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Nematodes may be protected by microbes attached to their surface

Nematodes and microbes co-exist in different environments, but they just might do more than that., In humans and insects, nematodes use the host and endosymbiotic microbes to assist them with parasitism. So, does the same thing happen to plant parasitic nematodes in the soil and within the roots of the plants they attack?

[Translate to English:] Unge nematoder. Foto: Md. Maniruzzaman Sikder

Nematodes come in various species and they live in very different environments, some are parasitic and others are beneficial for e.g. nutrient turnover. Some live in the soil and others are insect parasitic. Some have even been known to infect humans. Plant parasitic nematodes exist everywhere, and they remain a difficult problem to treat in many crops worldwide. 

“Plant parasitic nematodes can cause a lot of harm to crops resulting in large yield losses. Chemical nematicides have been mostly banned from the market to avoid their detrimental effects on the environment. Thus, biological control has become a great alternative, but the problem is that many biological agents have low efficiency,” post doc Olivera Topalovic from the Department of Agroecology explains. 

Native microbiota associate with nematodes

“We believe that part of the reason for the low efficiency of various biological agents may be caused by the native microbiota that are naturally associated with nematodes in the soil,” Olivera Topalovic says. 

However, the co-existence between plant parasitic nematodes and their associated microbiota has not been as extensively studied as for non-parasitic soil nematodes and human and insect parasites. By compiling studies of other types of nematodes researchers from the Department of Agroecology at Aarhus University have drawn parallels to the existing new knowledge on plant parasitic nematodes.  

Nematodes in humans

Within the field of human health there have been several studies on, what happens when nematodes parasitise humans.  

“Usually the nematodes that cause an infection are not interested in killing the host, as they want to live there. We found that up to 1.5 million people are infected by nematodes every year, and somehow the nematodes have found a way to suppress the human immune system in order to survive,” says Olivera Topalovic. 

Studies show that when nematodes are present within the human body there is an enrichment of certain human gut-microbes. This enrichment is not seen without the nematodes present, which could suggest that the two are working together, and that the microbes somehow help the nematode suppress the immune system and vice versa. 

Same picture in insects

Within entomology there have been similar studies showing insect pathogenic nematodes carrying symbiotic microbes within their gut, and once the nematodes enter what typically is the larva-stage in insects, the microbes are released and they produce toxins which kill the host.  

“These studies show that, besides the endosymbiotic bacteria that nematodes bring into the host, there might be some microbes within the host that assist them to be protected from the insect immune system. It indicates that certain microbes and nematodes collaborate, help, and protect each other,” says Olivera Topalovic.

In addition, studies with the most famous free-living model nematode, Caenorhabditis elegans, show that specific gut-microbes protect nematodes from pathogenic microbes, and that this protection is highly species-specific. 

By drawing parallels to studies within human health, entomology and free-living nematodes in soil, it is possible to create a picture of what might be going on with the less explored collaboration between plant parasitic nematodes and soil microbes. 

Microbes on the body surface of nematodes

“Plant parasitic nematodes have very small mouth part similar to very thin needles, so the opening is too small for the outside microbes to enter inside the body of the nematodes, which is why we focus on microbes attached to the nematode surface, and we believe they help protect the nematode in different ways,” says Olivera Topalovic. 

One way the microbes can protect nematodes from antagonistic microbes is by competing with the antagonistic microbes. 

“The nematode-protective microbiota can directly activate certain protective pathways in the nematode immune system. Other microbes could protect nematodes by outcompeting the antagonistic microbes by covering the entire body so no other microbiota can attach. In addition, the protective microbes could repress antagonistic microbes by producing toxins,” says Olivera Topalovic. 

A third way protecting microbiota can help nematodes comes into place, when the nematodes enter the plant roots with the microbes attached. 

“We believe that the microbes help suppress the plant immune system, or help by killing the nematode-antagonistic microbes present in the root system,” explains Olivera Topalovic. 

Paves the way for new research

The overview and parallels to previous research within different fields help pave the way for future research in nematodes and protective microbiota. 

“We have already started. Currently we are differentiating which microbes are attached to live and dead nematodes after being exposed to different treatments. In other words, we expose nematodes to soil microbes and certain bacteria, and after a few days some of them die and some are still alive. We use a next-generation sequencing approach to study which microbes are attached to the live and dead nematodes, and we assume that on the survived nematodes there will be some microbes with a protective function, while on the dead nematodes there will be more antagonistic ones. This is one way of discovering which microbes are protective and which are not,” says Olivera Topalovic. 

Understanding microbial-aided plant nematode performance is pivotal for efficient management, which is why the research is continuing beyond the review of the results in different research fields. 

Additional information
We strive to ensure that all our articles live up to the Danish universities' principles for good research communication (scroll down to find the English version on the web-site). Because of this the article will be supplemented with the following information:
Study type:Peer reviewed Opinion article (literature review)
Funding: Independent Research Fund Denmark, grant no. 9041-001398
Collaborators:Department of Agroecology
Read more: The article: “Can microorganisms assist the survival and parasitism of plant-parasitic nematodes?” is published in the journal Trends in Parasitology. It is written by Olivera Topalovic and Mette Vestergård
Conflict of interest: None

Post Doc Olivera Topalovic, Department of Agroecology, Aarhus University. Mail: otopalovic@agro.au.dk

Senior Researcher Mette Vestergård, Department of Agroecology, Aarhus University. Mail: mvestergard@agro.au.dk