fungi and even little squirming animals called roundworms.
Vampiro vibrio and Vampirococcus, true to their name, have cytoskeletal protrusions sometimes termed “fangs” that sink into the bodies of prey before draining the contents. Some of these are vampires actually vegetarian: Vampirovibrio chlorellavorus hunts green algae It’s so good at its job that it has become a pest in experimental algal bioreactors designed to produce biofuel. Once it finds its way into one of the reactors, almost 100 percent of the crop is dead within a day or two.
Others hunt in “wolf packs.” Herpetosiphon bacteria are huge gliding filaments that can exceed a millimeter in length and are apparently lapsed algae. Under the microscope they look like a swarm of snakes and use their collective muscle to punch holes in the colonies of their prey like a battering ram or bulldozer. Once inside, they torch the village by bursting prey with disruptive chemicals while others Herpetosiphon cells form a wall to prevent escapes. They all then feast on the entrails.
A group of gliding, spiral-shaped bacteria called Saprospira appear to have stumbled upon the strategy employed by some fungi, plants and lots of spiders: they catch prey with their adhesive bodies, sometimes even growing webs. Some can catch swimming bacteria by the tip of their tail, and those bacteria then spin futilely on their axis after being caught. These predators can even pick up food simply by moving like the sticky, tiny offspring of a sandworm and a piece of cavatappi pasta.
Yet this menagerie may only be the beginning. We still know little about the true diversity or importance of predatory bacteria, and the knowledge gap with animals is so large as to be grotesque. For those of you keeping track at home, Stolp’s 1962 discovery of Bdellovibrio is the equivalent of spotting wildebeest in 1676 (the year Dutch scientist Antonie van Leeuwenhoek first spotted bacteria) but not happening to notice lions until almost 300 years later.
To begin to grasp the true ecological power of predatory bacteria, a study published in mBio in April 2021 measured predators’ response to radioactively-labeled food added directly to their habitat in 15 wild sites across North America. It found that dedicated (“obligate”) predators such as Bdellovibrio and Vampiro vibrio grew, metabolized and fed much faster than nonpredatory bacteria when food was abundant And the more food there was, the more dominant they became in the ecosystem, a finding that also holds true for predatory animals.
The authors concluded that bacteria’s importance as soil predators rivals that of viruses and more complex microbes such as amoebas that we have studied much more extensively. If true, that’s a big deal. In this century we may find we care very much about the accuracy of our earth system models. If so, we’re going to want to know all we can about bacteria with vast powers over the productivity of soil and water.
But bacterial predators’ significance reaches beyond ecology. The biochemical arsenals of these microbes are rich targets for biotechnology: new antibiotics are sorely needed. Scientists have already proposed and tested the idea of using whole predatory bacteria in animals—applied topically, ingested or even injected—as living antibiotics. Remarkably, they so far seem to be both safe and effective in lab animals. Resistance may prove more elusive for pathogens against wily, evolvable predators than against the static biochemical antibiotics we currently employ.
If there is one lesson to take away from the story of Bdellovibrio and its ilk, it’s a familiar refrain: funding basic research on diverse organisms is vital, not only because it’s important in its own right but because momentous discoveries lurk where we least expect them. Scottish microbiologist Alexander Fleming was studying “a purely academic bacteriological problem” when he stumbled on penicillin. Microbiologist Emmanuelle Charpentier and biochemist Jennifer Doudna were just trying to understand how Streptococcus pyogenes defends itself from viruses when they finally understood the workings and profound uses of CRISPR-Cas9. And Stolp was looking for viral predators of a pathogen of beans—merely for classification purposes—when he discovered the world’s first dedicated bacterial predator.
This is an opinion and analysis article, and the views expressed by the author or authors are not necessarily those of Scientific American.
In 1962 Heinz Stolp, a researcher in Berlin, was searching for new viruses when he ran out of the filters that sifted them from these samples. So he substituted filters with slightly larger holes: 1.35 microns instead of 0.2 microns. No viruses, which normally reproduce very quickly, grew on the glass plates he had coated…
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