Modeling the Life Cycle of the Intramitochondrial Bacterium " Candidatus Midichloria mitochondrii" Using Electron Microscopy Data

Abstract

"Candidatus Midichloria mitochondrii" is a Gram-negative bacterium that lives in strict intracellular symbiosis with the hard tick Ixodes ricinus, forming one of the most intriguing endosymbiosis described to date. The bacterium is capable of durably colonizing the host mitochondria, a peculiar tropism that makes "Ca. Midichloria mitochondrii" a very interesting tool to study the physiology of these cellular organelles. The interaction between the symbiont and the organelle has, however, been difficult to characterize. A parallelism with the predatory bacterium Bdellovibrio bacteriovorus has been drawn, suggesting the hypothesis that "Ca. Midichloria mitochondrii" could prey on mitochondria and consume them to multiply. We studied the life cycle of the bacterium within the host oocytes using a multidisciplinary approach, including electron microscopy, molecular biology, statistics, and systems biology. Our results were not coherent with a predatory-like behavior by "Ca. Midichloria mitochondrii" leading us to propose a novel hypothesis for its life cycle. Based on our results, we here present a novel model called the "mitochondrion-to-mitochondrion hypothesis." Under this model, the bacterium would be able to move from mitochondrion to mitochondrion, possibly within a mitochondrial network. We show that this model presents a good fit with quantitative electron microscopy data. IMPORTANCE Our results suggest that "Candidatus Midichloria mitochondrii," the intramitochondrial bacterium, does not invade mitochondria like predatory bacteria do but instead moves from mitochondrion to mitochondrion within the oocytes of Ixodes ricinus. A better understanding of the lifestyle of "Ca. Midichloria mitochondrii" will allow us to better define the role of this bacterial symbiont in the host physiology.

Keywords: Bdellovibrio-like hypothesis; Ixodes ricinus; endosymbiosis; mitochondrial network; “Candidatus Midichloria mitochondrii.

FIG 1

Transmission electron microscopy observation of “Ca. Midichloria mitochondrii” bacteria in Ixodes ricinus oocytes. TEM images of mitochondria of Ixodes ricinus oocytes colonized by at least one (a), two (b), three (c), four (d), and five (e) “Ca. Midichloria mitochondrii” cells. In each photo, the red arrow indicates the mitochondrial membrane, the letter “a” indicates an intramitochondrial M. mitochondrii cell, and the letter “b” indicates the mitochondrial matrix. “Ca. Midichloria mitochondrii” cells possibly in replication within a mitochondrion (f) and in the cytoplasm (g); b, bacterium; m, mitochondrion.

FIG 2

Complementary cumulative distributions of the different levels of mitochondrial colonization. (a) Complementary cumulative distribution (CCD) of the mitochondrial colonization levels; x axis shows the number of intramitochondrial “Ca. Midichloria mitochondrii” (Mm) plus 1; y axis shows the frequency of mitochondria colonized by at least Mm plus 1 bacteria. (b) Log-log plot of the CCD. The Mm plus 1 scale (i.e., 1 for bacterium-free mitochondria, 2 for mitochondria colonized by one bacterium, and so on) has been used to allow the transformation of the CCD plot to the log-log plot. In both graphs, the line connecting the CCD points is colored blue, while the CCD confidence interval is reported in light blue. This graph shows that the distribution of the frequency of mitochondrial colonization levels follows a power law.

FIG 3

Scale-free, random, and small-world graphs characterized by different topologies.

FIG 4

Complementary cumulative distribution of the different levels of mitochondrial colonization obtained from TEM data and from simulations. The complementary cumulative distribution (CCD) of the mitochondrial colonization levels (i.e., the frequency of mitochondria colonized by at least Mm plus 1 bacteria) obtained from TEM data is reported in blue, and the relative confidence interval is in light blue. The CCDs calculated from the simulations performed to test the mitochondrial network hypothesis on different mitochondrial network topologies are also reported, colored red for scale-free topology, green for random topology, and brown for small-world topology. All the simulations fit well the frequencies of mitochondria colonized by at least zero, one, and two bacteria; indeed, the estimated values fall within the confidence interval of the TEM data. On the other hand, only the simulations of scale-free networks fit well the frequencies of mitochondria colonized by more than two bacterial cells.