In nature, organisms are constantly faced with not only the challenges of finding food and shelter, but also, with the challenge of avoiding becoming food for somebody else! As a survival mechanism, organisms have evolved various strategies: skunks bombard their enemies with a noxious substance, and opossums roll over and play dead.
But what about smaller organisms like bacteria? Are they at risk of predation, and if so, do they have a means of protecting themselves? The answer to both of these questions is YES. For bacteria that live in the soil, some of the products they release protect them from becoming a meal for bacterial feeders like the nematode (roundworm) Caenorhabditis elegans. But the question is: what can we learn from studying these bacteria?
Over the past several years, Teri de Kievit, along with her undergraduate and graduate students in the Department of Microbiology, has been studying bacteria (isolated from Manitoba soils by Dilantha Fernando in the Department of Plant Sciences) that are able to stop the growth of pathogenic fungi. The fungi they are particularly interested in are responsible for sclerotinia stem rot (Sclerotinia sclerotiorum) and black leg (Leptosphaeria maculans): devastating diseases for canola crops that can lead to millions of dollars of lost revenue globally.
Although these diseases are currently managed through the use of fungicides, there is always the chance that they will someday become fungicide resistant. Furthermore, with increasing public concern regarding the potential toxic effects associated with chemicals being released into the environment, scientists are looking for alternative ways of controlling plant diseases. Biocontrol (the use of living organisms to control pests like fungi) is an attractive option for such purposes.
The two bacteria that de Kievit’s team works with, Pseudomonas sp. DF41 and Pseudomonas chlororaphis PA23, have strong antifungal abilities. These bacteria each produce a battery of antibiotics and degradative enzymes that inhibit the growth of the fungi.
The bacterium PA23 produces two antibiotics, called phenazine and pyrrolnitrin; however, only pyrrolnitrin is required for inhibiting these two fungi. As for DF41, this bacterium produces a novel lipopeptide antibiotic that appears to function by poking holes in the fungal cells.
Lately scientists have been asking the question, “What’s in it for the bacteria?” Producing these compounds is energetically costly; consequently, there must be some fitness advantage for the bacteria to invest in making them.
The de Kievit’s team hypothesis was that the molecules benefit the bacteria by helping them avoid being consumed by grazing predators, like the nematode C. elegans. In collaboration with Karen Brassinga, Department of Microbiology, they tested Pseudomonas sp. DF41 to see whether they were a good food source for C. elegans.
They made some interesting discoveries. When the nematodes were feeding on the wild type DF41, they did not reproduce and the worms appeared to starve. Microscopic examination revealed that the bacteria were forming a sticky coating (called a biofilm) over the nematode mouth and body, which caused the worms to starve to death.
Analysis of a second mutant that no longer produced any antibiotics showed similar results; they were a poor food source and the worms starved due to biofilm formation. Thus, the presence of the antibiotic did not seem to affect bacterial interaction with the nematodes.
However, when provided with a third mutant that is unable to produce the antibiotic as well as a sticky polymer called alginate, the worms found the bacteria quite palatable and they increased in number. They also found no biofilm formation on the surface of the nematodes when growing on the alginate-deficient strain. Thus, by producing the alginate, the strain DF41 is able to avoid becoming a food source to bacterial feeders. As for why the bacteria are producing the antibiotic - it remains an unanswered question. It’s possible that the antibiotics deter other bacterial predators in the soil, like protozoa.
It’s good news! If bacteria are able to establish themselves in the environment and avoid predation, there is a greater likelihood that they will make an effective biocontrol agent. According to a recent Canola Council of Canada study, canola adds $15.4 billion dollars in economic activity to the Canadian economy. It is, therefore, essential that we strive to promote the health and vigor of this important crop. Understanding how biocontrol agents like DF41 and PA23 work to control pathogenic fungi as well as their effects on other soil organisms is critical if these bacteria are to become a viable alternative to conventional agrochemicals.
Dr. Teri de Kievit, Microbiology, is piecing together the puzzle aimed at understanding how biocontrol agents inhibit fungal diseases in plants.
(Photo courtesy of the Canola Council of Canada.)
On the left, canola plants were sprayed with saline (control); those on the right were sprayed with the biocontrol bacteria. 24 hours later, plants were challenged with spores of the fungal pathogen (Sclerotinia sclerotiorum). Note the significant stem rot on the plants treated with saline compared to canola plants pretreated with the bacteria. (Photo courtesy of Dilantha Fernando, Department of Plant Science.)
When growing on DF41, C. elegans gets trapped in the alginate and can no longer move easily through the bacteria. The nematodes become “glued” in place.