In the UK, thousands of oak trees are “bleeding,” oozing dark liquid from cracks on their bark. What lies underneath are fluid-filled necrotic cavities that disrupts their life force by impeding the normal flow of nutrients and water. Taking advantage of the weakened state of the trees, are bark-boring beetles, which lay their eggs in the cracks of the bark. These are the characteristic signs of Acute Oak Decline, a disease that can kill a tree within four or five years of symptom onset.
Acute Oak Decline made its appearance in 2008. Now, scientists have found that Acute Oak Decline is caused by a polymicrobial infection – in this case, a simultaneous infection of three bacterial species: Brenneria goodwinii, Gibbsiella quercinecans, and Rahnella victoriana. With the microbes (B. goodwinii and G. quercinecans) and beetle larvae in hand, the researchers were able to replicate the characteristic signs of Acute Oak Decline.
These researchers revisited principles developed in the 1800s designed to establish a causative relationship between a microbe and a disease: Koch’s postulates.
But there’s a twist. When Koch’s postulates were established, the paradigm of infectious disease was one organism causes one disease. Now, microbiologists are recognizing the importance of microbial communities and polymicrobial interactions, as well as environmental and hosts factors, in causing disease. The researcher’s modern adaptation of Koch’s postulates made it possible for them to identify the cause of Acute Oak Decline.
The first steps in solving this microbial mystery was to isolate and compare bacteria from healthy and diseased oak trees using conventional culturing techniques and next-generation sequencing. They found a significant co-occurrence of G. quercinecans and B. goodwinii on diseased tissues but not on healthy trees using culture-based methods. Using next-generation sequencing, they found that B. goodwinii, G. quercinecans, and R. victoriana were detected in all diseased trees.
Upon closer examination of the genome from the three species, the researchers found that they are related to organisms that parasitize plant hosts when the plant is compromised. And like plant pathogens, these three species also carry a stockpile of virulence genes.
After identifying potential culprits the researchers began to grow these microorganisms in the lab. They were only successful in growing G. quercinecans and B. goodwinii. By inoculating these microbes onto tree logs, the researchers showed that they caused necrosis of the tree’s tissue. When the bark-beetle eggs were added into the mix, damage increased significantly regardless of an individual or polymicrobial infection.
Finally, the researchers were then able to isolate the microorganisms from these newly infected trees. Through these steps, the scientists were able to prove that these microbes cause Acute Oak Decline.
The cause of infectious disease can be messy and their findings highlight the importance of incorporating contextual details in determining disease causation. By adding a modern flair to Koch’s postulates, the researchers showed that discovering the culprit of disease is not as simple as microbiologists once thought many decades ago.
