Microscopic Manipulators
How Tick Symbionts Influence Their Host’s Behavior
The past few decades of molecular biology research have delved deeply into the microbiomes of many species, examining the complex biochemical relationships between animals and the bacteria and parasites that live inside them. Insect microbiomes are of particular interest to science since insects are vectors of many devastating human, animal, and plant diseases. Numerous studies have revealed that the microscopic organisms that live inside insects have profound effects on their hosts’ biology. Many of these effects are directly tied to the microorganisms’ life history, increasing their likelihood of survival and proliferation.
Recently, a team of researchers in the Netherlands compared the effects of two symbiotic bacteria (Midichloria mitochondrii and Borrelia burgdorferi) on Ixodes ricinus ticks. These symbionts, which infect and live inside many European I. ricinus ticks, employ separate strategies to infect new hosts. Midichloria mitochondrii is a vertically transmitted bacterium — it exclusively infects ticks and is passed down “vertically” from one host to its offspring. Borrelia burgdorferi is a horizontally transmitted bacterium that is passed “horizontally” from infected ticks to the vertebrates that they bite. While M. mitochondrii is harmless, B. burgdorferi is pathogenic, causing Lyme disease in mammals. The researchers sought to determine whether the different transmission strategies of these two symbionts caused them to produce separate effects on the physiology and behavior of their insect hosts.
The researchers collected three groups of ticks (728, 607, and 165 individuals, respectively) from their natural habitat. The first and second groups were used in initial molecular and behavioral experiments. The third group was used to repeat the experiments, confirming the results obtained with the first and second groups.
The researchers used the first group of ticks to assess differences between weight and lipid fraction between ticks infected with different symbiotic bacteria. They humanely killed the ticks by freezing them, then weighed them before homogenizing them (crushing them up with a mortar and pestle). The homogenized tick samples were used to measure lipid fractions and determine the symbionts present in each tick.
The second group was used to assess the differences in activity patterns between ticks infected with different symbionts. The researchers filmed the ticks’ behavior within vertical polycarbonate tubes for 7 hours. After the behavior analysis, the ticks were killed, and DNA was extracted to determine the symbionts present in each sample.
The researchers found that M. mitochondrii and other vertically transmitted symbionts were associated with higher tick weights. This may indicate that these symbionts confer a fitness advantage on the ticks that facilitates their survival and reproduction. The researchers also discovered that B. burgdorferi and other horizontally transmitted symbionts were associated with lower tick weight and higher activity levels. Since these effects can render ticks more likely to seek out blood meals, they may indicate that these symbionts influence tick behavior and physiology to increase the likelihood of infecting vertebrates. Together, these results show that symbiotic bacteria in ticks influence host biology in different ways, which likely serve to facilitate the transmission of the bacteria to new hosts.
