There are all sorts of bacteria. Ones that make oxygen. Ones that consume oxygen. Ones that make the acetic acid in our vinegar and ones that consume this acetic acid and turn it into acetone and other off-flavoured compounds. There are bacteria that lurk in the oceans’ depths and live off the gases released by thermal vents. There are fellas floating in the stratosphere munching dust particles. There are bacteria that live in long, fungus-like mycelia. There are others that live as free-floating single cells. There are bacteria that live in pristine water and take decades to live out their life cycle, while guys like E. coli reproduce every twenty minutes in your gut. There are producers, degraders, parasites, symbionts, pathogens, commensals. And there are the hunters and the hunted.
I had never given much thought to bacteria that preyed on other bacteria. The few genera involved seemed more an evolutionary curiosity – a sideshow – than a vital component of our microbial ecosystem. Somehow, since my days of studying amoeba, paramecia and other protists, I had come to regard bacteria as the prey of “higher organisms”, ducks sitting at the very bottom of the food chain. However, bacterial hunters may be more common than we had previously thought and, because of their mode of action and the potential to exploit them as sources of novel antibiotics, they are receiving more attention from scientists.
A recent paper by Livingstone et al, which appears in the journal, Applied and Environmental Microbiology, sheds light on a genus of bacterial hunters called Corallococcus. These wee bugs live in the soil and derive their nutrition from consuming other microbes. They are hunters, although not in the usual manner we regard hunting. They don’t chase down a Lactobacillus as if it were a gazelle and they a lion. They don’t latch on to a prey snake-like, inject it with poison, and eat it piece by piece as it dies. They don’t even engulf an Escherichia as if imitating an amoeba. Nope: these guys kill their prey through chemical warfare – or more correctly biochemical warfare. And they cooperate with one another while they do it, resembling, nothing more than a tribe of hunter-gatherers taking down a woolly mammoth.
Corallococcus predators kill their prey by secreting antimicrobial substances which cause the lysis (or rupturing) of their quarry’s cells and release of their nutrients into the environment. It is speculated, but not proven, that Corallococcus cells work together when on the hunt, speculation which derives from our understanding of another interesting and unusual behaviour of the species: they respond cooperatively to starvation by forming multicellular fruiting bodies containing spores. Corallococcus hunt a wide range of other bacterial species. Livingstone et al showed that the eight isolates they characterised for the purposes of their study effectively killed E. coli, Pseudomonas aeruginosa, Klebsiella pneumoniae, Proteus mirabilis, Staphylococcus aureus, S. epidermidis, S. saphrophyticus, Enterococcus faecalis, Bacillus subtilis and Candida albicans. That’s one heck of an impressive range of action and explains why researchers are scrambling to characterise Corallococcus and other myxobacteria relatives.
Corallococcus and their cousins produce numerous weird and wonderful toxins to burst their prey open. Their genomes are jam-packed with genes for the production of what are known as “secondary metabolites” (substance which are not critical to the organism’s survival, but which give it a competitive advantage in its environment – a metabolism with optional extras or deluxe options, if you like) and perhaps some of these could find medical or veterinary use down the line. Apparently, predatory myxobacteria are ubiquitous in soil: us microbiologists just haven’t been looking for them or studying them very much. But watch this space. Perhaps the next generation of antibiotics against superbugs such as MRSA will come from these bacterial hunters.
The Bug Hunter 