Ixodes ricinus ticks have a functional association with Midichloria mitochondrii

Abstract

In addition to being vectors of pathogenic bacteria, ticks also harbor intracellular bacteria that associate with ticks over generations, aka symbionts. The biological significance of such bacterial symbiosis has been described in several tick species but its function in Ixodes ricinus is not understood. We have previously shown that I. ricinus ticks are primarily inhabited by a single species of symbiont, Midichloria mitochondrii, an intracellular bacterium that resides and reproduces mainly in the mitochondria of ovaries of fully engorged I. ricinus females. To study the functional integration of M. mitochondrii into the biology of I. ricinus, an M. mitochondrii-depleted model of I. ricinus ticks was sought. Various techniques have been described in the literature to achieve dysbiosed or apo-symbiotic ticks with various degrees of success. To address the lack of a standardized experimental procedure for the production of apo-symbiotic ticks, we present here an approach utilizing the ex vivo membrane blood feeding system. In order to deplete M. mitochondrii from ovaries, we supplemented dietary blood with tetracycline. We noted, however, that the use of tetracycline caused immediate toxicity in ticks, caused by impairment of mitochondrial proteosynthesis. To overcome the tetracycline-mediated off-target effect, we established a protocol that leads to the production of an apo-symbiotic strain of I. ricinus, which can be sustained in subsequent generations. In two generations following tetracycline administration and tetracycline-mediated symbiont reduction, M. mitochondrii was gradually eliminated from the lineage. Larvae hatched from eggs laid by such M. mitochondrii-free females repeatedly performed poorly during blood-feeding, while the nymphs and adults performed similarly to controls. These data indicate that M. mitochondrii represents an integral component of tick ovarian tissue, and when absent, results in the formation of substandard larvae with reduced capacity to blood-feed.

Tetracycline inhibits tick host reproduction by modulating bacterial microbiota, gene expression and metabolism levels

“…In many previous studies on ticks, antibiotic treatment has been demonstrated to affect tick host physiology by reducing the load of symbiotic bacteria, thereby rendering the bacteria unavailable for action 14,20,24–29 . In this study, we found that tetracycline treatment altered the bacterial community composition and that differences in composition increased with treatment time, but this did not affect the bacterial density in whole ticks.…”

“…longicornis were similar to those received by A . americanum , 24 but were among the higher doses of tetracycline used compared to similar studies in other tick species 20,24,25,28,29 . Tetracycline treatment inhibited the reproductive fitness of H .…”

“…haemaphysaloides and I . ricinus 25,29 . These findings suggest that it is necessary to explore in depth the mechanisms underlying the physiological abnormalities in ticks caused by antibiotic treatment.…”

“…For example, the ticks Amblyomma americanum, 24 Rhipicephalus haemaphysaloides, 25 Rhipicephalus sanguineus, 26 Rhipicephalus microplus 27 and H. longicornis 28 had reduced loads of Coxiella after antibiotic treatment, and the tick hosts showed blood-feeding and developmental abnormalities, confirming the essential role of Coxiella in these tick species. Likewise, the functions of Francisella in Ornithodoros moubata 14 and Midichloria mitochondrii in Ixodes ricinus 29 were investigated using antibiotic treatment and artificial feeding systems. It is noteworthy that these studies focused on the effects of antibiotic treatment on a single symbiotic bacterium; however, the effects on the bacterial community have been less well-studied.…”

“…For example, the use of tetracycline, an antibiotic with broad-spectrum antimicrobial activity, had immediate toxic effects in the ticks R. haemaphysaloides and I. ricinus. 25,29 These findings suggest that it is necessary to explore in depth the mechanisms underlying the physiological abnormalities in ticks caused by antibiotic treatment.…”