Ticks and the bacteria they carry: a dangerous combination
Ticks have a bad reputation, and this reputation is often well-deserved. These creatures are among the primary vectors of pathogens for humans, as well as for many other terrestrial vertebrates.
Olivier Duron, University of Montpellier
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The list of diseases associated with tick bites and the pathogens they transmit seems endless and, to be honest, rather unappetizing: Lyme disease, Crimean-Congo hemorrhagic fever, spotted fevers, relapsing fevers, tick-borne encephalitis, and many others. (For more information: Ticks, fleas: Lyme disease, a real time bomb)
However, ticks do not harbor only pathogens; the majority of the microorganisms they carry are actually quite harmless. Our research, led by the CNRS in collaboration with CIRAD, has recently established that certain bacteria are even necessary for the survival of ticks. Without these symbiotic bacteria, ticks experience premature developmental arrest, leading to a slow decline. This symbiosis, so vital to ticks, has its origins in their highly specialized diet.
From strict hematophagy to nutritional symbiosis
More than 900 species of ticks are known today! Some species are found in our woodlands, some in tropical forests, and others even in Antarctica. What they all have in common, however, is that they are blood-sucking: they feed on blood, just like mosquitoes and fleas.
Florian Binetruy, Author provided
Unlike the latter, ticks feed exclusively on blood, starting from the very beginning of their lives. They are strict blood feeders. This highly specialized diet is not without consequences, because while blood is rich in protein, it is relatively poor in certain nutrients, such as B vitamins. Eventually, the tick should therefore suffer from nutritional deficiencies and waste away. However, this is not the case, and the question remained as to why.
While animals generally lack the ability to synthesize B vitamins, many bacteria are actually quite capable of doing so. But could they do the same for ticks? Our initial studies on ticks quickly showed that, while they do harbor pathogens, they much more commonly host symbiotic bacteria.
These bacteria were found in nearly all of the ticks examined. The most curious aspect is the location of these symbiotic bacteria within their bodies. Contrary to expectations, these symbiotic bacteria are not part of the gut microbiota. Their lifestyle is much more specialized. They are obligate intracellular bacteria, unable to survive outside a tick cell. The mode of transmission of these symbiotic bacteria is equally intriguing and allows ticks to become infected even before they hatch from the egg.
Through a mode of transmission known as transovarian transmission, mother ticks pass these symbiotic bacteria on to their future offspring via their own ovaries. This process results in perfect transmission, in which all tick eggs carry symbiotic bacteria. The symbiotic relationship is thus maintained stably from one generation of ticks to the next. As long as this transmission chain remains unbroken, the association can persist for a long time. The oldest known example dates back more than 14 million years, continues to this day, and involves ticks commonly found in dogs’ ears. By comparison, 14 million years ago, the human lineage did not yet exist…
Supply of B vitamins by bacteria
It is precisely these symbiotic bacteria that enable ticks to be strictly hematophagous. Sequencing of their genomes has revealed the presence of genes involved in the biosynthesis—that is, the formation and production—of several types of B vitamins, such as biotin, folic acid, and riboflavin. These biosynthetic pathways are intact and functional in all tick symbiotic bacteria examined to date.
Marie Buysse, Author provided
These sources of B vitamins are vital for ticks, and anyone could come up with a unique way to kill a tick: all you have to do is deprive it of these symbiotic bacteria. Thus, adding a simple antibiotic like rifampicin to the blood that ticks feed on will quickly lead to the eradication of the symbiotic bacteria and, in the medium term, a vitamin B deficiency. The ticks then stop developing, are unable to molt, and exhibit physical abnormalities. These symptoms are those of acute nutritional deficiency.
None of these symptoms appear if ticks deprived of symbiotic bacteria are given an artificial supplement of B vitamins. By becoming strictly blood-feeding, ticks have thus become dependent on B vitamins and on the bacteria capable of synthesizing them. Far from being insignificant, this process has enabled the diversification of ticks and their spread throughout most terrestrial ecosystems. Without this symbiotic association, none of the ticks we know today would exist. And the same would likely be true for the diseases associated with them.
The origin of symbiosis revealed by genomes
The story doesn’t end there, however. Analyzing the genomes of symbiotic bacteria also provides a better understanding of how this symbiosis arose and allows us to trace it back to its origins. Most symbiotic bacteria found in ticks belong to bacterial genera well known to doctors and veterinarians: these genera, Francisella or Coxiella, include species that are particularly virulent to humans and other vertebrates.
These pathogenic bacteria are responsible for infectious diseases that can sometimes be fatal, such as tularemia and Q fever. Symbiotic tick bacteria originate specifically from these bacterial genera, but during evolution, while certain lineages of Francisella and Coxiella evolved toward symbiosis with ticks, others evolved toward virulence toward vertebrates, including humans.
Today, these two major lineages—symbiotic versus pathogenic—have diverged significantly, while retaining several common features. The first of these is their ability to colonize and multiply within the cells of their hosts—ticks in one case, humans and other vertebrates in the other.
The second point is the need for these bacteria to synthesize B vitamins, although the purpose of this production differs profoundly. For symbiotic bacteria, this synthesis of B vitamins is key to their mutualistic interactions with ticks. For pathogenic bacteria, this synthesis is absolutely essential for their replication—and ultimately for their virulence—against human cells or those of other vertebrates. The same mechanism—the synthesis of B vitamins—has thus enabled radically different evolutionary paths between symbiotic and pathogenic lineages.
What does this discovery tell us?
This symbiotic relationship illustrates the close interconnections that can form within living organisms. The interdependence between a tick and its symbionts is such that they form a single biological entity in which one cannot survive without the other.
This process is a powerful evolutionary driver of ecological innovations: by enabling the emergence of the highly specialized diet known as strict hematophagy, symbiosis allowed the ancestor of ticks to occupy new ecological niches.
This ancestor subsequently evolved into the many species we know today. In the context of the research we conduct in our laboratories, this discovery also shows us that studying ticks solely through the lens of their pathogens is too narrow an approach. It thus encourages us to explore new avenues of research to better understand how the diversity of the living world arises and is organized.
Olivier Duron, CNRS Research Fellow, University of Montpellier
The original version of this article was published on The Conversation.